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Unsaturated fats are a class of dietary lipids characterized by the presence of one or more carbon-carbon double bonds in the hydrocarbon chains of their constituent fatty acids, which distinguishes them from saturated fats that contain only single bonds between carbon atoms.^[1] These double bonds are typically in the cis configuration in natural unsaturated fats, though trans configurations also exist and have different properties. This structural feature renders unsaturated fats typically liquid at room temperature and more susceptible to oxidation compared to their solid, saturated counterparts.^[2] Found predominantly in plant-derived oils, nuts, seeds, avocados, and fatty fish, unsaturated fats serve as a primary energy source and play essential roles in cell membrane structure and function.^[2] Unsaturated fats are broadly categorized into monounsaturated fatty acids (MUFAs), which contain a single double bond, and polyunsaturated fatty acids (PUFAs), which have two or more double bonds.^[2] Common sources of MUFAs include olive oil, canola oil, avocados, and nuts like almonds and peanuts, while PUFAs are abundant in soybean oil, sunflower oil, walnuts, flaxseeds, and oily fish such as salmon and mackerel.^[2] Among PUFAs, omega-3 (e.g., alpha-linolenic acid or ALA) and omega-6 (e.g., linoleic acid or LA) fatty acids are deemed essential nutrients, as the human body lacks the enzymes to synthesize them and they must be obtained through diet to support physiological processes like inflammation regulation and neurological development.^[3] From a health perspective, monounsaturated and polyunsaturated fats are associated with beneficial effects on cardiovascular health, including lowering low-density lipoprotein (LDL) cholesterol levels and potentially raising high-density lipoprotein (HDL) cholesterol when they replace saturated fats in the diet.^[2] Epidemiological and clinical evidence supports that higher intake of PUFAs, particularly omega-3s, correlates with reduced risk of coronary heart disease and inflammation-related conditions, with recommendations from bodies like the American Heart Association to replace saturated fats with polyunsaturated fats in the diet.^[2] Additionally, omega-3 PUFAs like eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) contribute to brain health and may lower triglyceride levels, though their conversion from plant-based ALA is inefficient in humans.^[3]

Fundamentals

Definition

Unsaturated fats are a class of lipids in which the fatty acid chains contain one or more carbon-carbon double bonds (C=C), distinguishing them from saturated fats whose chains have only single bonds between carbon atoms.^[4] This structural feature allows unsaturated fatty acids to theoretically bind additional hydrogen atoms, hence the designation "unsaturated."^[5] Unsaturated fats typically exist as triglycerides, esters formed from glycerol and three unsaturated fatty acid molecules.^[6] The general formula for an unsaturated fatty acid is

\ce{CH3(CH2)_m(CH2CH=CH)_n(CH2)_pCOOH}

, where

m

n

, and

p

are non-negative integers that account for variations in chain length and position, with

n \geq 1

indicating at least one double bond.^[7] This notation captures the linear hydrocarbon backbone terminating in a carboxylic acid group, with the double bonds introducing kinks in the chain. The term "unsaturated" originated in the 19th century, applied to organic compounds capable of adding hydrogen across double bonds without disrupting the carbon skeleton, a property first systematically explored in fatty acid studies.^[8] Pioneering work by French chemist Michel Eugène Chevreul in the 1810s laid the groundwork, as he analyzed the saponification of animal fats—breaking them into glycerol and fatty acids like oleic acid—revealing their compositional complexity.^[9] While unsaturated fats focus on triglycerides with unsaturated fatty acids, they differ from other lipid categories such as phospholipids, which incorporate phosphate groups, or sterols like cholesterol, which feature a rigid ring structure rather than long-chain fatty acids.^[10]

