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Vitamin K2
Vitamin K2
Vitamin K2
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General structure of vitamin K2 (MK-n)

Vitamin K2 or menaquinone (MK) (/ˌmɛnəˈkwɪnoʊn/) is one of three types of vitamin K, the other two being vitamin K1 (phylloquinone) and K3 (menadione). K2 is both a tissue and bacterial product (derived from vitamin K1 in both cases) and is usually found in animal products or fermented foods.[1]

The number n of isoprenyl units in their side chain differs and ranges from 4 to 13, hence vitamin K2 consists of various forms.[2] It is indicated as a suffix (-n), e. g. MK-7 or MK-9.

  • The most common in the human diet is the short-chain, water-soluble menatetrenone (MK-4), which is commonly found in animal products. However, at least one published study concluded that "MK-4 present in food does not contribute to the vitamin K status as measured by serum vitamin K levels."[3] The MK-4 in animal (including human) tissue is made from dietary plant vitamin K1. This process can be accomplished by animal tissues alone, as it proceeds in germ-free rodents.[4]
  • Long-chain menaquinones (longer than MK-4) include MK-7, MK-8 and MK-9 and are more predominant in fermented foods such as natto and cheonggukjang.[5] They are bioavailable: oral consumption of MK-7 "significantly increases serum MK-7 levels and therefore may be of particular importance for extrahepatic tissues".[3]
  • Longer-chain menaquinones (MK-10 to MK-13) are produced by anaerobic bacteria in the colon, but they are not well absorbed at this level and have little physiological impact.[1]

When there are no isoprenyl side chain units, the remaining molecule is vitamin K3. This is usually made synthetically, and is used in animal feed. It was formerly given to premature infants, but due to inadvertent toxicity in the form of hemolytic anemia and jaundice,[failed verification] it is no longer used for this purpose.[1] K3 is now known to be a circulating intermediate in the animal production of MK-4: K1 is absorbed into the gut and converted into blood K3 and target tissues convert K3 into MK-4.[4]

Description

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See also: Vitamin K

Vitamin K2, the main storage form in animals, has several subtypes, which differ in isoprenoid chain length. These vitamin K2 homologues are called menaquinones, and are characterized by the number of isoprenoid residues in their side chains. Menaquinones are abbreviated MK-n, where M stands for menaquinone, the K stands for vitamin K, and the n represents the number of isoprenoid side chain residues. For example, menaquinone-4 (abbreviated MK-4) has four isoprene residues in its side chain. Menaquinone-4 (also known as menatetrenone from its four isoprene residues) is the most common type of vitamin K2 in animal products since MK-4 is normally synthesized from vitamin K1 in certain animal tissues (arterial walls, pancreas, and testes) by replacement of the phytyl tail with an unsaturated geranylgeranyl tail containing four isoprene units, thus yielding menaquinone-4 which is water soluble in nature. This homolog of vitamin K2 may have enzyme functions distinct from those of vitamin K1.

MK-7 and other long-chain menaquinones are different from MK-4 in that they are not produced by human tissue. MK-7 may be converted from phylloquinone (K1) in the colon by Escherichia coli bacteria.[6] However, these menaquinones synthesized by bacteria in the gut appear to contribute minimally to overall vitamin K status.[7][8] MK-4 and MK-7 are both found in the United States in dietary supplements for bone health.

All K vitamins are similar in structure: they share a "quinone" ring, but differ in the length and degree of saturation of the carbon tail and the number of repeating isoprene units in the "side chain".[9][full citation needed] The number of repeating units is indicated in the name of the particular menaquinone (e.g., MK-4 means that four isoprene units are repeated in the carbon tail). The chain length influences lipid solubility and thus transport to different target tissues.

Vitamin K structures. MK-4 and MK-7 are both subtypes of K2.

Mechanism of action

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The mechanism of action of vitamin K2 is similar to vitamin K1. K vitamins were first recognized as a factor required for coagulation, but the functions performed by this vitamin group were revealed to be much more complex. K vitamins play an essential role as cofactor for the enzyme γ-glutamyl carboxylase, which is involved in vitamin K-dependent carboxylation of the gla domain in "gla proteins" (i.e., in conversion of peptide-bound glutamic acid (glu) to γ-carboxy glutamic acid (Gla) in these proteins).[10]

Carboxylation reaction – the vitamin K cycle

Carboxylation of these vitamin K-dependent Gla-proteins, besides being essential for the function of the protein, is also an important vitamin recovery mechanism since it serves as a recycling pathway to recover vitamin K from its epoxide metabolite (KO) for reuse in carboxylation.

Several human Gla-containing proteins synthesized in several different types of tissue have been discovered:

  • Coagulation factors (II, VII, IX, X), as well as anticoagulation proteins (C, S, Z). These Gla-proteins are synthesized in the liver and play an important role in blood homeostasis.
  • Osteocalcin. This non-collagenous protein is secreted by osteoblasts and plays an essential role in the formation of mineral in bone.
  • Matrix gla protein (MGP). This calcification inhibitory protein is found in numerous body tissues, but its role is most pronounced in cartilage and in arterial vessel walls.
  • Growth arrest-specific protein 6 (GAS6). GAS6 is secreted by leucocytes and endothelial cells in response to injury and helps in cell survival, proliferation, migration, and adhesion.
  • Proline-rich Gla-proteins (PRGP), transmembrane Gla-proteins (TMG), Gla-rich protein (GRP) and periostin. Their precise functions are still unknown.

