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Types of fats in food
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Three-dimensional representations of several fatty acids. Saturated fatty acids have perfectly straight chain structure. Unsaturated ones are typically bent, unless they have a trans configuration.

In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28.[1] Fatty acids are a major component of the lipids (up to 70% by weight) in some species such as microalgae[2] but in some other organisms are not found in their standalone form, but instead exist as three main classes of esters: triglycerides, phospholipids, and cholesteryl esters. In any of these forms, fatty acids are both important dietary sources of fuel for animals and important structural components for cells.

History

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The concept of fatty acid (acide gras) was introduced in 1813 by Michel Eugène Chevreul,[3][4][5] though he initially used some variant terms: graisse acide and acide huileux ("acid fat" and "oily acid").[6]

Types of fatty acids

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Comparison of the trans isomer elaidic acid (top) and the cis isomer oleic acid (bottom)

Fatty acids are classified in many ways: by length, by saturation vs unsaturation, by even vs odd carbon content, and by linear vs branched.

Length of fatty acids

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Saturated fatty acids

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Main article: Saturated fat
For a more comprehensive list, see List of saturated fatty acids.

Saturated fatty acids have no C=C double bonds. They have the formula CH3(CH2)nCOOH, where n is some positive integer. An important saturated fatty acid is stearic acid (n = 16), which when neutralized with sodium hydroxide is the most common form of soap.

Arachidic acid, a saturated fatty acid
Examples of saturated fatty acids
Common name Chemical structure C :D [a]
Propionic acid CH3CH2COOH 3:0
Butyric acid CH3(CH2)2COOH 4:0
Caprylic acid CH3(CH2)6COOH 8:0
Capric acid CH3(CH2)8COOH 10:0
Lauric acid CH3(CH2)10COOH 12:0
Myristic acid CH3(CH2)12COOH 14:0
Palmitic acid CH3(CH2)14COOH 16:0
Stearic acid CH3(CH2)16COOH 18:0
Arachidic acid CH3(CH2)18COOH 20:0
Behenic acid CH3(CH2)20COOH 22:0
Lignoceric acid CH3(CH2)22COOH 24:0
Cerotic acid CH3(CH2)24COOH 26:0

Unsaturated fatty acids

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Main article: Unsaturated fat
For a more comprehensive list, see List of unsaturated fatty acids.

Unsaturated fatty acids have one or more C=C double bonds. The C=C double bonds can give either cis or trans isomers.

cis
A cis configuration means that the two hydrogen atoms adjacent to the double bond stick out on the same side of the chain. The rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in the cis configuration, the less flexibility it has. When a chain has many cis bonds, it becomes quite curved in its most accessible conformations. For example, oleic acid, with one double bond, has a "kink" in it, whereas linoleic acid, with two double bonds, has a more pronounced bend. α-Linolenic acid, with three double bonds, favors a hooked shape. The effect of this is that, in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be closely packed, and therefore can affect the melting temperature of the membrane or of the fat. Cis unsaturated fatty acids, however, increase cellular membrane fluidity, whereas trans unsaturated fatty acids do not.
trans
A trans configuration, by contrast, means that the adjacent two hydrogen atoms lie on opposite sides of the chain. As a result, they do not cause the chain to bend much, and their shape is similar to straight saturated fatty acids.

In most naturally occurring unsaturated fatty acids, each double bond has three (n−3), six (n−6), or nine (n−9) carbon atoms after it, and all double bonds have a cis configuration. Most fatty acids in the trans configuration (trans fats) are not found in nature and are the result of human processing (e.g., hydrogenation). Some trans fatty acids also occur naturally in the milk and meat of ruminants (such as cattle and sheep). They are produced, by fermentation, in the rumen of these animals. They are also found in dairy products from milk of ruminants, and may be also found in breast milk of women who obtained them from their diet.

The geometric differences between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, and in the construction of biological structures (such as cell membranes).

Examples of Unsaturated Fatty Acids
Common name Chemical structure Δx[b] C:D[a] IUPAC[10] n−x[c]
Omega−3:
Eicosapentaenoic acid CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH cis,cis,cis,cis,cis-Δ5,Δ8,Δ11,Δ14,Δ17 20:5 20:5(5,8,11,14,17) n−3
α-Linolenic acid CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH cis,cis,cis-Δ9,Δ12,Δ15 18:3 18:3(9,12,15) n−3
Docosahexaenoic acid CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)2COOH cis,cis,cis,cis,cis,cis-Δ4,Δ7,Δ10,Δ13,Δ16,Δ19 22:6 22:6(4,7,10,13,16,19) n−3
Omega−6:
Arachidonic acid CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOHNIST Archived 2009-03-04 at the Wayback Machine cis,cis,cis,cis-Δ5Δ8,Δ11,Δ14 20:4 20:4(5,8,11,14) n−6
Linoleic acid CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH cis,cis-Δ9,Δ12 18:2 18:2(9,12) n−6
Linoelaidic acid CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH trans,trans-Δ9,Δ12 18:2 18:2(9t,12t) n−6
Omega−9:
Oleic acid CH3(CH2)7CH=CH(CH2)7COOH cis-Δ9 18:1 18:1(9) n−9
Elaidic acid CH3(CH2)7CH=CH(CH2)7COOH trans-Δ9 18:1 18:1(9t) n−9
Erucic acid CH3(CH2)7CH=CH(CH2)11COOH cis-Δ13 22:1 22:1(13) n−9
Omega−5, 7, and 10:
Myristoleic acid CH3(CH2)3CH=CH(CH2)7COOH cis-Δ9 14:1 14:1(9) n−5
Palmitoleic acid CH3(CH2)5CH=CH(CH2)7COOH cis-Δ9 16:1 16:1(9) n−7
Vaccenic acid CH3(CH2)5CH=CH(CH2)9COOH trans-Δ11 18:1 18:1(11t) n−7
Sapienic acid CH3(CH2)8CH=CH(CH2)4COOH cis-Δ6 16:1 16:1(6) n−10

Even- vs odd-chained fatty acids

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Most fatty acids are even-chained, e.g. stearic (C18) and oleic (C18), meaning they are composed of an even number of carbon atoms. Some fatty acids have odd numbers of carbon atoms; they are referred to as odd-chained fatty acids (OCFA). The most common OCFA are the saturated C15 and C17 derivatives, pentadecanoic acid and heptadecanoic acid respectively, which are found in dairy products.[11][12] On a molecular level, OCFAs are biosynthesized and metabolized slightly differently from the even-chained relatives.

Branching

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Most common fatty acids are straight-chain compounds, with no additional carbon atoms bonded as side groups to the main hydrocarbon chain. Branched-chain fatty acids contain one or more methyl groups bonded to the hydrocarbon chain.

Nomenclature

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Carbon atom numbering

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See also: Essential fatty acid § Nomenclature and terminology
Numbering of carbon atoms. The systematic (IUPAC) C-x numbers are in blue. The omega-minus "ω−x" labels are in red. The Greek letter labels are in green.[d] Note that unsaturated fatty acids with a cis configuration are actually "kinked" rather than straight as shown here.

Most naturally occurring fatty acids have an unbranched chain of carbon atoms, with a carboxyl group (–COOH) at one end, and a methyl group (–CH3) at the other end.

The position of each carbon atom in the backbone of a fatty acid is usually indicated by counting from 1 at the −COOH end. Carbon number x is often abbreviated C-x (or sometimes Cx), with x = 1, 2, 3, etc. This is the numbering scheme recommended by the IUPAC.

Another convention uses letters of the Greek alphabet in sequence, starting with the first carbon after the carboxyl group. Thus carbon α (alpha) is C-2, carbon β (beta) is C-3, and so forth.

Although fatty acids can be of diverse lengths, in this second convention the last carbon in the chain is always labelled as ω (omega), which is the last letter in the Greek alphabet. A third numbering convention counts the carbons from that end, using the labels "ω", "ω−1", "ω−2". Alternatively, the label "ω−x" is written "n−x", where the "n" is meant to represent the number of carbons in the chain.[d]

In either numbering scheme, the position of a double bond in a fatty acid chain is always specified by giving the label of the carbon closest to the carboxyl end.[d] Thus, in an 18 carbon fatty acid, a double bond between C-12 (or ω−6) and C-13 (or ω−5) is said to be "at" position C-12 or ω−6. The IUPAC naming of the acid, such as "octadec-12-enoic acid" (or the more pronounceable variant "12-octadecanoic acid") is always based on the "C" numbering.

The notation Δx,y,... is traditionally used to specify a fatty acid with double bonds at positions x,y,.... (The capital Greek letter "Δ" (delta) corresponds to Roman "D", for Double bond). Thus, for example, the 20-carbon arachidonic acid is Δ5,8,11,14, meaning that it has double bonds between carbons 5 and 6, 8 and 9, 11 and 12, and 14 and 15.

In the context of human diet and fat metabolism, unsaturated fatty acids are often classified by the position of the double bond closest between to the ω carbon (only), even in the case of multiple double bonds such as the essential fatty acids. Thus linoleic acid (18 carbons, Δ9,12), γ-linolenic acid (18-carbon, Δ6,9,12), and arachidonic acid (20-carbon, Δ5,8,11,14) are all classified as "ω−6" fatty acids; meaning that their formula ends with –CH=CH–CH
2–CH
2–CH
2–CH
2–CH
3.

Fatty acids with an odd number of carbon atoms are called odd-chain fatty acids, whereas the rest are even-chain fatty acids. The difference is relevant to gluconeogenesis.

Naming of fatty acids

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The following table describes the most common systems of naming fatty acids.

Nomenclature Examples Explanation
Trivial Palmitoleic acid Trivial names (or common names) are non-systematic historical names, which are the most frequent naming system used in literature. Most common fatty acids have trivial names in addition to their systematic names (see below). These names frequently do not follow any pattern, but they are concise and often unambiguous.
Systematic cis-9-octadec-9-enoic acid
(9Z)-octadec-9-enoic acid 		Systematic names (or IUPAC names) derive from the standard IUPAC Rules for the Nomenclature of Organic Chemistry, published in 1979,[13] along with a recommendation published specifically for lipids in 1977.[14] Carbon atom numbering begins from the carboxylic end of the molecule backbone. Double bonds are labelled with cis-/trans- notation or E-/Z- notation, where appropriate. This notation is generally more verbose than common nomenclature, but has the advantage of being more technically clear and descriptive.
Δx cis-Δ9, cis-Δ12 octadecadienoic acid In Δx (or delta-x) nomenclature, each double bond is indicated by Δx, where the double bond begins at the xth carbon–carbon bond, counting from carboxylic end of the molecule backbone. Each double bond is preceded by a cis- or trans- prefix, indicating the configuration of the molecule around the bond. For example, linoleic acid is designated "cis-Δ9, cis-Δ12 octadecadienoic acid". This nomenclature has the advantage of being less verbose than systematic nomenclature, but is no more technically clear or descriptive.[citation needed]
n−x
(or ω−x) 		n−3
(or ω−3) 		n−x (n minus x; also ω−x or omega−x) nomenclature both provides names for individual compounds and classifies them by their likely biosynthetic properties in animals. A double bond is located on the xth carbon–carbon bond, counting from the methyl end of the molecule backbone. For example, α-linolenic acid is classified as a n−3 or omega−3 fatty acid, and so it is likely to share a biosynthetic pathway with other compounds of this type. The ω−x, omega−x, or "omega" notation is common in popular nutritional literature, but IUPAC has deprecated it in favor of n−x notation in technical documents.[13] The most commonly researched fatty acid biosynthetic pathways are n−3 and n−6.
Lipid numbers 18:3
18:3n3
18:3, cis,cis,cis-Δ9,Δ12,Δ15
18:3(9,12,15) 		Lipid numbers take the form C:D,[a] where C is the number of carbon atoms in the fatty acid and D is the number of double bonds in the fatty acid. If D is more than one, the double bonds are assumed to be interrupted by CH
2 units, i.e., at intervals of 3 carbon atoms along the chain. For instance, α-linolenic acid is an 18:3 fatty acid and its three double bonds are located at positions Δ9, Δ12, and Δ15. This notation can be ambiguous, as some different fatty acids can have the same C:D numbers. Consequently, when ambiguity exists this notation is usually paired with either a Δx or n−x term.[13] For instance, although α-linolenic acid and γ-linolenic acid are both 18:3, they may be unambiguously described as 18:3n3 and 18:3n6 fatty acids, respectively. For the same purpose, IUPAC recommends using a list of double bond positions in parentheses, appended to the C:D notation.[10] For instance, IUPAC recommended notations for α- and γ-linolenic acid are 18:3(9,12,15) and 18:3(6,9,12), respectively.