Chemical Structure

Unsaturated fatty acids are long-chain carboxylic acids featuring one or more carbon-carbon double bonds (C=C) within the hydrocarbon chain, typically spanning 12 to 24 carbon atoms in length.^[11] These double bonds introduce unsaturation by reducing the number of hydrogen atoms compared to saturated fats, and they occur primarily in the cis configuration in natural sources.^[12] The double bonds in unsaturated fatty acids exist as geometric isomers: cis and trans. In the cis isomer, the two hydrogen atoms attached to the carbons of the double bond lie on the same side, creating a kink or bend in the chain that disrupts straight alignment.^[12] Conversely, the trans isomer has the hydrogens on opposite sides, resulting in a straighter chain similar to that of saturated fatty acids.^[13] This restricted rotation around the C=C bond, due to the overlap of p-orbitals forming the pi bond, enforces these fixed geometries.^[14] Nomenclature for unsaturated fatty acids uses two primary systems to denote double bond positions. The delta (Δ) notation numbers the carbons from the carboxyl end, indicating the position of the double bond by the lower-numbered carbon involved (e.g., Δ9 for a double bond between carbons 9 and 10).^[7] The omega (ω) or n- notation counts from the methyl (ω) end of the chain, useful for classifying essential fatty acids like ω-3, where the first double bond starts at the third carbon from the methyl terminus.^[15] The C=C double bonds are shorter (approximately 1.34 Å) and stronger (bond dissociation energy of 614 kJ/mol) than single C-C bonds (1.54 Å and 348 kJ/mol), contributing to the rigidity and reactivity of unsaturated chains.^[16] This strength arises from the additional pi bond, while the shorter length affects molecular packing. A representative example is oleic acid, denoted as 18:1 cis-9 or (9Z)-octadec-9-enoic acid, with the structure CH₃(CH₂)₇CH=CH(CH₂)₇COOH, where the cis double bond is between carbons 9 and 10 from the carboxyl end.^[17] The cis configuration leads to poorer intermolecular packing due to the chain kink, which lowers the melting point of cis isomers compared to their trans counterparts or saturated analogs.^[18] Partial hydrogenation of unsaturated fats, a process that adds hydrogen across some double bonds, can convert cis isomers to trans, producing trans fats with straighter chains.^[19]

Classification

Monounsaturated Fats

Monounsaturated fatty acids (MUFAs) are a class of unsaturated fatty acids characterized by the presence of exactly one carbon-carbon double bond (C=C) in their aliphatic hydrocarbon chain, distinguishing them from saturated fatty acids that lack such bonds and polyunsaturated fatty acids that contain multiple double bonds. This single double bond results in the loss of two hydrogen atoms compared to their saturated counterparts, typically leading to a cis configuration that imparts a slight bend in the molecular structure. In standard notation, these fatty acids are designated by the total number of carbon atoms followed by the number of double bonds, such as 18:1 for an 18-carbon chain with one double bond. The most prevalent monounsaturated fatty acid in nature is oleic acid, denoted as 18:1 ω-9 or cis-9-octadecenoic acid, where the double bond is located between the ninth and tenth carbon atoms from the carboxyl end (or ω-9 from the methyl end). Oleic acid constitutes the majority of MUFAs in many biological systems due to its biosynthesis via the enzyme stearoyl-CoA desaturase, which introduces the cis double bond into stearic acid. Another common example is palmitoleic acid, or 16:1 ω-7 (cis-9-hexadecenoic acid), featuring a 16-carbon chain with the double bond between carbons 9 and 10; it is less abundant but plays roles in cellular signaling and membrane fluidity. Structurally, the single cis double bond in MUFAs creates a modest kink in the otherwise linear hydrocarbon chain, reducing van der Waals interactions and lowering the melting point compared to saturated fats, though this effect is less pronounced than in polyunsaturated fats with multiple kinks. These fatty acids are typically liquids at room temperature and solidify upon refrigeration, reflecting their intermediate packing efficiency. Monounsaturated fats are major components in various natural sources, with oleic acid comprising 70-80% of the fatty acids in olive oil, approximately 50-70% in avocados, and around 45% of the total fat in lard. Due to their single double bond, MUFAs exhibit lower susceptibility to oxidative degradation than polyunsaturated fats, as fewer sites are available for radical-initiated peroxidation reactions. Oleic acid also featured prominently in pioneering hydrogenation studies; in 1901, chemist Wilhelm Normann demonstrated the catalytic hydrogenation of its double bond to form stearic acid using nickel, laying the groundwork for industrial fat processing.