Health effects

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There is inconclusive clinical data whether specific vitamin K2 supplementation confers any beneficial effects compared to vitamin K1 which is the most common form in supplements.[1] In vitro studies show certain cellular effects of vitamin K2 in bone which are not observed with the K1 variant (including bone marrow stem cell (BMSC) proliferation, and stimulation of osteoblast differentiation).[1] The effects of vitamin K2 appear to be accentuated when combined with vitamin D and in the setting of osteoporosis.[1]

Research suggests that vitamin K2 (Menaquinone 7, MK-7]) may reduce the rate and severity of night time leg cramps.[11]

Absorption profile

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Vitamin K is absorbed along with dietary fat from the small intestine and transported by chylomicrons in the circulation.[12] Most of vitamin K1 is carried by triacylglycerol-rich lipoproteins (TRL) and rapidly cleared by the liver; only a small amount is released into the circulation and carried by LDL-C and HDL-C. MK-4 is carried by the same lipoproteins (TRL, LDL-C, and HDL-C) and cleared fast as well. The long-chain menaquinones are absorbed in the same way as vitamin K1 and MK-4 but are efficiently redistributed by the liver in predominantly LDL-C (VLDL-C). Since LDL-C has a long half-life in the circulation, these menaquinones can circulate for extended times resulting in higher bioavailability for extra-hepatic tissues as compared to vitamin K1 and MK-4. Accumulation of vitamin K in extra-hepatic tissues has direct relevance to vitamin K functions not related to hemostasis.[13]

Dietary intake in humans

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The European Food Safety Authority (EU) and the US Institute of Medicine, on reviewing existing evidence, have decided there is insufficient evidence to publish a dietary reference value for vitamin K or for K2. They have, however, published an Adequate Intake (AI) for vitamin K, but no value specifically for K2.[citation needed]

Parts of the scientific literature, dating back to 1998, suggest that the AI values are based only on the hepatic requirements (i.e. related to the liver).[14][15] This hypothesis is supported by the fact that the majority of the Western population exhibits a substantial fraction of undercarboxylated extra-hepatic proteins.[citation needed] Thus, complete activation of coagulation factors is satisfied, but there does not seem to be enough vitamin K2 for the carboxylation of osteocalcin in bone and MGP in the vascular system.[16][17]

There is no known toxicity associated with high doses of menaquinones (vitamin K2). Unlike the other fat-soluble vitamins, vitamin K is not stored in any significant quantity in the liver. All data available as of 2017 demonstrate that vitamin K has no adverse effects in healthy subjects.[citation needed] The recommendations for the daily intake of vitamin K, as issued recently by the US Institute of Medicine, also acknowledge the wide safety margin of vitamin K: "a search of the literature revealed no evidence of toxicity associated with the intake of either K1 or K2". Animal models involving rats, if generalisable to humans, show that MK-7 is well tolerated.[18]

Dietary sources

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Apart from animal livers, the richest dietary source of menaquinones are fermented foods (from bacteria, not molds or yeasts); sources include cheeses consumed in Western diets (e.g., containing MK-9, MK-10, and MK-11) and fermented soybean products (e.g., in traditional nattō consumed in Japan, containing MK-7 and MK-8).[citation needed] (Here and following it is noteworthy that most food assays measure only fully unsaturated menaquinones.[citation needed])

MK-4 is synthesized by animal tissues and is found in meat, eggs, and dairy products.[19] Cheeses have been found to contain MK-8 at 10–20 μg per 100 g and MK-9 at 35–55 μg per 100 g.[13] In one report, no substantial differences in MK-4 levels were observed between wild game, free-range animals, and factory farm animals.[20]

In addition to its animal origins, menaquinones are synthesized by bacteria during fermentation and so, as stated, are found in most fermented cheese and soybean products.[21][non-primary source needed] As of 2001, the richest known source of natural K2 was nattō fermented using the nattō strain of Bacillus subtilis,[22] which is reportedly a good source of long-chain MK-7.[citation needed] In nattō, MK-4 is absent as a form of vitamin K, and in cheeses it is present among the vitamins K only in low proportions.[relevant?][23][better source needed] Still it is unknown whether B. subtilis will produce K2 using other legumes (e.g., chickpeas, or lentils) or even B. subtilis fermented oatmeal. According to Rebecca Rocchi et al., 2024, creating natto by using Bacillus subtilis to ferment boiled red lentils, chickpeas, or green peas produced greater amounts of MK-7 than creating natto by using Bacillus subtilis to ferment boiled soybeans, lupins, or brown beans.[24]

Food frequency questionnaire-derived estimates of relative intakes of vitamins K in one northern European country suggest that for that population, about 90% of total vitamin K intakes are provided by K1, about 7.5% by MK-5 through MK-9 and about 2.5% by MK-4[citation needed]