Free fatty acids

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See also: Deep frying § Oil deterioration and chemical changes

When circulating in the plasma (plasma fatty acids), not in their ester, fatty acids are known as non-esterified fatty acids (NEFAs) or free fatty acids (FFAs). FFAs are always bound to a transport protein, such as albumin.[15]

FFAs also form from triglyceride food oils and fats by hydrolysis, contributing to the characteristic rancid odor.[16] An analogous process happens in biodiesel with risk of part corrosion.

Production

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Industrial

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Fatty acids are usually produced industrially by the hydrolysis of triglycerides, with the removal of glycerol (see oleochemicals). Phospholipids represent another source. Some fatty acids are produced synthetically by hydrocarboxylation of alkenes.[17]

By animals

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Main article: Fatty acid synthesis

In animals, fatty acids are formed from carbohydrates predominantly in the liver, adipose tissue, and the mammary glands during lactation.[18]

Carbohydrates are converted into pyruvate by glycolysis as the first important step in the conversion of carbohydrates into fatty acids.[18] Pyruvate is then decarboxylated to form acetyl-CoA in the mitochondrion. However, this acetyl CoA needs to be transported into cytosol where the synthesis of fatty acids occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA, citrate (produced by the condensation of acetyl-CoA with oxaloacetate) is removed from the citric acid cycle and carried across the inner mitochondrial membrane into the cytosol.[18] There it is cleaved by ATP citrate lyase into acetyl-CoA and oxaloacetate. The oxaloacetate is returned to the mitochondrion as malate.[19] The cytosolic acetyl-CoA is carboxylated by acetyl-CoA carboxylase into malonyl-CoA, the first committed step in the synthesis of fatty acids.[19][20]

Malonyl-CoA is then involved in a repeating series of reactions that lengthens the growing fatty acid chain by two carbons at a time. Almost all natural fatty acids, therefore, have even numbers of carbon atoms. When synthesis is complete the free fatty acids are nearly always combined with glycerol (three fatty acids to one glycerol molecule) to form triglycerides, the main storage form of fatty acids, and thus of energy in animals. However, fatty acids are also important components of the phospholipids that form the phospholipid bilayers out of which all the membranes of the cell are constructed (the cell wall, and the membranes that enclose all the organelles within the cells, such as the nucleus, the mitochondria, endoplasmic reticulum, and the Golgi apparatus).[18]

The "uncombined fatty acids" or "free fatty acids" found in the circulation of animals come from the breakdown (or lipolysis) of stored triglycerides.[18][21] Because they are insoluble in water, these fatty acids are transported bound to plasma albumin. The levels of "free fatty acids" in the blood are limited by the availability of albumin binding sites. They can be taken up from the blood by all cells that have mitochondria (with the exception of the cells of the central nervous system). Fatty acids can only be broken down in mitochondria, by means of beta-oxidation followed by further combustion in the citric acid cycle to CO2 and water. Cells in the central nervous system, although they possess mitochondria, cannot take free fatty acids up from the blood, as the blood–brain barrier is impervious to most free fatty acids,[citation needed] excluding short-chain fatty acids and medium-chain fatty acids.[22][23] These cells have to manufacture their own fatty acids from carbohydrates, as described above, in order to produce and maintain the phospholipids of their cell membranes, and those of their organelles.[18]

Variation between animal species

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Studies on the cell membranes of mammals and reptiles discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids (DHA, omega−3 fatty acid) than reptiles.[24] Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega−3 fatty acids as compared to omega−6 for a given body size.[25] This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions (H+ & Na+), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitant warm-bloodedness of mammals and birds.[24] However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In fish increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lower temperatures.[26][27]

Fatty acids in dietary fats

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The following table gives the fatty acid, vitamin E and cholesterol composition of some common dietary fats.[28][29]

Saturated Monounsaturated Polyunsaturated Cholesterol Vitamin E
g/100g g/100g g/100g mg/100g mg/100g
Animal fats
Duck fat[30] 33.2 49.3 12.9 100 2.70
Lard[30] 40.8 43.8 9.6 93 0.60
Tallow[30] 49.8 41.8 4.0 109 2.70
Butter 54.0 19.8 2.6 230 2.00
Vegetable fats
Coconut oil 85.2 6.6 1.7 0 .66
Cocoa butter 60.0 32.9 3.0 0 1.8
Palm kernel oil 81.5 11.4 1.6 0 3.80
Palm oil 45.3 41.6 8.3 0 33.12
Cottonseed oil 25.5 21.3 48.1 0 42.77
Wheat germ oil 18.8 15.9 60.7 0 136.65
Soybean oil 14.5 23.2 56.5 0 16.29
Olive oil 14.0 69.7 11.2 0 5.10
Corn oil 12.7 24.7 57.8 0 17.24
Sunflower oil 11.9 20.2 63.0 0 49.00
Safflower oil 10.2 12.6 72.1 0 40.68
Hemp oil 10 15 75 0 12.34
Canola/Rapeseed oil 5.3 64.3 24.8 0 22.21

Reactions of fatty acids

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Fatty acids exhibit reactions like other carboxylic acids, i.e. they undergo esterification and acid-base reactions.

Transesterification

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All fatty acids transesterify. Typically, transesterification is practiced in the conversion of fats to fatty acid methyl esters. These esters are used for biodiesel. They are also hydrogenated to give fatty alcohols. Even vinyl esters can be made by transesterification using vinyl acetate.[31]

Acid-base reactions

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Fatty acids do not show a great variation in their acidities, as indicated by their respective pKa. Nonanoic acid, for example, has a pKa of 4.96, being only slightly weaker than acetic acid (4.76). As the chain length increases, the solubility of the fatty acids in water decreases, so that the longer-chain fatty acids have minimal effect on the pH of an aqueous solution. Near neutral pH, fatty acids exist at their conjugate bases, i.e. oleate, etc.

Solutions of fatty acids in ethanol can be titrated with sodium hydroxide solution using phenolphthalein as an indicator. This analysis is used to determine the free fatty acid content of fats; i.e., the proportion of the triglycerides that have been hydrolyzed.

Neutralization of fatty acids, like saponification, is a widely practiced route to metallic soaps.[32]

Hydrogenation and hardening

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Hydrogenation of unsaturated fatty acids is widely practiced. Typical conditions involve 2.0–3.0 MPa of H2 pressure, 150 °C, and nickel supported on silica as a catalyst. This treatment affords saturated fatty acids. The extent of hydrogenation is indicated by the iodine number. Hydrogenated fatty acids are less prone toward rancidification. Since the saturated fatty acids are higher melting than the unsaturated precursors, the process is called hardening. Related technology is used to convert vegetable oils into margarine. The hydrogenation of triglycerides (vs fatty acids) is advantageous because the carboxylic acids degrade the nickel catalysts, affording nickel soaps. During partial hydrogenation, unsaturated fatty acids can be isomerized from cis to trans configuration.[17]

More forcing hydrogenation, i.e. using higher pressures of H2 and higher temperatures, converts fatty acids into fatty alcohols. Fatty alcohols are, however, more easily produced from simpler fatty acid esters, like the fatty acid methyl esters ("FAME"s).

Decarboxylation

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Ketonic decarboxylation is a method useful for producing symmetrical ketones from carboxylic acids. The process involves reactions of the carboxylic acid with an inorganic base. Stearone is prepared by heating magnesium stearate.[33]

Chemistry of saturated vs unsaturated acids

[edit]

The reactivity of saturated fatty acids is usually associated with the carboxylic acid or the adjacent methylene group By conversion to their acid chlorides, they can be converted to the symmetrical fatty ketone laurone (O=C(CnH(2n+1))2).[34] Treatment with sulfur trioxide gives the α-sulfonic acids.[35]

The reactivity of unsaturated fatty acids is often dominated by the site of unsaturation. These reactions are the basis of ozonolysis, hydrogenation, and the iodine number. Ozonolysis (degradation by ozone) is practiced in the production of azelaic acid ((CH2)7(CO2H)2) from oleic acid.[17]

Circulation

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Digestion and intake

[edit]
Main article: Digestion § Fat digestion

Short- and medium-chain fatty acids are absorbed directly into the blood via intestine capillaries and travel through the portal vein just as other absorbed nutrients do. However, long-chain fatty acids are not directly released into the intestinal capillaries. Instead they are absorbed into the fatty walls of the intestine villi and reassemble again into triglycerides. The triglycerides are coated with cholesterol and protein (protein coat) into a compound called a chylomicron.

From within the cell, the chylomicron is released into a lymphatic capillary called a lacteal, which merges into larger lymphatic vessels. It is transported via the lymphatic system and the thoracic duct up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left subclavian vein. At this point the chylomicrons can transport the triglycerides to tissues where they are stored or metabolized for energy.

Metabolism

[edit]
Main article: Fatty acid metabolism

Fatty acids are broken down to CO2 and water by the intra-cellular mitochondria through beta oxidation and the citric acid cycle. In the final step (oxidative phosphorylation), reactions with oxygen release a lot of energy, captured in the form of large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose, but fatty acids release more energy per gram. Fatty acids (provided either by ingestion or by drawing on triglycerides stored in fatty tissues) are distributed to cells to serve as a fuel for muscular contraction and general metabolism.

Essential fatty acids

[edit]
Main article: Essential fatty acid

Fatty acids that are required for good health but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has a double bond three carbon atoms away from the methyl end; the other has a double bond six carbon atoms away from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side.[36] Two essential fatty acids are linoleic acid (LA) and alpha-linolenic acid (ALA). These fatty acids are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer-chain omega-3 fatty acids — eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which can also be obtained from fish. Omega−3 and omega−6 fatty acids are biosynthetic precursors to endocannabinoids with antinociceptive, anxiolytic, and neurogenic properties.[37]

Distribution

[edit]
Main article: Blood fatty acids

Blood fatty acids adopt distinct forms in different stages in the blood circulation. They are taken in through the intestine in chylomicrons, but also exist in very low density lipoproteins (VLDL) and low density lipoproteins (LDL) after processing in the liver. In addition, when released from adipocytes, fatty acids exist in the blood as free fatty acids.