Polyunsaturated Fats

Polyunsaturated fatty acids (PUFAs) are a class of lipids characterized by the presence of two or more carbon-carbon double bonds in their hydrocarbon chain, distinguishing them from monounsaturated fatty acids that contain only one such bond.^[3] These double bonds are typically in the cis configuration, and the fatty acids are denoted using a shorthand notation that indicates the total number of carbon atoms followed by the number of double bonds, such as 18:2 for an 18-carbon chain with two double bonds.^[20] PUFAs play critical roles in cellular membranes and signaling pathways due to their structural flexibility.^[21] Prominent examples of PUFAs include linoleic acid (18:2 ω-6), an essential omega-6 fatty acid; alpha-linolenic acid (18:3 ω-3), an essential omega-3 fatty acid; arachidonic acid (20:4 ω-6), a precursor to eicosanoids; eicosapentaenoic acid (EPA, 20:5 ω-3), involved in anti-inflammatory processes; and docosahexaenoic acid (DHA, 22:6 ω-3), crucial for neural development.^[22] The omega (ω) nomenclature classifies PUFAs based on the position of the first double bond from the methyl (omega) end of the chain: ω-3 fatty acids have their initial double bond between carbons 3 and 4, while ω-6 fatty acids have it between carbons 6 and 7.^[23] Both ω-3 and ω-6 families are essential for humans because the body lacks the enzymes to insert double bonds at these specific positions, necessitating dietary intake of precursors like linoleic and alpha-linolenic acids.^[24] The multiple cis double bonds in PUFAs enhance membrane fluidity by introducing kinks that prevent tight packing of lipid chains, contributing to cellular adaptability.^[25] However, these same structural features make PUFAs particularly susceptible to lipid peroxidation, as the allylic hydrogens adjacent to the double bonds are easily abstracted by reactive oxygen species, leading to oxidative chain reactions.^[26] The essentiality of PUFAs was first demonstrated in the late 1920s through experiments on rats fed fat-free diets, which developed scaly skin, growth retardation, and reproductive failure; supplementation with linoleic acid reversed these symptoms, establishing fatty acids as vital nutrients.^[27] Furthermore, the dietary ratio of ω-6 to ω-3 PUFAs is significant for health, with modern Western diets often exceeding 10:1—far higher than the ancestral 1:1 to 4:1—potentially promoting inflammation and chronic disease risk, whereas a lower ratio supports balanced eicosanoid production and anti-inflammatory effects.^[28]

Properties

Physical Properties

Unsaturated fats are characteristically liquid at room temperature, a physical state attributable to the kinks introduced by cis double bonds in their fatty acid chains, which disrupt molecular packing and inhibit crystallization. This contrasts with saturated fats, which tend to be solid due to their straight chains allowing tighter alignment. For example, olive oil, predominantly composed of monounsaturated fats, exhibits a melting point of approximately -6°C, while butter, rich in saturated fats, melts around 32°C.^[12]^[29] The density of unsaturated fats typically ranges from 0.91 to 0.93 g/cm³ at 20°C, rendering them less dense than water but comparable to or marginally lower than many saturated fats owing to the irregular chain conformations. These fats are hydrophobic and insoluble in water, yet they dissolve readily in nonpolar organic solvents like hexane and chloroform, facilitating their extraction and analysis in laboratory settings.^[30]^[31] Viscosity in unsaturated fats decreases with increasing unsaturation, promoting greater fluidity and ease of flow, which is advantageous in industrial processing and culinary applications. Olive oil, for instance, displays a dynamic viscosity of about 84 mPa·s at 20°C, lower than that of more saturated counterparts like lard. Additionally, the refractive index rises with the number of double bonds, generally falling between 1.46 and 1.47 for vegetable oils such as soybean and sunflower, serving as an optical indicator of unsaturation levels.^[32]^[33]^[34] The iodine value quantifies unsaturation by measuring iodine absorption capacity, expressed as grams of iodine per 100 g of fat; higher values indicate greater double bond presence. Corn oil, with significant polyunsaturated content, has an iodine value of 107–128, compared to coconut oil's low 6–11 due to its saturated profile.^[31] Thermally, polyunsaturated fats exhibit lower smoke points from enhanced volatility and oxidative instability at heat. Linseed oil, highly polyunsaturated, smokes at approximately 107°C (225°F), limiting its use in high-temperature cooking.^[35]