Analysis of foods

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Food Vitamin K2 (μg per 100 g
or μg/100 ml)[20]: Table 2  		Proportion of compounds
Nattō, fermented 1,034.0 0% MK-4, 1% MK-5, 1% MK-6, 90% MK-7, 8% MK-8
Goose liver pâté 369.0 100% MK-4
Hard cheeses (15 samples) 76.3 6% MK-4, 2% MK-5, 1% MK-6, 2% MK-7, 22% MK-8, 67% MK-9
Cheddar 23.5 (235 ng/g)[25] (ng/g) 51.2 MK-4, 3.8 MK-6, 18.8 MK-7, 36.4 MK-8, 125 MK-9
Eel 63.1[25] 100% MK-4
Eel 2.2[20]: Table 2  1.7 MK-4, 0.1 MK-6, 0.4 MK-7
Soft cheeses (15 samples) 56.5 6.5% MK-4, 0.5% MK-5, 1% MK-6, 2% MK-7, 20% MK-8, 70% MK-9
Camembert 68.1 (681 ng/g)[25] (ng/g) 79.5 MK-4, 13.4 MK-5, 10.1 MK-6, 32.4 MK-7, 151 MK-8, 395 MK-9
Milk (4% fat, USA)† 38.1[26] 2% MK-4, 46% MK-9, 7% MK-10, 45% MK-11
Egg yolk (Netherlands) 32.1 98% MK-4, 2% MK-6
Goose leg 31.0 100% MK-4
Curd cheeses (12 samples) 24.8 2.6% MK-4, 0.4% MK-5, 1% MK-6, 1% MK-7, 20% MK-8, 75% MK-9
Egg yolk (USA) 15.5[27] 100% MK-4
Butter 15.0 100% MK-4
Chicken liver (pan-fried) 12.6[27] 100% MK-4
Chicken leg 8.5 100% MK-4
Ground beef (medium fat) 8.1[27] 100% MK-4
Calf's liver (pan-fried) 6.0[27] 100% MK-4
Hot dog 5.7[27] 100% MK-4
Bacon 5.6[27] 100% MK-4
Whipping cream 5.4 100% MK-4
Sauerkraut 4.8 8% MK-4, 17% MK-5, 31% MK-6, 4% MK-7, 17% MK-8, 23% MK-9
Pork steak 3.7 57% MK-4, 13% MK-7, 30% MK-8
Duck breast 3.6 100% MK-4
Buttermilk 2.5 8% MK-4, 4% MK-5, 4% MK-6, 4% MK-7, 24% MK-8, 56% MK-9
Beef 1.1 100% MK-4
Buckwheat bread 1.1 100% MK-7
Whole milk yogurt 0.9 67% MK-4, 11% MK-5, 22% MK-8
Whole milk (Netherlands)† 0.9 89% MK-4, 11% MK-5
Egg white 0.9 100% MK-4
Salmon 0.5 100% MK-4
Cow's liver (pan-fried) 0.4[27] 100% MK-4
Mackerel 0.4 100% MK-4
Skimmed milk yogurt 0.1 100% MK-8

Notes:

  • † – The reported amounts in comparable milk from the USA and the Netherlands differ by more than 40 times, so these numbers should be considered suspect.

Anticoagulants

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Recent studies found a clear association between long-term oral (or intravenous) anticoagulant treatment (OAC) and reduced bone quality due to reduction of active osteocalcin. OAC might lead to an increased incidence of fractures, reduced bone mineral density or content, osteopenia, and increased serum levels of undercarboxylated osteocalcin.[28]

Furthermore, OAC is often linked to undesired soft-tissue calcification in both children and adults.[29][30] This process has been shown to be dependent upon the action of K vitamins. Vitamin K deficiency results in undercarboxylation of MGP. Also in humans on OAC treatment, two-fold more arterial calcification was found as compared to patients not receiving vitamin K antagonists.[31][32] Among consequences of anticoagulant treatment: increased aortic wall stiffness, coronary insufficiency, ischemia, and even heart failure. Arterial calcification might also contribute to systolic hypertension and ventricular hypertrophy.[33][34] Anticoagulant therapy is usually instituted to avoid life-threatening diseases, and high vitamin K intake interferes with anticoagulant effects.[citation needed] Patients on warfarin (Coumadin) or being treated with other vitamin K antagonists are therefore advised not to consume diets rich in K vitamins.[citation needed]

In other organisms

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Many bacteria synthesize menaquinones from chorismic acid. They use it as a part of the electron transport chain, playing a similar role as other quinones such as ubiquinone. Oxygen, heme, and menaquinones are needed for many species of lactic acid bacteria to conduct respiration.[35]

Variations in biosynthetic pathways mean that bacteria also produce analogues of vitamin K2. For example, MK9(II-H), which replaces the second geranylgeranyl unit with a saturated phytyl, is produced by Mycobacterium phlei. There also exists a possibility of cis–trans isomerism due to the double bonds present. In M. phlei, the 3'-methyl-cis MK9(II-H) form seems to be more biologically active than trans MK9(II-H).[36] However, with human enzymes, the naturally abundant trans form is more efficient.[37]

One hydrogenated MK that sees relevant amounts of human consumption is MK-9(4H), found in cheese fermented by Propionibacterium freudenreichii. This variation has the second and third units replaced with phytyl.[38]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Vitamin K2