It is proposed that the blend of fatty acids exuded by mammalian skin, together with lactic acid and pyruvic acid, is distinctive and enables animals with a keen sense of smell to differentiate individuals.[38]

Skin

[edit]

The stratum corneum – the outermost layer of the epidermis – is composed of terminally differentiated and enucleated corneocytes within a lipid matrix.[39] Together with cholesterol and ceramides, free fatty acids form a water-impermeable barrier that prevents evaporative water loss.[39] Generally, the epidermal lipid matrix is composed of an equimolar mixture of ceramides (about 50% by weight), cholesterol (25%), and free fatty acids (15%).[39] Saturated fatty acids 16 and 18 carbons in length are the dominant types in the epidermis,[39][40] while unsaturated fatty acids and saturated fatty acids of various other lengths are also present.[39][40] The relative abundance of the different fatty acids in the epidermis is dependent on the body site the skin is covering.[40] There are also characteristic epidermal fatty acid alterations that occur in psoriasis, atopic dermatitis, and other inflammatory conditions.[39][40]

Analysis

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The chemical analysis of fatty acids in lipids typically begins with an interesterification step that breaks down their original esters (triglycerides, waxes, phospholipids etc.) and converts them to methyl esters, which are then separated by gas chromatography[41] or analyzed by gas chromatography and mid-infrared spectroscopy.

Separation of unsaturated isomers is possible by silver ion complemented thin-layer chromatography.[42] Other separation techniques include high-performance liquid chromatography (with short columns packed with silica gel with bonded phenylsulfonic acid groups whose hydrogen atoms have been exchanged for silver ions). The role of silver lies in its ability to form complexes with unsaturated compounds.

Industrial uses

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Fatty acids are mainly used in the production of soap, both for cosmetic purposes and, in the case of metallic soaps, as lubricants. Fatty acids are also converted, via their methyl esters, to fatty alcohols and fatty amines, which are precursors to surfactants, detergents, and lubricants.[17] Other applications include their use as emulsifiers, texturizing agents, wetting agents, anti-foam agents, or stabilizing agents.[43]

Esters of fatty acids with simpler alcohols (such as methyl-, ethyl-, n-propyl-, isopropyl- and butyl esters) are used as emollients in cosmetics and other personal care products and as synthetic lubricants. Esters of fatty acids with more complex alcohols, such as sorbitol, ethylene glycol, diethylene glycol, and polyethylene glycol are consumed in food, or used for personal care and water treatment, or used as synthetic lubricants or fluids for metal working.

Fatty acids[44] and their derivatives like dimer acids[45] have also been used by scientists to prepare polyurethane coatings of bio-based or bio-derived coatings.

See also

[edit]
Wikimedia Commons has media related to Fatty acids.

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Fatty acid

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from Grokipedia
A fatty acid is an organic consisting of a chain attached to a terminal group, typically featuring an even number of carbon atoms ranging from 14 to 24 in biological systems. These compounds serve as the primary structural components of complex such as triglycerides, phospholipids, and sterols, playing essential roles in , integrity, and signaling pathways. In nature, fatty acids are classified based on chain length (short-, medium-, or long-chain), degree of saturation, and the position of double bonds, with most occurring as cis isomers in living organisms. Fatty acids are broadly categorized into saturated and unsaturated types. Saturated fatty acids contain no carbon-carbon s, resulting in a straight chain that allows them to pack tightly, often appearing solid at ; examples include (16:0) and (18:0), commonly found in animal fats and tropical oils like coconut oil. Unsaturated fatty acids, in contrast, feature one or more s: monounsaturated types have a single (e.g., , 18:1n-9, abundant in ), while polyunsaturated fatty acids (PUFAs) have multiple s (e.g., , 18:2n-6). These s introduce kinks in the chain, making unsaturated fats liquid at and more fluid in biological membranes. Trans fatty acids, which have trans-configured s, occur rarely in nature but are produced industrially through partial of oils, contributing to adverse effects like elevated LDL . Biologically, fatty acids are indispensable for maintaining cellular and physiological functions. They form the backbone of phospholipids in cell membranes, influencing and permeability, and are stored as triglycerides in for long-term energy reserves. Certain polyunsaturated fatty acids, known as essential fatty acids, cannot be synthesized by humans due to the absence of specific desaturase enzymes and must be obtained through diet; these include omega-6 () and omega-3 alpha-linolenic acid (ALA), which serve as precursors for longer-chain derivatives like arachidonic acid (AA), (EPA), and (DHA). These essential fatty acids are critical structural elements in neural tissues (e.g., DHA in the and ) and generate bioactive mediators such as eicosanoids, which regulate , blood clotting, and immune responses. Deficiencies in essential fatty acids can impair growth, skin integrity, and cardiovascular health, underscoring their role in preventing chronic diseases.

History

Early Discovery and Isolation

The early discovery of fatty acids traces back to the work of French chemist in the early . In 1811, Chevreul began systematic investigations into the composition of s derived from animal fats, prompted by his mentor Nicolas-Louis Vauquelin. By acidifying soap solutions, he isolated crystalline substances that displayed acidic properties and could form salts with bases, leading him to coin the term "acides gras" (fatty acids) to describe these compounds extracted from natural fats. His observations marked the first recognition of fatty acids as distinct chemical entities separable from the glycerol backbone of fats. Chevreul's experiments in the 1810s and 1820s focused on saponifying various animal and plant lipids to liberate the free fatty acids, followed by purification techniques such as recrystallization of their metal salts (e.g., or lead salts) to achieve separation based on differences. From these efforts, he isolated and named several key fatty acids, including from mutton fat in 1817, from and fats around 1819, and margaric acid (later identified as a mixture) from various sources in 1816. These isolations revolutionized the understanding of fat chemistry, demonstrating that natural fats were esters of and these organic acids, and enabling practical applications in and production through a 1825 patent with for stearic acid-based products. Throughout the , refinements in experimental methods advanced the isolation of individual fatty acids from complex mixtures in animal tallows, plant oils, and other . —boiling fats with alkali hydroxides to hydrolyze esters into and fatty acid salts—emerged as the foundational technique, with subsequent acidification yielding the free acids; this process, formalized by Chevreul, allowed scalable extraction from natural sources. Complementary advancements included of the freed acids under reduced pressure to separate them by differences, particularly effective for unsaturated acids like oleic. These methods facilitated broader access to pure compounds for and industry, with early applications in refining animal fats for production by the mid-century. Notable milestones in specific isolations during this period include , obtained in 1840 by French chemist Edmond Frémy through of , highlighting the diversity of plant-derived fatty acids. Similarly, was first isolated in 1841 by British chemist Lyon Playfair from () butter via and . The nature of these compounds was empirically confirmed through their salt-forming behavior, akin to known acids like acetic, and further validated in the by oxidation studies conducted by and contemporaries, which degraded the acids to , , and simpler carboxylates consistent with a -COOH at one end of an aliphatic chain.

Key Milestones in Research and Classification

In 1929, George O. Burr and Mildred Burr demonstrated that rats on a fat-free diet developed severe symptoms, including growth retardation and skin lesions, which could only be alleviated by supplementing with specific unsaturated fats, thereby establishing (an omega-6 polyunsaturated fatty acid) as an essential nutrient that mammals cannot synthesize de novo. Their subsequent work in the early extended this finding to alpha-linolenic acid (an omega-3 polyunsaturated fatty acid), confirming it as another required for preventing deficiency symptoms like scaly skin and reproductive failure. This breakthrough shifted the understanding of dietary fats from mere energy sources to vital components for integrity and physiological function. During the 1950s, Eugene P. Kennedy and Albert L. Lehninger elucidated the mitochondrial beta-oxidation pathway, revealing how fatty acids are sequentially shortened by two-carbon units to generate for production via the and . Their experiments with isolated rat liver mitochondria demonstrated that fatty acid oxidation is tightly coupled to ATP synthesis, providing a mechanistic link between catabolism and cellular that explained the high caloric yield of fats. This work built on earlier hypotheses and laid the foundation for studying metabolic disorders involving defective beta-oxidation. In the , and Bengt I. Samuelsson identified eicosanoids, a class of bioactive derived from polyunsaturated fatty acids like , including prostaglandins that mediate , pain, and vascular regulation. Their structural elucidation of these compounds, showing how they arise from enzymatic oxidation of C20 polyunsaturated fatty acids, highlighted their roles in physiological signaling and disease. This research earned them the 1982 in Physiology or Medicine (shared with John R. Vane), transforming fatty acids from structural molecules into precursors of potent regulatory mediators. In 2023, researchers at (QUT) employed ozone-enabled to identify 103 previously unknown unsaturated fatty acids in human plasma, , and samples, effectively doubling the catalog of known human-derived unsaturated fatty acids. This discovery revealed unexpected structural diversity, including branched and cyclic variants, and underscored the need for advanced tools to map the full human lipidome, potentially aiding discovery for metabolic and neurological conditions. From 2023 to 2025, studies have advanced the understanding of omega-3 polyunsaturated fatty acids' roles in health maintenance, with a comprehensive review indicating that supplementation preserves muscle strength in older adults by modulating inflammation and supporting protein synthesis, showing small but significant effects in randomized trials. Concurrently, research reported in highlighted that higher circulating levels of omega-3 fatty acids were associated with better lung function and slower decline in individuals with and without (COPD), suggesting benefits for maintaining respiratory health.

Definition and Structure

Chemical Composition

Fatty acids are aliphatic s consisting of a chain attached to a carboxyl group. The general formula for saturated fatty acids is CH3(CH2)nCOOHCH_3(CH_2)_nCOOH, where n≥2n \geq 2, comprising a polar carboxylic acid head (−COOH-COOH) and a nonpolar tail. Naturally occurring fatty acids typically feature unbranched carbon chains of 4 to 28 atoms in length, with even-numbered chains predominating due to their from successive two-carbon units. At physiological , the carboxyl group (−COOH-COOH) deprotonates to form a anion (−COO−-COO^-), as exemplified by stearate derived from . This combination of a charged, hydrophilic head and a hydrophobic tail renders fatty acids amphipathic.