Chemical Properties

Unsaturated fats exhibit significant chemical reactivity primarily due to their carbon-carbon double bonds, which make them prone to addition reactions and oxidation processes. One of the most important reactions is auto-oxidation, a free radical chain mechanism that leads to the degradation of these lipids. This process begins with initiation, where reactive oxygen species (ROS), such as hydroxyl radicals (.OH), abstract a hydrogen atom from a methylene group (-CH₂-) adjacent to a double bond in the fatty acid chain, generating a lipid radical (L•). The double bonds weaken these adjacent C-H bonds, facilitating radical formation, and the lipid radical quickly reacts with molecular oxygen to form a peroxyl radical (LOO•). Propagation follows, in which the peroxyl radical abstracts a hydrogen from another lipid molecule, producing a lipid hydroperoxide (LOOH) and regenerating a lipid radical, creating an autocatalytic chain that propagates oxidation. This step is enhanced by trace metals like iron or copper. Termination occurs when two radicals combine to form non-radical products, such as stable dimers (LOOL) or when antioxidants intervene to break the chain. Polyunsaturated fats are particularly susceptible to auto-oxidation because of bis-allylic positions (methylene groups flanked by two double bonds), which have even weaker C-H bonds, leading to faster radical formation and ultimately rancidity characterized by off-flavors and odors from decomposition products.^[36]^[37] Hydrogenation is another key reaction involving the addition of hydrogen across the double bonds of unsaturated fats, converting them to saturated fats. This process typically requires a catalyst, such as finely divided nickel, and elevated temperatures around 150°C under pressure to facilitate the reaction. Complete hydrogenation saturates all double bonds, producing fully saturated fats with increased stability and higher melting points, while partial hydrogenation stops at monounsaturated intermediates and can generate trans fats due to isomerization of cis double bonds to trans configurations during the process. The general equation for hydrogenation of an alkene in a fatty acid chain is:

\mathrm{R-CH=CH-R' + H_2 \xrightarrow{\text{Ni, 150°C}} R-CH_2-CH_2-R'}

R−CH=CH−R′+H2Ni, 150°C

R−CH2−CH2−R′ ^[38]^[39] Halogenation serves as a qualitative test for unsaturation in fats, involving the electrophilic addition of halogens like bromine (Br₂) or iodine (I₂) across the double bonds. In the bromine water test, unsaturated fats decolorize the reddish-brown bromine solution as the halogen adds to the double bond, forming a colorless dibromide derivative, confirming the presence of unsaturation; saturated fats do not react.^[40] The chemical stability of unsaturated fats is influenced by factors that mitigate oxidation, particularly the use of antioxidants such as tocopherols (vitamin E), which act as chain-breaking agents by donating hydrogen to peroxyl radicals, forming stable hydroperoxides and preventing further propagation of peroxidation. These antioxidants extend shelf life by inhibiting radical formation, with α-tocopherol showing significant reduction in peroxide values during storage. High-polyunsaturated oils, such as fish oil rich in omega-3 fatty acids, have inherently shorter shelf lives due to their multiple double bonds, often oxidizing within months under ambient conditions if unprotected, compared to monounsaturated oils.^[41]^[42]

Health and Nutrition

Nutritional Role

Unsaturated fats play a critical role in human nutrition, primarily through their provision of essential fatty acids that the body cannot synthesize. Linoleic acid, an omega-6 polyunsaturated fatty acid, and alpha-linolenic acid, an omega-3 polyunsaturated fatty acid, are indispensable dietary components, serving as precursors to longer-chain fatty acids such as arachidonic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). These longer chains are vital for the production of eicosanoids, including prostaglandins and leukotrienes, which regulate inflammation, blood clotting, and other physiological processes.^[23]^[43] Minimum requirements to prevent deficiency are met with approximately 1-2% of total caloric intake from linoleic acid and 0.5% from alpha-linolenic acid, though adequate intake recommendations are higher (e.g., 5-10% for total polyunsaturated fats including these essentials).^[43]^[44] Beyond their role in precursor synthesis, unsaturated fats serve multiple essential functions in the body. They provide a dense energy source at 9 kcal per gram, supporting overall caloric requirements during periods of high demand. Additionally, they facilitate the absorption of fat-soluble vitamins A, D, E, and K in the intestines, enhancing their bioavailability and utilization. As structural components, unsaturated fatty acids integrate into cell membranes, influencing fluidity and function, particularly in neural tissues and the retina where DHA predominates.^[43]^[23] Dietary guidelines emphasize the importance of unsaturated fats in balanced nutrition. The World Health Organization (WHO) and Food and Agriculture Organization (FAO) recommend that total fat intake constitute less than 30% of energy (as updated by WHO in 2023), with polyunsaturated fatty acids contributing 6-11% of total energy to meet essential needs while minimizing health risks. An optimal omega-6 to omega-3 ratio of 4:1 to 1:1 is advised to support balanced eicosanoid production and reduce potential inflammatory imbalances.^[45]^[46]^[28] Deficiency in essential unsaturated fatty acids, though rare in humans due to widespread dietary availability, manifests distinctly based on early research. Studies from the 1930s by George and Mildred Burr demonstrated in rats that deprivation led to symptoms such as scaly dermatitis, growth retardation, and reproductive issues, which were reversed by linoleic acid supplementation. In humans, deficiencies are linked to skin disorders like dry, scaly rashes and impaired wound healing, often observed in cases of severe malnutrition or prolonged fat-free parenteral nutrition.^[27]^[47]^[43] The body's ability to derive longer-chain omega-3 fatty acids from alpha-linolenic acid is limited, with conversion efficiency to EPA and DHA typically under 5-10%, influenced by factors such as high omega-6 intake that competes for enzymatic pathways. This inefficiency underscores the recommendation for direct dietary sources of EPA and DHA, particularly from marine origins, to ensure sufficient levels for optimal physiological functions.^[23]