View on Grokipedia
from Grokipedia
Vitamin K2, also known as menaquinone, is a fat-soluble essential for the activation of proteins involved in blood clotting, bone metabolism, and the prevention of vascular . It comprises a family of compounds designated as MK-4 through MK-13 based on the length of their unsaturated isoprenyl side chains, with MK-4, MK-7, and MK-9 being the most commonly studied forms. Unlike K1 (phylloquinone), which is primarily obtained from green leafy vegetables, K2 is predominantly sourced from fermented foods and animal products, and it can also be synthesized in the human body from K1 by certain tissues. The primary physiological function of vitamin K2 is to serve as a cofactor for gamma-carboxylation of vitamin K-dependent proteins, such as prothrombin for and for health, as well as (MGP) that inhibits . Dietary sources include natto (a fermented product containing high levels of MK-7, up to 850 mcg per 3 ounces), certain cheeses, and meats from animals fed , which yields MK-4. Supplements often provide MK-4 or MK-7, with typical doses ranging from 45 mcg to over 4,000 mcg daily, though adequate intake recommendations for total are set at 90–120 mcg per day for adults without a specific upper limit due to low toxicity risk. A dose of 400 mcg of vitamin K2 as MK-7 per day is fully safe; the Council for Responsible Nutrition (CRN) indicates a highest observed intake of 375 mcg/day for supplements based on clinical trials showing no adverse effects, and no toxicity has been reported even at 1000 mcg/day or higher in studies. Research highlights vitamin K2's role in reducing fracture risk in , with meta-analyses showing that 45 mg/day of MK-4 decreases vertebral fractures by 60% and hip fractures by 77% in postmenopausal women. Observational studies have associated higher intakes of menaquinones with reduced coronary artery calcification and a lower risk of coronary heart disease mortality, potentially supporting cardiovascular health through inhibition of vascular calcification; for instance, intakes above 32 mcg/day have been associated with a 57% reduced risk of coronary heart disease mortality in one population study. However, randomized controlled trials have shown mixed results regarding the effects of vitamin K2 supplementation on vascular calcification progression and other cardiovascular outcomes. As of 2025, emerging evidence from recent reviews and studies suggests additional benefits for brain (including cognitive function and potential role in symptoms), inflammation reduction, metabolic disorders (with no scientific evidence that vitamin K2 causes weight gain or adverse effects on body weight; human intervention studies, including a 3-year randomized placebo-controlled trial in postmenopausal women using 180 mcg/day MK-7, found no overall effect on body composition but supported reductions in abdominal and visceral fat in subgroups with strong increases in carboxylated osteocalcin; obesity is associated with poorer circulating vitamin K status and higher vitamin K accumulation in adipose tissue, but no causal link exists for vitamin K2 causing weight gain), and further confirmation of bone improvements, though more clinical trials are needed to confirm these effects.

Overview and Chemistry

Definition and Forms

Vitamin K2, collectively referred to as menaquinones, comprises a subfamily of fat-soluble vitamins within the broader group. These compounds are essential for the post-translational γ-carboxylation of glutamate residues in specific proteins, converting them into γ-carboxyglutamate () residues that enable calcium binding and support various physiological functions, including and mineral metabolism. Menaquinones are designated as MK-n, where n denotes the number of isoprenoid (prenyl) units in their , typically varying from 4 to 13. The short-chain variant MK-4 (menaquinone-4), with four units, is primarily obtained from animal tissues such as meat and eggs, and can also be produced endogenously in humans through the conversion of vitamin K1. Longer-chain forms, like MK-7 (menaquinone-7) with seven units, are synthesized by and are prominent in fermented foods, including natto and certain cheeses. The identification of vitamin K2 traces to the 1930s, when Danish biochemist Henrik Dam discovered a fat-soluble factor promoting normal blood coagulation in chicks deprived of dietary fats during studies, which otherwise caused subcutaneous and muscular hemorrhages. Dam termed this substance , derived from the German "Koagulation." Subsequently, American biochemist Edward A. Doisy isolated the compound in pure form, distinguishing the plant-derived phylloquinone (K1) from the menaquinone (K2) found in animal and bacterial sources, and elucidated their chemical structures. For these pioneering contributions, Dam and Doisy shared the 1943 in Physiology or Medicine. Vitamin K2's carboxylation activity is particularly important for calcium , activating proteins that direct calcium deposition into bones while preventing its accumulation in vascular and soft tissues.

Comparison to Vitamin K1

Vitamin K1 (phylloquinone) and vitamin K2 (menaquinones) share a common 2-methyl-1,4-naphthoquinone ring structure but differ in their side chains: K1 features a phytyl side chain derived from plants, while K2 contains polyisoprenoid side chains of varying lengths (typically 4 to 13 isoprenyl units), produced by or tissues. This structural variation influences their metabolic pathways and tissue targeting. In terms of primary functions, vitamin K1 is predominantly utilized in the liver to support blood by serving as a cofactor for the γ- of hepatic vitamin K-dependent proteins, such as clotting factors II, VII, IX, and X. In contrast, vitamin K2 primarily activates extrahepatic proteins, including for bone mineralization and (MGP) to inhibit vascular calcification. Although both forms contribute to carboxylation processes, K2's role extends beyond to broader physiological regulation. Bioavailability differs markedly, with vitamin K2 exhibiting superior absorption and distribution compared to K1; for instance, long-chain forms like MK-7 achieve 10-fold higher postprandial serum concentrations and have a longer (approximately 72 hours) versus K1's shorter (1-3 hours), allowing better accumulation in extrahepatic tissues such as bones and arteries. Both are fat-soluble vitamins requiring dietary for optimal uptake, but K1's rapid clearance limits its systemic reach. Dietarily, vitamin K1 is abundant in green leafy vegetables like spinach and kale, while vitamin K2 occurs mainly in fermented foods (e.g., natto) and animal products such as cheese and liver. Together, they meet overall vitamin K requirements (90-120 μg/day for adults), though deficiencies can be form-specific due to limited dietary overlap and conversion inefficiencies from K1 to K2.