Physical and Chemical Properties

Fatty acids display a range of physical states at that depend on their carbon chain length. containing 4 to 6 carbon atoms, such as , exist as colorless liquids with s below 0°C; for example, has a of -7.9°C. Medium-chain fatty acids with 8 to 12 carbons are typically oily liquids or waxy solids with low melting points, while long-chain fatty acids with 14 or more carbons are white solids; , for instance, melts at 69.3°C. These trends arise because longer chains enable greater van der Waals interactions, raising melting points progressively with chain length. Regarding solubility, fatty acids are poorly soluble in due to the hydrophobic of their nonpolar alkyl chains, which dominate over the polar group, leading to insolubility for chains longer than about 10 carbons. In contrast, they dissolve readily in nonpolar organic solvents like , , and , where the tails interact favorably. At higher concentrations in aqueous media, fatty acids can function as , self-assembling into micelles above their (CMC), which varies with chain length but typically falls in the millimolar range for medium- to long-chain acids. Chemically, fatty acids behave as weak carboxylic acids with pKa values of approximately 4.5 to 5.0, rendering them weaker than simple carboxylic acids like acetic acid (pKa 4.76) because the extended alkyl chain exerts an electron-donating that stabilizes the neutral form. This acidity is described by the ionization equilibrium: \ceR−COOH⇌R−COO−+H+\ce{R-COOH ⇌ R-COO^- + H^+} where R represents the alkyl chain. Trends in and are influenced by saturation level and chain length. generally decreases with unsaturation due to looser molecular packing from cis double bonds; for example, (C18:1) has a density of 0.89 g/cm³ at 25°C (), while saturated (C18:0) has a density of 0.94 g/cm³ at 20°C (solid). follows a similar pattern, with unsaturated fatty acids showing reduced values compared to their saturated counterparts owing to decreased intermolecular forces.

Classification

By Carbon Chain Length

Fatty acids are classified by the length of their carbon chain, which influences their physical properties, metabolic pathways, and biological roles. (SCFAs) contain 2 to 6 carbon atoms, medium-chain fatty acids (MCFAs) have 8 to 12 carbons, long-chain fatty acids (LCFAs) range from 14 to 20 carbons, and very long-chain fatty acids (VLCFAs) exceed 20 carbons. This categorization highlights how chain length affects volatility, absorption rates, and incorporation into cellular structures. Short-chain fatty acids (SCFAs), with 2 to 6 carbons, are volatile compounds primarily produced through microbial of dietary fibers in the gut. Acetic acid (C2:0), a key SCFA, is the main component of and contributes to its characteristic odor. Butyric acid (C4:0) is found in , where it constitutes about 4% of total fatty acids, and plays a role in gut health by serving as an energy source for colonocytes. These SCFAs are rapidly metabolized and influence host physiology, including immune modulation. Medium-chain fatty acids (MCFAs), spanning 8 to 12 carbons, are distinguished by their rapid absorption and oxidation, bypassing the need for carnitine-dependent transport into mitochondria. (C8:0), abundant in , exemplifies MCFAs and is a primary component of (MCT) oils used for quick energy provision, particularly in for disorders. MCFAs provide immediate energy due to their efficient hepatic . Long-chain fatty acids (LCFAs), with 14 to 20 carbons, predominate in human diets and form the structural backbone of most membrane . Palmitic acid (C16:0) is the most abundant saturated LCFA in the diet, comprising about 55% of dietary saturated fats, and is integral to phospholipids in cell membranes. LCFAs are essential for energy storage and signaling but require specific transport mechanisms for utilization. Very long-chain fatty acids (VLCFAs), with more than 20 carbons, are enriched in specialized tissues such as and sheaths, where they constitute significant portions of and glycerophospholipids. (C24:0) is a prominent VLCFA in these structures, supporting and neural insulation. Accumulation of VLCFAs, including , is a hallmark of X-linked adrenoleukodystrophy, a peroxisomal disorder leading to demyelination and . The length of the fatty acid chain critically impacts beta-oxidation, as different enzymes exhibit specificity for chain lengths: short- and medium-chain acyl-CoA dehydrogenases handle SCFAs and MCFAs, while long- and very long-chain variants process LCFAs and VLCFAs, primarily in peroxisomes for the latter. This enzymatic partitioning ensures efficient energy extraction tailored to chain size.

By Degree of Unsaturation

Fatty acids are classified by based on the number of carbon-carbon double bonds in their chain, which influences their chemical reactivity, physical properties, and biological roles. Saturated fatty acids contain no double bonds, making their chains fully hydrogenated and linear, which allows tight molecular packing and results in higher melting points compared to unsaturated counterparts. A representative example is , denoted as 16:0, with 16 carbon atoms and zero double bonds, commonly found in animal fats and . Monounsaturated fatty acids feature exactly one carbon-carbon , typically in the cis configuration, introducing a kink in the chain that disrupts packing and lowers the . , or 18:1 Δ9 cis, exemplifies this class, comprising the majority of fatty acids in and contributing to its liquid state at . Polyunsaturated fatty acids (PUFAs) possess two or more s, most often cis, leading to multiple kinks that further reduce packing efficiency and increase susceptibility to oxidation due to the reactive allylic positions adjacent to the double bonds. (18:2 Δ9,12), an -6 PUFA, and alpha-linolenic acid (ALA; 18:3 Δ9,12,15), an -3 PUFA, illustrate this category, with the designation indicating the position of the first double bond from the methyl end of the chain. The standard notation for fatty acids integrates chain length and unsaturation as "total carbons:number of double bonds" (e.g., 18:3 for ALA), often followed by double bond positions using the delta (Δ) system, which numbers from the carboxyl carbon, or the omega (ω) system from the methyl end; cis or trans isomerism is specified, as trans configurations promote straighter chains and better packing similar to saturated acids. This notation also links to chain length classification by specifying total carbons upfront. In 2023, researchers identified 103 previously unknown unsaturated fatty acids in samples using ozonolysis-mass spectrometry, including novel polyunsaturated variants with unconventional double bond patterns, effectively doubling the cataloged diversity of these in plasma.

By Chain Configuration

Fatty acids are classified by chain configuration into even-chain, odd-chain, branched-chain, and cyclic forms, each arising from distinct biosynthetic pathways and serving specialized roles in organisms. Even-chain fatty acids, such as (C16:0) and (C18:0), predominate in most biological systems and are synthesized via the complex using as the initial primer unit, followed by sequential additions of two-carbon units. This process results in chains with an even number of carbon atoms, which form the structural backbone of lipids and in animals, , and microorganisms. In contrast, odd-chain fatty acids, exemplified by pentadecanoic acid (C15:0) and heptadecanoic acid (C17:0), are less common and initiate synthesis with propionyl-CoA as the starter unit instead of , leading to chains terminating in an odd number of carbons after extensions. These fatty acids occur in minor proportions in most tissues but are more prevalent in ruminant-derived products, such as fat and , due to microbial in the that generates propionyl-CoA from and . For instance, odd-chain fatty acids constitute about 4-6% of total fatty acids in bovine , reflecting the unique gut of ruminants. Branched-chain fatty acids deviate from linear structures through methyl substitutions along the chain, with iso- and anteiso- forms being prominent in bacterial membranes. Iso-branched fatty acids, such as isopalmitic acid (14-methylpentadecanoic acid), feature a methyl group at the penultimate carbon, while anteiso- forms, like anteisoheptadecanoic acid (12-methylhexadecanoic acid), have the branch at the antepenultimate position; both are produced by bacteria using branched-chain acyl-CoA primers derived from amino acid catabolism to adjust membrane fluidity and packing. In ruminants, these bacterial-derived branched chains transfer to host tissues, comprising up to 4% of milk fat. Another notable branched fatty acid is phytanic acid (3,7,11,15-tetramethylhexadecanoic acid), a highly branched saturated chain originating from the phytol tail of chlorophyll in plant forages, which ruminant microbes cleave and incorporate into lipids before absorption by the host. Cyclic fatty acids represent a rare configuration, primarily featuring small ring structures integrated into the chain to enhance stability. In , cyclopropane fatty acids incorporate a three-membered ring adjacent to the carboxyl group or at internal positions, formed post-synthesis by cyclopropane fatty acid synthases that transfer a from S-adenosylmethionine to an unsaturated precursor . These rings increase rigidity and impermeability, allowing like to maintain fluidity under environmental stresses such as low pH or without altering overall chain length or saturation. Cyclic forms are scarce in eukaryotes but can arise in certain pathological conditions or from dietary sources. The metabolic implications of chain configuration extend beyond , influencing outcomes in higher organisms. Odd-chain fatty acids, particularly C15:0 and C17:0, have been epidemiologically linked to reduced risk of , with higher circulating levels associated with 14-24% lower risk in prospective cohorts, potentially due to their roles in mitochondrial function and signaling. Branched-chain fatty acids similarly modulate metabolism through altered and membrane dynamics. Cyclic fatty acids, while primarily microbial, underscore how chain variations fine-tune biophysical properties like phase transitions in bilayers.

Nomenclature

Systematic Naming Conventions

The systematic nomenclature of fatty acids follows the recommendations of the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Biochemistry and (IUPAC-IUBMB), which provide a structured approach based on the carbon chain length, degree of saturation, and configuration of double bonds. For saturated fatty acids, the IUPAC name is derived from the corresponding by replacing the "-ane" ending with "-anoic acid," where the carboxyl carbon is designated as carbon 1 (C-1). For example, the 18-carbon saturated fatty acid, commonly known as , is systematically named octadecanoic acid. Unsaturated fatty acids incorporate the suffix "-enoic acid" for one (or "-dienoic acid" for two, and so on), with locants indicating the positions of the double bonds relative to C-1. The geometry of each is specified using the E/Z designation, where corresponds to cis configuration and to trans. A representative example is , named (9)-octadec-9-enoic acid, indicating an 18-carbon chain with a cis double bond between carbons 9 and 10. Double bond positions can also be denoted using delta (Δ) notation, which marks the lower-numbered carbon of the counting from C-1 (e.g., Δ^9 for a between C-9 and C-10), or (ω) notation, which counts from the methyl terminus (e.g., ω-3 for a between C-3 and C-4 from the end). These notations are often used in shorthand alongside the systematic name, such as 18:2(Δ^9,Δ^{12}) for . For polyunsaturated acids with multiple double bonds, all positions and configurations are listed in ascending order, as in (9Z,12Z)-octadeca-9,12-dienoic acid for . Trivial names for fatty acids often originate from their natural sources or historical isolation contexts. For instance, oleic acid derives its name from the Latin oleum, meaning oil, reflecting its abundance in olive and other plant oils. Similarly, arachidonic acid's name stems from arachidic acid, which was first isolated from peanut oil (Arachis hypogaea), with the prefix "arach-" adapted from the genus name.