Health Implications

Unsaturated fats, particularly when used to replace saturated fats in the diet, have been associated with significant cardiovascular benefits. Meta-analyses indicate that substituting saturated fats with polyunsaturated fats lowers low-density lipoprotein (LDL) cholesterol levels and reduces the risk of cardiovascular disease events by approximately 20-30%. For instance, a 2020 Cochrane systematic review of randomized controlled trials found that reducing saturated fat intake and replacing it with polyunsaturated fats decreased cardiovascular events with a relative risk of 0.79 (95% CI 0.66-0.93), highlighting the protective role of this substitution. Similarly, the American Heart Association's 2017 advisory emphasized that replacing saturated fats with unsaturated fats, including monounsaturated and polyunsaturated varieties, lowers LDL cholesterol and decreases overall cardiovascular disease risk. Omega-3 polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), exhibit anti-inflammatory effects by reducing the production of proinflammatory cytokines like interleukin-6 and tumor necrosis factor-alpha. Clinical studies have linked higher intake of these omega-3 fats to alleviation of symptoms in inflammatory conditions, including rheumatoid arthritis, where supplementation decreased joint pain and morning stiffness in patients. A 2023 review confirmed that EPA and DHA modulate signaling pathways to inhibit proinflammatory cytokine expression, supporting their role in managing chronic inflammation. However, potential risks arise from imbalanced or improper consumption of unsaturated fats. Excessive intake of omega-6 polyunsaturated fatty acids, relative to omega-3s, can promote a proinflammatory state due to the overproduction of arachidonic acid-derived eicosanoids, contributing to chronic inflammation when the omega-6 to omega-3 ratio exceeds recommended levels. Additionally, heating polyunsaturated oils to high temperatures during frying can generate oxidation products, such as aldehydic lipid peroxidation toxins, which animal and in vitro studies from the 2020s suggest may elevate cancer risk through DNA damage and cellular toxicity, though human evidence remains emerging. Partially hydrogenated oils, used in industrial food processing to create unsaturated fats with improved stability, produce harmful artificial trans fats that elevate LDL cholesterol and cardiovascular risk, leading the U.S. Food and Drug Administration to determine in 2015 (with full revocation effective December 2023) that these oils are not generally recognized as safe.^[48] In contrast, naturally occurring trans fats in ruminant products, such as those from dairy and meat, occur in minimal amounts (typically 2-5% of total fat) and do not show the same adverse associations with coronary heart disease as industrial trans fats. Epidemiological evidence further supports the health implications of unsaturated fats through dietary patterns like the Mediterranean diet, which is rich in monounsaturated fats from sources such as olive oil and has been linked to increased longevity and reduced cardiovascular mortality. The PREDIMED trial, a large randomized controlled study published in 2013, demonstrated that adherence to a Mediterranean diet supplemented with extra-virgin olive oil reduced the incidence of stroke by 30% compared to a low-fat control diet, underscoring the protective effects against cerebrovascular events.