Molecular Structure

Vitamin K2, collectively known as menaquinones (MK), shares a core 2-methyl-1,4- ring structure with other forms of , featuring a at position 2 and two carbonyl groups at positions 1 and 4 of the naphthoquinone moiety. At the 3-position of this ring, a polyisoprenoid is attached, consisting of repeating isoprenoid (4-carbon) units that vary in length to define the specific MK-n subtypes, where n denotes the number of units. For instance, MK-4 contains four isoprenoid units, while MK-7 has seven. The molecular formula of menaquinones depends on the length; MK-4 is C₃₁H₄₀O₂, and MK-7 is C₄₆H₆₄O₂. As a fat-soluble compound, vitamin K2 typically appears as a light yellow crystalline solid or microcrystalline plates at room temperature, with melting points around 51–54 °C for common forms like MK-4 and MK-7. Its stability is relatively high toward heat, oxygen, and mild acidic conditions but is compromised by exposure to UV light, , and strong acids. In natural sources, menaquinones predominantly exhibit an all-trans (all-E) configuration in the double bonds of their isoprenoid side chains, which contributes to their linear molecular shape and .

Sources and Production

Dietary Sources

Vitamin K2, also known as menaquinone, is primarily obtained from dietary sources that include fermented foods, animal products, and certain organ meats, with content varying by the specific menaquinone subtype (e.g., MK-4 to MK-10). High-content foods such as natto, a fermented product, provide substantial amounts of MK-7, reaching up to 998 μg per 100 g, making it one of the richest sources. Goose liver stands out among organ meats with approximately 369 μg per 100 g, predominantly as MK-4, while hard cheeses like Gouda contain 45–55 μg per 100 g of longer-chain menaquinones such as MK-8 and MK-9. Animal products contribute primarily MK-4, with egg yolks offering 15–64 μg per 100 g, butter providing 13–21 μg per 100 g, and meats like containing 13–32 μg per 100 g, though levels depend on the animal's diet. Fermented vegetables such as yield lower amounts, around 2–3 μg per 100 g total menaquinones (mainly MK-6 and MK-9), while provides variable traces of menaquinones from bacterial , typically less than 5 μg per 100 g. These sources are supplemented by endogenous production in the gut, which can contribute to overall vitamin K2 status. Plant-based calcium sources, such as tofu, lack significant vitamin K2 content, with levels typically around 1 μg per 28 g serving. While there is a theoretical risk of arterial or soft tissue calcification from high calcium intake in the context of severe vitamin K2 deficiency, no direct evidence links tofu or other plant-based calcium sources to increased calcification risk; such risks are more associated with low vitamin K2 diets overall. Plant-based eaters often obtain abundant vitamin K1 from greens, which the body partially converts to vitamin K2 (particularly MK-4), helping to mitigate potential deficiencies. Regional dietary patterns influence intake; for instance, Asian diets often feature higher vitamin K2 consumption due to regular natto intake, potentially exceeding 200 μg daily, compared to Western diets relying more on cheese and , averaging 30–60 μg daily. Factors such as fermentation duration and bacterial strains in products like natto and cheese can increase menaquinone levels by up to 20–50%, with longer enhancing MK-7 yield. Similarly, enriched with precursors boosts MK-4 in meat, eggs, and , leading to 2–5 times higher content in products from supplemented . The bioavailability of vitamin K2 from these foods varies by subtype, with longer-chain forms like MK-7 from natto exhibiting superior absorption and longer half-life compared to MK-4 from animal sources.
Food SourcePrimary Form(s)Content (μg/100 g)Notes
NattoMK-7882–1039Highest source; fermentation-dependent.
Goose LiverMK-4369Organ meat; consistent high levels.
Hard Cheese (e.g., Gouda)MK-8, MK-945–55Ripening duration affects yield.
Egg YolkMK-415–64Varies by hen feed.
ButterMK-413–21From grass-fed cows higher.
Chicken MeatMK-413–32Influenced by poultry feed.
SauerkrautMK-6, MK-92–3Low but variable from bacteria.
KimchiVariable MK<5Traces from fermentation.