Common Names and Shorthand Notations

Fatty acids are frequently referred to by common names derived from their primary natural sources, facilitating their identification in nutritional, biochemical, and industrial contexts. For instance, is named after , where it constitutes about 40% of the fatty acids; derives from or animal fat, comprising 5-40% in fats; from , its major constituent; and from , present in virtually all seed oils. These names provide a practical bridge to their systematic IUPAC equivalents, such as hexadecanoic acid for , as detailed in formal conventions. In biochemical and nutritional , fatty acids are commonly denoted using notations that indicate chain length and degree of unsaturation. The general format is C_n:m, where n represents the number of carbon atoms and m the number of double bonds; for example, is abbreviated as 18:2, signifying 18 carbons and 2 s. Double bond positions can be specified using Δ notation from the carboxyl end, such as 18:2(Δ9,12) for , or omitted when contextually clear. In biological systems, unsaturated fatty acids are typically assumed to have all-cis configurations unless otherwise stated. An alternative notation, particularly useful in and , is the (ω) or n- system, which counts the position of the first from the methyl (ω) end of the chain. This highlights the family classification, such as ω-3 for alpha-linolenic acid (ALA, 18:3 n-3), where the double bonds begin at the third carbon from the methyl terminus. Similarly, is classified as 18:2 n-6. This system is essential for distinguishing families like n-3 and n-6 polyunsaturated fatty acids (PUFAs). The following table summarizes major dietary fatty acids, categorized by saturation, with representative examples, their shorthand notations, and primary sources:
CategoryCommon NameShorthand NotationPrimary Dietary Sources
Saturated (SFA)12:0Coconut and palm kernel oils
Saturated (SFA)16:0Palm oil, meat, dairy
Saturated (SFA)18:0Animal fats,
Monounsaturated (MUFA)18:1 n-9, avocados, nuts
Polyunsaturated (PUFA)18:2 n-6Seed oils (e.g., , sunflower)
Polyunsaturated (PUFA)Alpha-linolenic acid (ALA)18:3 n-3Flaxseed, chia seeds, walnuts
Polyunsaturated (PUFA) (DHA)22:6 n-3Fatty fish (e.g., ), algae oils

Sources and Production

Biological Biosynthesis in Organisms

In eukaryotic organisms, the primary site of de novo fatty acid biosynthesis is the , where the multifunctional (FAS) complex catalyzes the iterative assembly of saturated fatty acids from and precursors. This type I FAS system operates through seven cycles of , reduction, , and further reduction, starting with the priming of and incorporating seven units to yield palmitate (16:0), the most common product. The overall reaction is: 8 acetyl-CoA+7 ATP+14 NADPH→palmitate+14 NADP++8 CoA+7 ADP+7 Pi+7 CO28 \text{ acetyl-CoA} + 7 \text{ ATP} + 14 \text{ NADPH} \rightarrow \text{palmitate} + 14 \text{ NADP}^+ + 8 \text{ CoA} + 7 \text{ ADP} + 7 \text{ P}_i + 7 \text{ CO}_2 This process requires energy input from ATP for malonyl-CoA formation via acetyl-CoA carboxylase and reducing equivalents from NADPH, primarily generated by the pentose phosphate pathway. Post-synthesis modifications occur in the endoplasmic reticulum (ER) or mitochondria, where elongases add two-carbon units from malonyl-CoA to the growing acyl chain, extending palmitate to longer fatty acids such as stearate (18:0). These elongases, including ELOVL family members in animals and plants, facilitate the production of very-long-chain fatty acids essential for membrane structure and signaling. Desaturation introduces double bonds via desaturase enzymes, which are oxygen-dependent and cytochrome b5-supported in eukaryotes. Plants possess Δ12 and Δ15 desaturases that enable synthesis of polyunsaturated fatty acids (PUFAs) like linoleic (18:2 ω-6) and α-linolenic (18:3 ω-3) acids from oleate and linoleate precursors, respectively, contributing to their high ω-3 content. In contrast, animals lack Δ12 and Δ15 desaturases, limiting de novo PUFA production and rendering ω-6 and ω-3 fatty acids essential in their diets. Microbial fatty acid biosynthesis exhibits diversity, with employing a dissociated type II FAS system comprising individual enzymes in the to produce primarily straight-chain saturated and monounsaturated fatty acids. Many , such as those in the genus Bacillus, generate branched-chain fatty acids (e.g., iso- and anteiso-forms) by initiating synthesis with branched primers like isobutyryl-CoA derived from , which enhances under stress. In like Schizochytrium species, a polyketide synthase-like PUFA synthase pathway enables efficient de novo production of (DHA, 22:6 ω-3), serving as a rich natural source for this long-chain ω-3 PUFA. Species-specific variations further diversify fatty acid profiles; for instance, plants accumulate abundant ω-3 PUFAs due to their desaturase repertoire, supporting integrity. In ruminants, rumen microbial biohydrogenation converts dietary unsaturated fatty acids to even-chain saturated forms, such as transforming to via and by like Butyrivibrio fibrisolvens, thereby altering the fatty acid composition absorbed in the .

Industrial Production Methods

Industrial production of fatty acids primarily involves the of triglycerides from natural fats and oils, yielding mixtures of saturated and unsaturated fatty acids alongside as a . Alkaline , or , reacts triglycerides with sodium or under heat to form fatty acid salts (soaps) and ; subsequent acidification liberates the free fatty acids. This method is commonly applied to sources like palm and soy oils, which provide high volumes of mixed fatty acids for oleochemical applications. Acid , often catalyzed by or conducted via high-pressure steam splitting, directly cleaves triglycerides into free fatty acids and without soap intermediates, achieving near-complete conversion (up to 99% yield) and is favored for large-scale production due to its efficiency. Raw materials include animal , rich in saturated fatty acids like palmitic and stearic acids, and oils such as soy and palm, which yield unsaturated fatty acids including oleic and linoleic acids. Following , fatty acids are purified via under vacuum, separating components by ; for instance, tall oil fatty acids—comprising oleic and linoleic acids—are isolated from crude tall oil, a wood pulping byproduct, through this process. Synthetic routes complement natural extraction for specialized fatty acids. Oxidation of hydrocarbons, such as n-paraffins with air or oxygen, produces linear fatty acids used in detergents, while the Koch reaction carbonylaates olefins with and water under acidic conditions to yield branched carboxylic acids. , particularly cross-metathesis of natural unsaturated fatty acids with terminal alkenes, enables production of tailored polyunsaturated fatty acids (PUFAs) for nutraceuticals and polymers. Recent advances emphasize , with enzymatic using immobilized lipases catalyzing breakdown under mild conditions (40–60°C, pH 7–8), achieving 90–95% yields from waste oils while minimizing energy use and compared to chemical methods. The global fatty acids market is estimated at USD 33.8 billion in 2025 (as of September 2025), propelled by demand where fatty acids serve as precursors for .

Metabolism and Physiology

Digestion, Absorption, and Transport

The digestion of dietary fatty acids primarily occurs through the hydrolysis of triglycerides, the main form in which fats are ingested. In the oral cavity and , lingual and gastric lipases initiate the process by partially hydrolyzing triglycerides into diglycerides and free fatty acids, though this step accounts for only about 10-30% of total lipid digestion. The majority of hydrolysis takes place in the , where pancreatic , in conjunction with colipase, efficiently cleaves triglycerides at the sn-1 and sn-3 positions, yielding free fatty acids and 2-monoglycerides. Colipase anchors the lipase to the lipid-water interface, counteracting the inhibitory effects of salts. Following , the lipolytic products—free fatty acids and 2-monoglycerides—are rendered soluble by salts secreted from the liver and stored in the . These amphipathic salts form mixed micelles (approximately 4-8 nm in diameter) that incorporate the hydrophobic fatty acids and monoglycerides, along with and other , facilitating their transport to the of enterocytes in the . Absorption into enterocytes occurs primarily via passive diffusion across the unstirred water layer, with contributions from transmembrane proteins such as / and fatty acid transport protein 4 (FATP4). Within the enterocytes, absorbed fatty acids and 2-monoglycerides are rapidly re-esterified into via the monoacylglycerol pathway, involving enzymes like monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT). These are then packaged with apolipoprotein B-48 (apoB-48), phospholipids, and esters into chylomicrons in the and Golgi apparatus, a process dependent on microsomal triglyceride transfer protein (MTP). Chylomicrons are exocytosed from enterocytes into the lacteals of the villi and enter the (), bypassing the to deliver directly into the systemic bloodstream. In contrast, short- and medium-chain fatty acids (typically 2-12 carbons) do not require formation; they are absorbed directly by enterocytes and transported via the to the liver bound to , due to their higher water solubility. The process is regulated by enteroendocrine hormones, notably cholecystokinin (CCK), which is released from I-cells in the and in response to fatty acids and in the . CCK stimulates contraction for release and pancreatic secretion of and colipase, optimizing emulsification and .

Catabolic Pathways

Fatty acids are activated in the by synthetases, which catalyze the reaction between the fatty acid, (CoA), and ATP to form , AMP, and ; this activation step consumes the equivalent of two ATP molecules due to the subsequent of to two inorganic phosphates. Following activation, long-chain esters are transported into the via the carnitine shuttle system, a prerequisite detailed in fatty acid absorption and processes. The principal catabolic pathway for fatty acids is β-oxidation, a repetitive four-step cycle that sequentially removes two-carbon units as , primarily occurring in the for long-chain fatty acids (LCFA, 12–20 carbons) and in peroxisomes for very long-chain fatty acids (VLCFA, >20 carbons). The cycle begins with dehydrogenation of to form trans-Δ²-enoyl-CoA, catalyzed by acyl-CoA dehydrogenases (e.g., very long-chain, medium-chain, or short-chain variants) and producing FADH₂. This is followed by hydration to L-3-hydroxyacyl-CoA via enoyl-CoA hydratase (crotonase), oxidation to 3-ketoacyl-CoA by 3-hydroxyacyl-CoA dehydrogenase using NAD⁺ to yield NADH and H⁺, and finally thiolysis by (e.g., mitochondrial trifunctional protein or β-ketothiolase) to produce and a shortened that re-enters the cycle. Each turn of the β-oxidation cycle generates one FADH₂ and one NADH, which yield a net of 4 ATP upon oxidation in the (assuming P/O ratios of 1.5 for FADH₂ and 2.5 for NADH). For the saturated even-chain fatty acid palmitate (C16:0), complete β-oxidation requires seven cycles, yielding eight acetyl-CoA units that can enter the citric acid cycle for further energy production. The overall reaction is: \ceC15H31COOH+7CoA+7FAD+7NAD++7H2O−>8acetyl−CoA+7FADH2+7NADH+7H+\ce{C15H31COOH + 7 CoA + 7 FAD + 7 NAD+ + 7 H2O -> 8 acetyl-CoA + 7 FADH2 + 7 NADH + 7 H+} This process, minus the 2 ATP equivalents for activation, provides a net energy yield of approximately 106 ATP molecules when accounting for the oxidation of reduced coenzymes and acetyl-CoA through oxidative phosphorylation. Unsaturated fatty acids require additional enzymatic steps during β-oxidation to handle double bonds: for monounsaturated fatty acids like oleate, Δ³-cis-enoyl-CoA is isomerized to trans-Δ²-enoyl-CoA by 2,4-dienoyl-CoA Δ³,Δ²-isomerase (DCI), allowing continuation of the cycle; polyunsaturated fatty acids, such as linoleate, additionally involve reduction by 2,4-dienoyl-CoA reductase (DECR1) to remove conjugated double bonds. Odd-chain fatty acids, less common in diets but present in some microbial , undergo β-oxidation to yield propionyl-CoA as the final three-carbon unit, which is carboxylated to D-methylmalonyl-CoA by propionyl-CoA carboxylase (using and ATP), racemized to L-methylmalonyl-CoA, and rearranged to by (vitamin B12-dependent), entering the as a gluconeogenic precursor. When β-oxidation produces excess beyond the liver's capacity, particularly during or prolonged exercise, it is diverted to in hepatic mitochondria to generate (acetoacetate and β-hydroxybutyrate) for export to extrahepatic tissues as an alternative fuel source. This pathway begins with the reversible condensation of two to acetoacetyl-CoA by acetoacetyl-CoA , followed by addition of another to form 3-hydroxy-3-methylglutaryl-CoA () via HMG-CoA synthase (the rate-limiting enzyme, induced by ), and cleavage by HMG-CoA lyase to acetoacetate, which is partially reduced to β-hydroxybutyrate by β-hydroxybutyrate dehydrogenase.