Biological Functions

Role in Cell Membranes

Unsaturated fatty acids are primarily incorporated into the sn-2 position of phospholipids, such as phosphatidylcholine and phosphatidylethanolamine, which form the core lipid bilayer of cell membranes.^[49] In mammalian cells, plasma membranes exhibit a higher degree of unsaturation, with approximately 50-60% of fatty acyl chains being unsaturated, compared to lower levels (around 40%) in mitochondrial inner membranes, reflecting specialized functional requirements.^[49]^[50] This composition ensures the structural integrity and dynamic properties of membranes across cellular compartments. The cis configuration of double bonds in unsaturated fatty acids creates kinks in the acyl chains, disrupting tight packing and promoting a liquid-crystalline phase that enhances overall membrane fluidity.^[49] Through homeoviscous adaptation, organisms adjust the unsaturation level of membrane lipids in response to environmental temperatures, increasing cis-unsaturation at lower temperatures to counteract rigidification and maintain optimal viscosity.^[51] The 1972 Singer-Nicolson fluid mosaic model underscores this fluidity as essential for the dynamic organization of membranes, where lipids serve as a solvent for embedded proteins.^[52] This fluidity has critical functional implications, enabling lateral mobility of integral proteins, proper gating of ion channels, and facilitation of membrane curvature during endocytosis.^[49] In neurons, elevated levels of docosahexaenoic acid (DHA), an omega-3 polyunsaturated fatty acid, in synaptic membranes support rapid signal transmission by optimizing receptor dynamics and synaptic vesicle fusion.^[53] Membrane composition varies across species; for instance, fish possess higher omega-3 unsaturation in their membranes than mammals, enhancing cold tolerance through preserved fluidity in low-temperature environments.^[54] Deficiencies in unsaturated fatty acids result in membrane stiffening due to increased saturated chain dominance, which impairs ion and nutrient transport across the bilayer.^[55] The membrane pacemaker concept, developed in the late 20th and early 21st centuries, posits that higher fatty acid unsaturation acts as a regulator of metabolic rate by influencing membrane-bound enzyme activities and proton leak, linking lipid composition to cellular energy dynamics.^[56]

Metabolic Processes

Unsaturated fatty acids undergo beta-oxidation in mitochondria, a process modified to accommodate double bonds through auxiliary enzymes such as 3,2-trans-enoyl-CoA isomerase, which repositions the double bond to allow continuation of the standard degradation pathway.^[57] This isomerase converts cis-Δ3-enoyl-CoA intermediates to trans-Δ2-enoyl-CoA, enabling hydration and subsequent cleavage. For example, the complete beta-oxidation of oleic acid (18:1, n-9), a monounsaturated fatty acid, yields approximately 118.5 moles of ATP per mole, compared to 120 moles for stearic acid (18:0), as the double bond at position 9 leads to one fewer FADH₂ after three cycles, while still yielding nine acetyl-CoA units and the necessary reducing equivalents.^[58] Post-dietary unsaturated fatty acids, particularly essential polyunsaturated ones like linoleic acid (18:2, n-6), are further modified through elongation and desaturation in the liver's endoplasmic reticulum.^[59] Elongases, such as ELOVL2 and ELOVL5, add two-carbon units using malonyl-CoA, while desaturases (e.g., Δ6-desaturase and Δ5-desaturase) introduce double bonds at specific positions.^[60] This sequential process converts linoleic acid to gamma-linolenic acid (18:3, n-6), then to dihomo-gamma-linolenic acid (20:3, n-6), and finally to arachidonic acid (20:4, n-6), supporting the synthesis of bioactive lipids.^[61] Arachidonic acid serves as a key precursor in eicosanoid synthesis, undergoing oxidation via cyclooxygenase (COX-1 and COX-2) to form prostaglandins and thromboxanes, or via lipoxygenase (LOX) pathways to produce leukotrienes and lipoxins.^[62] These eicosanoids mediate inflammation, hemostasis, and immune responses; for instance, COX-derived prostaglandin E2 (PGE2) promotes vasodilation, while 5-LOX-derived leukotriene B4 (LTB4) recruits neutrophils.^[63] Omega-3 polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA), compete with arachidonic acid for these enzymes, shunting metabolism toward less inflammatory resolvins and protectins, thereby reducing pro-inflammatory eicosanoid production.^[64]^[65] Very-long-chain unsaturated fatty acids (>C22) are primarily oxidized in peroxisomes via a beta-oxidation pathway that shortens chains before transfer to mitochondria for complete degradation.^[66] This process involves acyl-CoA oxidases and auxiliary enzymes handling double bonds, generating hydrogen peroxide as a byproduct that links to reactive oxygen species (ROS) management through peroxisomal catalases.^[67] Peroxisomal oxidation thus prevents lipotoxicity from accumulated very-long-chain fatty acids while contributing to cellular redox homeostasis.^[68] The metabolism of unsaturated fatty acids is regulated by peroxisome proliferator-activated receptors (PPARs), particularly PPARα, which are activated by ligands including polyunsaturated fatty acids to induce gene expression for lipid catabolism.^[69] PPARα binds to peroxisome proliferator response elements, upregulating enzymes like carnitine palmitoyltransferase-1 and acyl-CoA oxidases to enhance beta-oxidation during fasting.^[70] Additionally, fasting increases desaturase activity, such as Δ5- and Δ6-desaturase expression via PPARα and SREBP-1c interplay, promoting conversion of essential fatty acids to longer-chain derivatives.^[71] This transcriptional control ensures adaptive lipid utilization under nutrient deprivation.^[72]

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