Microbial and Endogenous Production

Vitamin K2, in the form of menaquinones (MK-n), is primarily synthesized by the human through the menaquinone biosynthetic pathway, which begins with chorismate as the precursor molecule derived from the . This process involves a series of enzymatic steps, including the conversion of chorismate to isochorismate by MenF, followed by reactions catalyzed by MenD, MenC, MenH, MenB, and to form o-succinylbenzoate and ultimately demethylmenaquinone, which is then methylated to menaquinone. Key producers among the include species from the genera (e.g., and Bacteroides thetaiotaomicron) and (e.g., Eubacterium lentum), which generate various MK homologs such as MK-6 to MK-13 to support their electron transport chains during . These bacteria predominantly reside in the , where the anaerobic environment favors menaquinone production, but the distal location limits absorption efficiency, with only a fraction of the synthesized MKs entering systemic circulation via passive or incorporation. In addition to microbial synthesis, humans endogenously produce MK-4, a short-chain menaquinone, through the tissue-specific conversion of phylloquinone (vitamin K1) in extrahepatic organs such as the brain, pancreas, and testes. This conversion occurs via a two-step process: first, oxidative cleavage of the phytyl side chain of K1 to form menadione (vitamin K3) in the intestine or target tissues, followed by prenylation of menadione with geranylgeranyl pyrophosphate by the enzyme UbiA prenyltransferase domain-containing protein 1 (UBIAD1), which is localized to the endoplasmic reticulum of non-hepatic cells. Unlike longer-chain MKs from bacteria, MK-4 accumulates preferentially in extrahepatic tissues, supporting local carboxylation functions without relying on hepatic storage. Several factors influence the rate and extent of microbial and endogenous production. use disrupts the composition, reducing populations of menaquinone-producing bacteria like and , thereby decreasing colonic MK synthesis and potentially contributing to insufficiency. Dietary composition modulates diversity and function; for instance, low-vitamin K diets alter microbial community structure, favoring bacteria with incomplete menaquinone pathways, while high-fiber or fermented food intake promotes producers. Endogenous MK-4 production is less affected by but depends on K1 availability and UBIAD1 expression levels in tissues. Recent research through 2025 has explored interventions to enhance MK-7 production, a long-chain menaquinone with high . Studies using strains, isolated from fermented sources, have optimized conditions—such as adjusting carbon (e.g., at 6 g/L) and (e.g., at 12 g/L) sources, , and incubation time—to achieve MK-7 yields exceeding 440 mg/L, far surpassing baseline production of around 67 mg/L. These advancements suggest potential for supplementation to boost gut-derived MK-7 in humans, with further increasing precursor flux in producer strains.

Absorption, Transport, and Metabolism

Absorption and Bioavailability

Vitamin K2, a fat-soluble vitamin, is primarily absorbed in the through a process that involves incorporation into mixed micelles formed by salts, pancreatic lipases, and dietary , facilitating uptake by enterocytes. This micellar solubilization is essential, as the absence of salts, such as in cholestatic conditions, significantly impairs absorption. Once absorbed, vitamin K2 is packaged into chylomicrons and enters the before reaching the bloodstream. Among the forms of vitamin K2, menaquinone-7 (MK-7) demonstrates superior compared to menaquinone-4 (MK-4) and vitamin K1 (phylloquinone), primarily due to its longer plasma of approximately 72 hours, versus 6-8 hours for MK-4 and 1-2 hours for phylloquinone. This extended allows for more sustained plasma concentrations, with MK-7 reaching peak levels around 6 hours post-ingestion and remaining detectable for up to 48 hours after a single dose. In contrast, MK-4 exhibits poor systemic at nutritional doses, often undetectable in serum even after supplementation, due to its shorter half-life leading to rapid clearance. Several factors influence the efficiency of vitamin K2 absorption. Dietary fat markedly enhances uptake, with increasing up to threefold when consumed with , as require fatty acids for stability. Age-related declines in production and gut can reduce absorption efficiency, while conditions impairing gut health, such as or , further compromise formation and function, leading to lower vitamin K status. Bioavailability is typically assessed by measuring post-ingestion plasma levels, where studies indicate near-complete absorption for long-chain menaquinones like MK-7—approaching 90-100% efficiency in the presence of —compared to only 5-10% for phylloquinone from sources. Recent analyses confirm that MK-7 derived from fermented sources exhibits enhanced over synthetic forms, with higher and more stable plasma elevations due to better micellar integration and reduced degradation in the gut.

Distribution and Excretion

Following absorption in the intestine, vitamin K2 is transported in the bloodstream primarily bound to lipoproteins, including triglyceride-rich lipoproteins (TRL) such as very low-density lipoproteins (VLDL), as well as low-density lipoproteins (LDL) and high-density lipoproteins (HDL) for certain forms like menaquinone-4 (MK-4). This lipoprotein-dependent transport facilitates the delivery of vitamin K2 to peripheral tissues, with preferential accumulation occurring in extrahepatic sites such as s and arteries, where it supports local processes. For instance, MK-7 is efficiently transported to tissue via remnants, contributing to about 20% of hepatic uptake before redistribution. Tissue distribution of vitamin K2 varies by menaquinone form and organ. MK-4, the predominant short-chain form, accumulates at high levels in extrahepatic tissues, including the (2.8 ng/g wet weight), kidneys (2.8 ng/g), and , where it exceeds concentrations of phylloquinone (vitamin K1) in these sites. In contrast, longer-chain menaquinones such as MK-7 through MK-11 are more abundant in the liver compared to vitamin K1, reflecting their higher hydrophobicity and retention in hepatic stores. This differential distribution underscores the role of chain length in targeting specific tissues, with MK-4 favoring peripheral organs and longer forms concentrating centrally. Excretion of vitamin K2 occurs mainly through the biliary route into , accounting for 40%–50% of elimination, while a smaller portion (approximately 20%) is excreted via as metabolites. Following hepatic metabolism, which involves ω-hydroxylation, β-oxidation, and , these metabolites are secreted into for fecal elimination, with minor urinary clearance. An pathway recycles a portion of vitamin K2, allowing from the gut and contributing to of body pools despite rapid turnover. Pharmacokinetic profiles differ markedly between vitamin K2 forms, influencing their and duration of action. MK-4 exhibits a short of 6–8 hours, with serum levels dropping to near zero within 24 hours post-intake, necessitating frequent dosing for sustained effects. In comparison, MK-7 has a longer elimination of approximately 3 days, enabling prolonged circulation and higher extrahepatic accumulation. Chronic intake of vitamin K2 leads to steady-state plasma levels, particularly for longer-chain forms like MK-7, which plateau after several weeks of supplementation. For example, daily consumption of 75–180 μg MK-7 from fortified or capsules achieves plasma concentrations of 2–2.3 ng/mL after 6 weeks, reflecting biphasic with an initial of ~3 days and a terminal phase exceeding 8 days. These steady-state models highlight the importance of consistent dietary or supplemental intake to maintain tissue saturation, as acute dosing primarily reflects recent exposure rather than long-term status.