Anabolic Pathways and Essential Fatty Acids

In anabolic pathways, fatty acids serve as building blocks for the synthesis of more complex , including longer-chain polyunsaturated fatty acids (PUFAs) through processes like elongation and desaturation. These pathways occur primarily in the and peroxisomes of mammalian cells, where enzymes add carbon atoms via elongation or introduce s via desaturation. Following the initial biosynthesis of saturated fatty acids like palmitate, further modification of essential PUFAs— (LA, 18:2 ω-6) and (ALA, 18:3 ω-3)—relies on alternating cycles of desaturation and elongation to produce bioactive longer-chain PUFAs such as (AA, 20:4 ω-6) and (DHA, 22:6 ω-3). The key rate-limiting enzymes include Δ6-desaturase (FADS2), which initiates the conversion by introducing a double bond at the Δ6 position, and Δ5-desaturase (FADS1), which acts later in the pathway; elongases such as ELOVL2 and ELOVL5 add two-carbon units between these steps. Humans and other mammals lack the Δ12- and Δ15-desaturases needed to insert double bonds at the ω-6 and ω-3 positions, respectively, making LA and ALA essential fatty acids that must be obtained from the diet. These precursors are then metabolized into longer-chain PUFAs critical for production, , and neural development. Deficiency in essential fatty acids arises from inadequate dietary intake, leading to symptoms such as scaly , poor , and growth retardation in children, as observed in cases of prolonged without lipid supplementation. The conversion pathways from LA and ALA highlight the competitive nature of these anabolic processes, as both ω-6 and ω-3 substrates vie for the same desaturase and elongase enzymes, often favoring ω-6 metabolism due to higher dietary availability. The ω-6 pathway proceeds as follows:
  • LA (18:2 ω-6) → γ-linolenic (GLA, 18:3 ω-6) via Δ6-desaturase
  • → dihomo-γ-linolenic (DGLA, 20:3 ω-6) via elongation
  • DGLA → AA (20:4 ω-6) via Δ5-desaturase
Similarly, the ω-3 pathway is:
  • ALA (18:3 ω-3) → stearidonic (SDA, 18:4 ω-3) via Δ6-desaturase
  • SDA → eicosatetraenoic (ETA, 20:4 ω-3) via elongation
  • → (EPA, 20:5 ω-3) via Δ5-desaturase
  • EPA → docosapentaenoic (DPA, 22:5 ω-3) via elongation
  • DPA → DHA (22:6 ω-3) via peroxisomal Δ4-desaturase or further elongation/desaturation
These sequences enable the production of signaling molecules like prostaglandins from AA and resolvins from EPA/DHA. However, the efficiency of converting ALA to EPA and DHA in humans is notably low, estimated at less than 5% for DHA and 5-10% for EPA, influenced by factors such as high dietary ω-6 intake, which competes for enzymatic resources, and genetic variations in FADS1/2. This inefficiency underscores the recommendation for direct dietary sources of EPA and DHA, such as fatty fish, rather than relying solely on plant-derived ALA from sources like flaxseed. Recent research from 2023-2025 has explored ω-3 supplementation's role in supporting anabolic processes related to muscle and health. A 2024 meta-analysis found that omega-3 fatty acid supplementation combined with resistance training significantly enhanced muscle strength in healthy adults. In health, a 2024 indicated that omega-3 supplementation increased body weight and quality-of-life scores in patients with advanced non-small cell experiencing .

Chemical Reactions

Esterification is the between a fatty acid and an alcohol to form an and water, represented by the equilibrium equation: RCOOH+R’OH⇌RCOOR’+H2O\text{RCOOH} + \text{R'OH} \rightleftharpoons \text{RCOOR'} + \text{H}_2\text{O} where R is the chain of the fatty acid and R' is the from the alcohol. This reversible process is typically catalyzed by acids, such as , or enzymes like lipases, which protonate the carbonyl oxygen to facilitate nucleophilic attack by the alcohol. To drive the equilibrium toward ester formation, an excess of alcohol is commonly employed, shifting the reaction via . In approaches, recyclable zinc(II) salts have been developed as catalysts for solvent-free esterification of fatty acids with medium- to long-chain alcohols, yielding high conversions under mild conditions. Transesterification involves the exchange of alcohol groups in an , such as a , with another alcohol, producing a new and the original alcohol. In , triglycerides from vegetable oils or animal fats react with in the presence of a base like to form fatty acid methyl esters (FAME) and : [Triglyceride](/page/Triglyceride)+3CH3OH→3FAME+[Glycerol](/page/Glycerol)\text{[Triglyceride](/page/Triglyceride)} + 3\text{CH}_3\text{OH} \rightarrow 3\text{FAME} + \text{[Glycerol](/page/Glycerol)} This process proceeds rapidly at mild temperatures (around 60°C) and requires excess to achieve near-complete conversion, with the base catalyst activating the methoxide ion for nucleophilic attack on the carbonyl carbon of the . The resulting FAME serves as the primary component, while is recovered as a byproduct. In phospholipid synthesis, fatty acids are incorporated via activated intermediates. The process begins with the of glycerol-3-phosphate by glycerol-3-phosphate acyltransferase (GPAT), using to form at the sn-1 position, typically with a saturated fatty acid chain. A second at the sn-2 position, catalyzed by acyltransferase, adds another -derived chain—often unsaturated—to yield , the precursor to various like and . These enzymatic steps ensure stereospecific assembly of . Saponification is the base-catalyzed hydrolysis of esters, such as those in triglycerides, representing the reverse of esterification. In this reaction, alkali like sodium hydroxide cleaves the ester bonds to produce carboxylate salts (soaps) and glycerol: Triglyceride+3NaOH→3RCOONa+Glycerol\text{Triglyceride} + 3\text{NaOH} \rightarrow 3\text{RCOONa} + \text{Glycerol} The mechanism involves hydroxide ion addition to the carbonyl, forming a tetrahedral intermediate that expels the alkoxide, followed by proton transfer. This irreversible process under basic conditions has been industrially applied since ancient times for soap production from fats. Industrially, fat splitting recovers free fatty acids from triglycerides through high-pressure , often using steam at 245–255°C and 55–60 bar in countercurrent towers. The fatty acids, being less dense, separate at the top, while the aqueous solution (sweet water) is collected at the bottom for further purification. This hydrolytic process, distinct from enzymatic methods, enables efficient production of oleochemicals like soaps and from natural oils and fats.

Hydrogenation, Hardening, and Oxidation

Hydrogenation is a chemical reaction in which hydrogen gas (H₂) is added across the carbon-carbon double bonds of unsaturated fatty acids, converting them to saturated forms. This process typically employs a nickel-based catalyst, such as Raney nickel, under controlled temperature and pressure conditions to facilitate the addition. Partial hydrogenation, where not all double bonds are saturated, is commonly used in the food industry to modify vegetable oils for products like margarines, resulting in a semi-solid consistency with improved stability. During this reaction, cis double bonds can isomerize to trans configurations; for instance, oleic acid (cis-9-octadecenoic acid) may form elaidic acid (trans-9-octadecenoic acid), producing trans fatty acids that were historically prevalent in processed foods. Hardening refers to the industrial application of partial to increase the of liquid oils, transforming them into or semi-solid fats suitable for shortenings and spreads. For example, , rich in polyunsaturated fatty acids like , undergoes selective hydrogenation that preferentially targets less substituted (more isolated) double bonds, yielding a product with a higher proportion of saturated and monounsaturated fatty acids. This selectivity is achieved by optimizing catalyst activity and reaction conditions, such as lower temperatures to minimize trans isomer formation while raising the fat content for and applications. The resulting hardened fats, like those used in commercial shortenings, exhibit enhanced oxidative stability and texture compared to their liquid precursors. Auto-oxidation of fatty acids is a free radical that occurs spontaneously in the presence of oxygen, particularly affecting unsaturated fatty acids and leading to rancidity in oils and fats. The mechanism proceeds in three stages: , , and termination. In , hydroperoxides (ROOH) decompose into alkoxy (RO•) and hydroxyl (•OH) radicals, often triggered by , , or metal ions: ROOH→RO•+•OH\text{ROOH} \rightarrow \text{RO•} + \text{•OH} involves the abstraction of a from the fatty acid (RH) by a peroxyl radical (ROO•), forming a radical (R•) that then reacts with oxygen to regenerate ROO•: RH+ROO•→R•+ROOH\text{RH} + \text{ROO•} \rightarrow \text{R•} + \text{ROOH} R•+O2→ROO•\text{R•} + \text{O}_2 \rightarrow \text{ROO•} Termination occurs when radicals combine to form non-radical products, such as ROOR or R-R. This process generates volatile off-flavors and odors, compromising food quality. Polyunsaturated fatty acids (PUFAs) are especially susceptible to peroxidation, where hydroperoxides (ROOH) form at allylic positions adjacent to double bonds, further decomposing into secondary products like aldehydes. Antioxidants such as vitamin E (α-tocopherol) inhibit peroxidation by scavenging peroxyl radicals, donating a hydrogen atom to form a stable phenoxyl radical and interrupting the chain reaction. Saturated fatty acids are highly stable against oxidation due to the absence of double bonds, whereas unsaturated ones are prone, with reactivity increasing with the number of double bonds. For example, (18:2) oxidizes approximately 10 times faster than (18:1) under similar conditions, highlighting the vulnerability of PUFAs in biological and food systems.