Biological Functions

Mechanism of Action

Vitamin K2, primarily in the form of menaquinones, serves as a cofactor in the post-translational γ-carboxylation of specific glutamate residues on vitamin K-dependent proteins (VKDPs), enabling their calcium-binding capacity and biological activity. This process is catalyzed by the enzyme γ-glutamyl carboxylase (GGCX), which utilizes the reduced form of vitamin K2 (KH₂) as a cofactor, along with (CO₂) and molecular oxygen (O₂), to convert glutamate (Glu) residues to γ-carboxyglutamate () residues. The reaction can be represented as: Protein-Glu+CO2+O2+KH2→Protein-Gla+KO+H2O\text{Protein-Glu} + \text{CO}_2 + \text{O}_2 + \text{KH}_2 \rightarrow \text{Protein-Gla} + \text{KO} + \text{H}_2\text{O} where KO denotes the oxidized epoxide form of vitamin K2. The carboxylation reaction oxidizes KH₂ to vitamin K epoxide (KO), which must be recycled to sustain the cycle. This recycling occurs through a two-step reduction: first, vitamin K epoxide reductase (VKOR) converts KO back to the quinone form (K), and then vitamin K quinone reductases (such as VKOR itself or other reductases like DT-diaphorase) reduce K to KH₂. The entire vitamin K cycle thus depends on these reductase enzymes to regenerate the active hydroquinone form, ensuring continuous carboxylation. Anticoagulants like warfarin inhibit VKOR, disrupting the cycle and impairing Gla formation on VKDPs. The mechanism is highly specific to VKDPs, which contain a propeptide sequence recognized by GGCX; examples include in bone and (MGP) in vascular tissues, where is essential for their function in calcium regulation. Beyond , vitamin K2 activates the and receptor (SXR, also known as PXR), a that modulates gene transcription related to and detoxification. Recent research as of 2025 suggests vitamin K2 may also exert epigenetic effects, such as influencing and modifications, potentially through SXR-mediated pathways to regulate in tissues like the .

Physiological Roles

Vitamin K2 exerts its physiological effects primarily through the post-translational gamma- of extrahepatic vitamin K-dependent proteins (VKDPs), enabling them to interact with calcium ions and regulate key biological processes. , produced by osteoblasts, is a prominent extrahepatic VKDP that requires by vitamin K2 to achieve its fully active form, which then binds crystals to facilitate mineralization and maintain skeletal integrity. Similarly, (MGP), expressed in vascular cells and chondrocytes, undergoes vitamin K2-dependent to inhibit ectopic by sequestering calcium in soft tissues, thereby preventing vascular and renal deposition. These proteins highlight vitamin K2's targeted role in tissues outside the liver, contrasting with vitamin K1's dominance in hepatic functions. Beyond these VKDPs, vitamin K2 contributes to calcium by directing calcium trafficking toward bone deposition while averting its accumulation in arteries and other soft tissues, a process mediated through carboxylated MGP and . It also modulates responses via growth arrest-specific protein 6 (Gas6), another VKDP that promotes cell survival and inhibits in vascular cells through activation of the receptor pathway. In cell growth regulation, vitamin K2 influences proliferation and differentiation while suppressing osteoclastogenesis, partly by inhibiting signaling to balance . Although vitamin K2 supports the of coagulation factors II, VII, IX, and X, its contribution here is minor compared to vitamin K1, which predominates in the liver for these hepatic proteins. Tissue-specific functions of vitamin K2 extend to the , where menaquinone-4 (MK-4) correlates with the synthesis of , including sulfatides and , which are vital for sheath formation and neuronal membrane integrity. In dental health, carboxylated from odontoblasts and periodontal osteoblasts aids in mineralization and supports alveolar stability, mirroring its role in skeletal . Vitamin K2 exhibits synergy with vitamins D and A to optimize these roles; for instance, it enhances vitamin D-induced expression and for improved calcium utilization in , while cooperating with retinoids ( derivatives) to regulate differentiation and cell growth. Specifically, the combination of vitamin D3 and vitamin K2 supports bone and cardiovascular health: vitamin D3 enhances intestinal calcium absorption, while vitamin K2 activates proteins such as osteocalcin and matrix Gla protein to direct calcium to bones for mineralization and prevent its deposition in arteries, thereby reducing the risk of arterial calcification. This synergy has been shown to optimize calcium utilization, increase bone mineral density, and support cardiovascular integrity, as demonstrated in randomized controlled trials and meta-analyses. Studies and systematic reviews show that combining D3 and K2 improves bone mineral density (especially in postmenopausal women), reduces bone loss, and may benefit cardiovascular health by decreasing arterial stiffness or calcification; however, there is no conclusive evidence that moderate D3 alone is harmful without K2, and some reviews find insufficient proof for routine combination in all populations.