Decarboxylation and Other Transformations

Decarboxylation reactions of fatty acids involve the loss of , often facilitating synthetic transformations or structural modifications. In , beta-keto acids derived from fatty acid precursors undergo thermal to yield , proceeding via a six-membered cyclic that releases CO₂ and forms an intermediate, which tautomerizes to the . For example, a beta-keto acid of the form \ceR−CO−CH2−COOH\ce{R-CO-CH2-COOH} upon heating to produce \ceR−CO−CH3+CO2\ce{R-CO-CH3 + CO2}, a process analogous to the final step in acetoacetic synthesis, which extends carbon chains in fatty acid analogs by three atoms. Similarly, derivatives, relevant for building even-chain fatty acids, after to add a two-carbon unit, as seen in where \ceROOC−CH2−COOR\ce{ROOC-CH2-COOR} alkylated at the alpha position yields \ceR′−CH2−COOH\ce{R'-CH2-COOH} post-. These reactions are thermally driven, typically requiring heat above 100°C, and are widely used in laboratory-scale preparation of shorter-chain fatty acid derivatives. Kolbe electrolysis provides an electrochemical route for decarboxylative coupling of fatty acid carboxylates, generating symmetric hydrocarbons from two carboxylate ions at the . The process involves anodic oxidation to form radicals, which dimerize, as represented by the equation \ce2RCOO−−>[anode]R−R+2CO2+2e−\ce{2 RCOO^- ->[anode] R-R + 2 CO2 + 2 e^-}, where R is the alkyl chain from the fatty acid. For instance, electrolysis of laurate (from , C12) produces docosane (C24H50), a useful in lubricants and , with carbon efficiencies up to 92% under optimized conditions like alkaline media and temperatures above 45°C for longer chains. This method is particularly valuable for converting renewable biomass-derived fatty acids into biofuels or specialty chemicals, offering a sustainable alternative to thermal processes with lower energy demands for medium-chain acids. Ozonolysis cleaves the carbon-carbon double bonds in unsaturated fatty acids, resulting in carbonyl compounds and enabling structural analysis or production of dicarboxylic acids. The reaction proceeds via addition of ozone to form a molozonide, which rearranges to an ozonide and is then reductively cleaved, often with dimethyl sulfide or triphenylphosphine. For oleic acid (cis-9-octadecenoic acid), ozonolysis yields nonanal (a C9 aldehyde) and 9-oxononanoic acid (a C9 aldehydo-acid) as primary products, with nonanal appearing predominantly in the gas phase (yield ~95%) and 9-oxononanoic acid contributing to the particulate phase. Yields increase with humidity due to enhanced hydrolysis, and secondary products like nonanoic acid and azelaic acid (HOOC-(CH2)7-COOH) form under certain conditions, making this transformation key for degrading unsaturated fatty acids in analytical chemistry or synthesizing shorter-chain acids for polymers. Olefin metathesis rearranges the carbon skeletons of unsaturated fatty acids through carbene-catalyzed exchange of alkylidene groups, producing new alkenes or dienes for industrial applications. In self-metathesis of methyl oleate, the internal exchanges to form 9-octadecene and dimethyl 9-octadecenedioate, a diester used in lubricants and polyesters. Cross-metathesis, such as ethenolysis with , shortens chains to generate terminal alkenes like 1-decene and methyl 9-decenoate from methyl oleate, facilitating production of even-chain fatty acid derivatives for precursors (e.g., nylon-12). Industrial-scale examples include the Elevance refinery's processing of with to yield and biofuels at 180,000 metric tons per year, highlighting metathesis's role in upgrading oil-derived fatty acids into high-value oleochemicals. Alpha-ation of fatty acids introduces a at the carbon adjacent to the carboxyl group, typically under acidic conditions, to form derivatives for further functionalization, though it remains a less common transformation due to the inertness of saturated chains. Using (TCCA) as a halogenating agent, alpha-chlorination of saturated fatty acids like stearic or proceeds solvent-free at 80°C with , achieving 96-97% yields of alpha-chloro fatty acids, which can then be hydrolyzed to alpha-hydroxy fatty acids for use in or pharmaceuticals. This method valorizes waste fatty acid biomasses by enabling selective substitution, contrasting with base-promoted enolization in more reactive carbonyl analogs.

Biological Functions

Structural Roles in Lipids and Membranes

Fatty acids serve as essential building blocks in the formation of complex that contribute to cellular architecture. In triglycerides, three fatty acid molecules are esterified to a central backbone, creating neutral lipids that predominate in for structural storage purposes. The incorporation of saturated fatty acids, with their unbranched hydrocarbon chains, enables tight molecular packing, resulting in a solid or semi-solid state at body temperature that supports efficient lipid deposition in fat cells. Phospholipids represent another key class where fatty acids play a structural role, with two fatty acyl chains typically esterified to the sn-1 and sn-2 positions of , paired with a hydrophilic head group; serves as a prominent example due to its prevalence in eukaryotic membranes. Unsaturated fatty acids in these positions introduce double bonds that create kinks in the acyl chains, reducing van der Waals interactions and thereby promoting greater molecular disorder. This unsaturation is vital for maintaining the dynamic properties of lipid assemblies. Within cellular bilayers, the fatty acid composition of phospholipids dictates packing density and phase behavior. Saturated fatty acids align in extended, straight conformations, facilitating close packing into a phase characterized by high order and reduced fluidity, which can occur at lower temperatures or in cholesterol-poor environments. Conversely, unsaturated fatty acids disrupt this alignment through cis double bonds, favoring a crystalline phase where chains exhibit rotational freedom and lateral mobility, essential for flexibility and protein function. modulates these transitions by intercalating between acyl chains, broadening the phase range and preventing abrupt shifts between and states to preserve optimal . Sphingolipids incorporate fatty acids into structures via an linkage to a sphingoid base, contributing to specialized membrane domains. In sheaths surrounding neuronal axons, ceramides enriched with very long-chain fatty acids (VLCFAs, often C22–C26) enhance bilayer thickness and stability, providing electrical insulation that supports rapid nerve impulse propagation through tight molecular interactions. In the of the , ceramides bearing ω-hydroxy fatty acids form extracellular lamellae that constitute the primary skin barrier. These elongated, hydroxylated chains enable covalent cross-linking with adjacent ceramides, creating a robust, brick-and-mortar-like matrix with and free fatty acids that restricts and shields against external pathogens.

Energy Storage and Metabolism

Fatty acids serve as the primary form of in animals, primarily in the form of triglycerides within . These triglycerides provide a high caloric of approximately 9 kcal per gram, more than double the 4 kcal per gram yielded by carbohydrates, making them an efficient means of storing surplus without the osmotic drawbacks associated with other molecules. , the main repository for this , consists primarily of adipocytes filled with triglycerides, which comprise about 80-90% of the tissue's by , allowing for compact, long-term energy reserves. During periods of energy demand, such as or exercise, stored triglycerides are mobilized through , where hydrolyzes them into free fatty acids (FFAs) and . This is activated by hormonal signals, releasing FFAs into the bloodstream for transport to energy-consuming tissues like muscle and liver. The process ensures a steady supply of fuel when stores are depleted, supporting prolonged activity or survival without intake. Once mobilized, FFAs undergo beta-oxidation to produce , which enters the tricarboxylic acid (TCA) cycle and subsequently fuels the for ATP generation (as detailed in Catabolic Pathways). For example, the complete theoretical oxidation of one molecule of palmitate, a common 16-carbon fatty acid, yields 106 ATP molecules, highlighting the substantial energy potential of fatty acid catabolism compared to other substrates. This integration into central metabolic pathways underscores fatty acids' role as a versatile energy source, contributing to under aerobic conditions. The balance between fatty acid storage and mobilization is tightly regulated by hormones. Insulin, released in response to elevated blood glucose, promotes and esterification into triglycerides, favoring energy storage in during fed states. In contrast, , secreted during , stimulates by activating , releasing FFAs to meet energy needs. Chronic excess caloric intake disrupts this regulation, leading to expansion and through unchecked triglyceride accumulation. Compared to , another key energy reserve stored mainly in liver and muscle, fatty acids offer greater storage capacity—virtually unlimited in —versus the limited glycogen reserves that can sustain only short-term needs. However, fatty acid mobilization and oxidation occur more slowly than glycogen breakdown and , making fats ideal for sustained, low-to-moderate energy demands rather than rapid bursts of activity.

Signaling, Regulation, and Health Implications

Fatty acids play crucial roles in cellular signaling, particularly through the production of eicosanoids derived from , an omega-6 polyunsaturated fatty acid (PUFA). Eicosanoids are bioactive lipid mediators synthesized via enzymatic pathways, including enzymes that generate prostaglandins (PGs) and thromboxanes (TXs), and enzymes that produce leukotrienes (LTs). These molecules regulate by modulating immune cell responses, , and signaling; for instance, prostaglandins like PGE2 promote and fever, while leukotrienes such as LTB4 attract neutrophils to sites of . Thromboxanes, primarily TXA2, induce platelet aggregation and , contributing to but also exacerbating inflammatory conditions when dysregulated. In addition to eicosanoid-mediated signaling, fatty acids act as ligands for peroxisome proliferator-activated receptors (PPARs), a family of nuclear receptors that regulate related to . Long-chain fatty acids, including both saturated and unsaturated types, bind to PPARα, PPARγ, and PPARδ, activating transcription of genes involved in fatty acid oxidation, , and . PPARα, predominantly expressed in the liver, enhances mitochondrial β-oxidation and reduces circulating triglycerides, while PPARγ in promotes lipid storage and insulin sensitivity. This ligand-dependent activation allows dietary fatty acids to fine-tune metabolic responses, influencing and energy balance at the transcriptional level. Health implications of fatty acid signaling and regulation are profound, with imbalances linked to various diseases. Omega-3 PUFAs, such as (EPA) and (DHA), reduce (CVD) risk by lowering triglycerides and improving endothelial function; meta-analyses show EPA/DHA supplementation decreases by 6-13% and triglycerides by over 30% at doses of 4 g/day. Recent meta-analyses from 2023-2025 indicate a small but significant positive effect of omega-3s on muscle strength in older adults, potentially via signaling and enhanced protein synthesis, though effects on lean mass remain inconsistent. In non-alcoholic fatty liver disease (NAFLD), pharmacotherapies targeting fatty acid metabolism, such as PPAR agonists like pioglitazone and emerging agents like , improve by promoting β-oxidation and reducing de novo . Deficiencies in essential fatty acids, particularly linoleic and alpha-linolenic acids, manifest as skin disorders like eczema due to impaired and increased . Conversely, excess saturated fatty acids (SFAs) promote by elevating LDL and inducing endothelial dysfunction, increasing plaque formation risk. Recent research (as of 2025) also links fatty acid oxidation to cancer and DHA to fetal brain development. Emerging research highlights novel roles for PUFAs in , with 2023 studies elucidating their conversion to that resolve beyond traditional eicosanoids.

Applications and Uses

Nutritional and Dietary Aspects

Fatty acids are essential components of diets, obtained primarily from and sources. Saturated fatty acids (SFAs) are predominantly found in meats, products, and tropical oils such as and , with dietary guidelines recommending that they constitute less than 10% of total caloric intake to reduce cardiovascular risk. Monounsaturated fatty acids (MUFAs) are rich in foods like avocados, nuts, seeds, and , contributing to heart-healthy eating patterns. Polyunsaturated fatty acids (PUFAs), including omega-6 and omega-3 types, occur in fatty , flaxseeds, walnuts, and certain oils; these support effects. Major health organizations provide specific recommendations for fatty acid intake. The (AHA) advises limiting SFAs to less than 6% of total daily calories and encourages replacement with unsaturated fats. For omega-3 PUFAs, particularly (EPA) and (DHA), the AHA recommends at least 250 mg per day for cardiovascular health, ideally from two servings of fatty fish weekly. Vegans face a higher risk of omega-3 deficiency due to low conversion of plant-based alpha-linolenic acid (ALA) to EPA and DHA, often necessitating algae-derived supplements. As of 2025, the WHO's REPLACE initiative has led to the elimination of industrially-produced trans fatty acids in over 50 countries, with a global target by 2030. In common dietary fats, fatty acids are esterified as triglycerides with varying compositions. For instance, typically contains about 40% (a MUFA), alongside higher proportions of palmitic and stearic acids. Trans fatty acids, largely from partially hydrogenated oils, should be limited to less than 1% of total energy intake to minimize coronary heart disease risk. Food labeling regulations focus on broad categories rather than detailed profiles. , Facts panels require disclosure of total fat, , and per serving, but do not mandate listing of individual unsaturated fatty acids or subtypes like MUFAs and PUFAs.

Industrial and Commercial Applications

Fatty acids serve as fundamental raw materials in the production of s and detergents, primarily through , where they react with alkalis like to form sodium salts that act as anionic . These salts, such as derived from , provide cleansing and emulsifying properties essential for removing dirt and oils. , a of fat rich in palmitic and stearic acids, has historically been a major source for manufacturing in the United States, often combined with or oils for optimal performance. In , fatty acids function as emollients and emulsifiers, enhancing skin hydration and product stability. , a saturated fatty acid, is commonly used as an emollient in creams and lotions due to its ability to form a protective barrier on the skin, while derivatives from , such as those containing , serve as emulsifiers in formulations like emulsions and ointments. Recent trends show a shift toward plant-based sources, including palm and soy-derived fatty acids, to meet demands for sustainable and ingredients in skincare products. Fatty acids are key feedstocks for via , where triglycerides from vegetable oils or animal fats are converted into fatty acid methyl esters (FAME) using and a catalyst. This process yields a renewable that can be blended with petroleum diesel; in the , comprises up to 7% of blends under EN 590 standards, promoting reduced emissions in transportation. In lubricants and paints, fatty acids (TOFA), derived from pine wood byproducts, are polymerized to produce resins, which form the basis of durable coatings and varnishes. TOFA's mixture of oleic and linoleic acids provides flexibility and in these applications, with up to 50% substitution for traditional oil fatty acids maintaining performance in long-oil systems. Other commercial uses include candles made from , a hardened mixture of stearic and palmitic acids that improves burn quality and reduces dripping when added to . In plastics, dicarboxylic acids derived from fatty acids, such as from , are esterified to create non-phthalate plasticizers, such as , enhancing flexibility in (PVC) without compromising material integrity.

Medical and Therapeutic Applications

Fatty acids play a significant role in medical supplements, particularly omega-3 fatty acids derived from , which are prescribed for managing . Clinical evidence indicates that a daily dose of 4 grams of (EPA) and (DHA) from reduces triglyceride levels by at least 30% in patients with very high triglycerides, while also increasing cholesterol levels. For vegan populations, algal-derived DHA supplements provide a sustainable alternative, effectively raising plasma, serum, and cellular DHA concentrations to comparable levels as fish sources, thereby addressing potential deficiencies in long-chain omega-3 intake without relying on marine animal products. In therapeutic applications, omega-3 fatty acids have demonstrated benefits in reducing (NSAID) use among patients with . Meta-analyses show that dosages exceeding 2.7 grams per day for more than three months lower NSAID consumption, likely due to effects that alleviate and tenderness. Recent advancements include drugs targeting fatty acid oxidation for (NAFLD), such as , a hormone receptor-beta approved by the FDA in March 2024 for adults with noncirrhotic (NASH) and moderate to advanced ; it promotes hepatic fatty acid β-oxidation to reduce accumulation. For inherited disorders of fatty acid oxidation (FAO), treatments focus on mitigating metabolic crises. In medium-chain (MCAD) deficiency, the most common FAO disorder, management includes carnitine supplementation to support acyl-carnitine transport when levels are low, alongside strict avoidance of to prevent and lethargy during illness or stress. In X-linked (X-ALD), characterized by very long-chain fatty acid (VLCFA) accumulation, —a mixture of oleic (18:1) and erucic (22:1) acids—normalizes plasma VLCFA levels and has been associated with reduced risk of abnormalities in asymptomatic boys when initiated early. Serum fatty acid profiles serve as diagnostic tools for , revealing characteristic patterns such as elevated saturated fatty acids (e.g., ) and altered polyunsaturated fatty acid ratios that correlate with and cardiovascular risk. These profiles help identify individuals at risk by highlighting imbalances in free fatty acid composition linked to and . Emerging research highlights how ocean acidification, as part of broader climate change effects, may diminish omega-3 fatty acid availability in seafood, impacting dietary nutrition in vulnerable regions. A 2024 study modeling climate scenarios predicts substantial declines in omega-3 supply from wild-capture fisheries in Southeast Asia and Pacific Island countries—up to 70-92% under no-mitigation pathways—due to reduced fish stocks from acidification, warming, and deoxygenation, potentially exacerbating micronutrient deficiencies in reliant populations.

Analysis and Detection

Separation and Identification Techniques

The separation of fatty acids from complex biological or natural mixtures relies on their distinct physical properties, such as polarity, volatility, and , which enable isolation based on differences in and interaction with stationary or mobile phases. Extraction techniques are fundamental for obtaining fatty acids from lipid-containing samples, typically involving initial isolation of total followed by . The Folch method, developed in 1957, uses a chloroform-methanol- mixture (20:10:8 v/v) to extract from animal tissues, achieving high recovery rates of over 95% for total , which can then be hydrolyzed under acidic or basic conditions to release free fatty acids. Similarly, the Bligh and Dyer method, introduced in 1959, employs a single-phase chloroform-methanol system diluted with for moist samples like or microbial tissues, offering efficient extraction (up to 98% yield) with reduced solvent volumes compared to Folch, particularly suited for aqueous matrices. These solvent-based approaches are widely adopted due to their simplicity and reproducibility, though they require subsequent or to liberate fatty acids from glycerolipids. Chromatographic methods provide effective separation of fatty acids based on volatility and polarity. (TLC) is commonly used for preliminary classification of fatty acids into saturated and unsaturated fractions, employing plates with developing solvents like hexane-diethyl ether-acetic acid (80:20:1 v/v); unsaturated fatty acids are visualized by their reaction with iodine vapor, forming brown spots due to addition across double bonds, while saturated ones remain unstained. This technique allows rapid screening of lipid classes but is limited for quantitative analysis. For higher resolution, (GC) of fatty acid methyl esters (FAMEs) exploits differences in boiling points and chain length, with FAMEs prepared via acid- or base-catalyzed ; non-polar columns like DB-5 separate over 30 FAMEs from C8 to C24 in under 30 minutes, enabling identification by retention time comparison to standards. GC is the gold standard for volatile derivatives, achieving baseline separation for most common fatty acids in biological samples. High-performance liquid chromatography (HPLC) offers versatility for non-volatile or polar fatty acids. Reverse-phase HPLC, using C18 columns and mobile phases like acetonitrile-water gradients, separates fatty acids by chain length and hydrophobicity, with retention times increasing for longer chains (e.g., elutes before ); detection at 205 nm quantifies mixtures with resolutions exceeding 1.5 for C16-C20 homologs. Recent developments include HPLC with array (PDA) detection for , providing sensitive analysis in biological samples as of 2025. For unsaturation-specific separation, silver-ion HPLC employs cation-exchange columns impregnated with Ag+ ions, which form reversible complexes with double bonds, allowing order based on the number and position of unsaturations (e.g., saturated > monounsaturated > polyunsaturated); this method resolves geometric isomers like cis- and trans-18:1 in under 20 minutes using hexane-dichloromethane gradients. These HPLC variants are particularly useful for complex mixtures where GC derivatization is impractical. Capillary electrophoresis (CE) has emerged as an efficient technique for fatty acid separation, particularly for charged species. CE, including modes like capillary zone electrophoresis and non-aqueous CE, separates fatty acids based on electrophoretic mobility in an , often with UV or conductivity detection. Recent advances, such as CE coupled to contactless conductivity detection (CE-C4D), enable rapid analysis of underivatized fatty acids in and biological samples, achieving separations in under 10 minutes with limits of detection in the nanomolar range as of 2024. This method is advantageous for its low sample volume requirements and minimal solvent use, complementing chromatographic approaches. Supercritical fluid with CO2 (SFE-CO2) represents a alternative for isolating fatty acids from oils and seeds, operating at pressures of 200-400 bar and temperatures of 40-60°C to achieve solvent-like extraction without organic residues. This method selectively extracts non-polar , yielding up to 10% oil from sources like rosehip seeds with preserved fatty acid profiles, including high polyunsaturated content, and is scalable for industrial applications due to its tunable density. For preparative-scale purification, urea complexation exploits the ability of urea to form crystalline adducts with straight-chain saturated and monounsaturated fatty acids, leaving polyunsaturated ones in the liquid phase. The process involves dissolving fatty acids in , adding (typically 3:1 w/w ratio), and cooling to 0°C for crystallization; filtration separates the non-complexed polyunsaturated fraction (e.g., enriching to >70%), with recoveries of 80-90% for target unsaturates, making it cost-effective for concentrating omega-3 fatty acids from fish oils. This technique is non-chromatographic and widely used in and production.

Quantification and Structural Elucidation Methods

Gas chromatography with flame ionization detection (GC-FID) is a widely used method for quantifying total fatty acids in biological samples, often following derivatization to fatty acid methyl esters (FAMEs) to enhance volatility and separation. In GC-FID, the detector response is proportional to the carbon content, with a relative response factor approximately 1 for hydrocarbons and similar for FAMEs, enabling accurate total fatty acid determination without individual standards for each species. Nuclear magnetic resonance (NMR) spectroscopy, particularly 1H NMR integration of characteristic proton signals, provides an orthogonal approach for assessing fatty acid purity and composition in oils and extracts, offering non-destructive quantification based on signal areas relative to an internal standard. Mass spectrometry coupled with gas chromatography (GC-MS) facilitates both quantification and structural elucidation by providing molecular weights and fragmentation patterns of fatty acid derivatives under electron ionization (EI). In EI-GC-MS, the McLafferty rearrangement produces characteristic ions, such as m/z 74 for methyl esters, aiding identification of chain length and functional groups through fragment analysis. For intact lipids, liquid chromatography-mass spectrometry (LC-MS) preserves molecular ions, enabling direct profiling of fatty acids within complex lipid classes without hydrolysis, with electrospray ionization (ESI) enhancing sensitivity for low-abundance species. Recent advances in chemical derivatization for mass spectrometry, particularly for unsaturated fatty acids, involve reagents that fix double bond positions, improving structural identification and quantification in biological samples through enhanced fragmentation patterns, as reviewed in 2025. Structural elucidation of fatty acids relies on 1H and 13C NMR to determine chain length and unsaturation. In 1H NMR, the terminal (CH3) resonates around 0.9 ppm, while allylic protons adjacent to double bonds appear at approximately 2.0 ppm, allowing precise counting of methylene chains and olefinic positions. 13C NMR complements this by resolving carbonyl carbons and unsaturated sites with higher dispersion, facilitating assignment in mixtures. (IR) provides rapid confirmation of functional groups, with the carbonyl (C=O) stretch of free fatty acids at about 1710 cm⁻¹ and the C=C stretch of alkenes around 1650 cm⁻¹, though overlap in complex samples limits its standalone use. Advanced lipidomics approaches, such as shotgun , enable simultaneous identification and quantification of hundreds of fatty acids directly from total lipid extracts using high-resolution instruments like or time-of-flight analyzers, bypassing chromatographic separation for high-throughput analysis. High-resolution MS has recently uncovered novel polyunsaturated fatty acids (PUFAs), including very-long-chain variants up to 44 carbons in tissues, through accurate determination and fragmentation. Stable isotope labeling with 13C-enriched precursors traces in vivo and in vitro, monitoring incorporation into specific chains via MS or NMR to elucidate biosynthetic pathways and flux rates. This technique distinguishes from dietary sources, providing quantitative insights into oxidation and elongation processes.

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