Health Implications

Bone and Cardiovascular Health

Vitamin K2 plays a crucial role in bone health by facilitating the γ-carboxylation of osteocalcin, a vitamin K-dependent protein synthesized by osteoblasts, which enables it to bind calcium and integrate into the bone matrix, thereby promoting mineralization and structural integrity. This activation process ensures that calcium is directed toward bone tissue rather than soft tissues, reducing the risk of demineralization. Clinical studies, including randomized controlled trials with menaquinone-7 (MK-7), have demonstrated that supplementation improves bone mineral density (BMD) and enhances bone quality and strength, particularly in postmenopausal women. A systematic review and meta-analysis of randomized controlled trials further confirmed that vitamin K2 supplementation maintains or improves BMD and reduces fracture risk, with effects more pronounced in forms like MK-7. For instance, intervention studies using MK-4, a shorter-chain form of K2, have shown reductions in fracture incidence and improvements in BMD. Recent research presented at the 7th Osteoporosis Congress in Padova (March 2025) highlighted MK-7's potential in reducing fractures and enhancing density through these mechanisms. In cardiovascular health, vitamin K2 activates proteins such as matrix Gla protein (MGP), a potent inhibitor of vascular calcification, by carboxylation, which prevents calcium deposition in arterial walls and soft tissues while directing calcium to bones. This process inhibits the progression of arterial stiffening and coronary artery calcification (CAC), key contributors to cardiovascular disease. Meta-analyses of randomized controlled trials indicate that vitamin K2 supplementation significantly slows CAC progression, with one analysis reporting a mean difference of -17.37 in CAC scores. Higher dietary intake of menaquinone (K2) has been linked to lower CAC scores and reduced cardiovascular risk. There is no direct evidence linking tofu or plant-based calcium sources to increased arterial calcification; theoretical risks exist only in cases of severe K2 deficiency combined with very high calcium intake, but such risks are more associated with supplements or low K2 diets. However, evidence from human clinical trials remains mixed. While observational studies consistently link higher vitamin K2 intake with cardiovascular benefits, many randomized controlled trials have failed to demonstrate significant changes in vascular calcification progression or other outcomes, particularly in certain populations such as those with chronic kidney disease. Vitamin K2 supplementation, particularly with MK-7, has been associated with reduced arterial stiffness as measured by pulse wave velocity, which may indirectly support lower blood pressure and improved vascular health. Limited evidence from small studies indicates increased maximal cardiac output (approximately 12%) and a trend toward higher heart rate during exercise, but no consistent direct effects on resting heart rate or blood pressure have been observed. Some preclinical evidence suggests improved endothelial function through nitric oxide-dependent mechanisms, potentially aiding vasodilation, though no reliable data exist on effects on blood volume. The longitudinal Rotterdam Study, involving over 4,800 participants, found that higher dietary menaquinone intake was associated with a 57% reduced relative risk of incident coronary heart disease compared to lower intake, alongside decreased all-cause and cardiovascular mortality. Supplementation trials support these observational findings; for example, daily doses of 180 μg MK-7 over three years improved arterial elasticity and reduced vascular calcification in healthy adults. Doses in this range (180–360 μg/day) are effective for both bone and cardiovascular benefits by enhancing carboxylation and preventing ectopic calcification.

Neurological and Other Effects

Vitamin K2 has been implicated in epigenetic within the , particularly through modulation of modifications that influence related to neuronal function and . Specifically, menaquinone-4 (MK-4), a form of vitamin K2, may promote by inhibiting 6 (HDAC6) or enhancing butyrate production, thereby supporting processes like and cognitive health. This mechanism draws from observations in non-neuronal cells, such as HL-60 lines where vitamin K analogues induce hyperacetylation, suggesting potential applicability to neurodegenerative contexts. Recent 2025 reviews highlight vitamin K2's neuroprotective effects against (AD) and (PD), primarily through anti-apoptotic, anti-inflammatory, and mitochondrial support pathways. In AD models, elevated MK-4 levels correlate with a 17-20% reduced risk of dementia and , alongside decreased amyloid-beta accumulation and pathology. For PD, vitamin K2-MK4 improves mitochondrial dynamics and locomotor function in PINK1 mutant models, with ongoing clinical trials exploring its therapeutic potential. studies further demonstrate these benefits, as MK-4 and MK-7 reduce amyloidogenesis and in SK-N-BE neuronal cells via DNA hypermethylation and suppression of pro-inflammatory cytokines. Human evidence remains limited to observational data showing higher vitamin K2 intake associated with better cognitive performance in older adults, though randomized controlled trials are sparse and indicate modest improvements in memory scores with supplementation. Beyond neurological roles, vitamin K2 exhibits anti-cancer potential by inducing in cells, particularly through mitochondria-dependent pathways that activate cascades and disrupt cell viability. experiments with androgen-dependent and independent prostate cell lines, such as VCaP and PC-3, show that MK-4 suppresses proliferation and promotes at concentrations of 10-50 μM, independent of status. Emerging evidence links vitamin K2 to dental health by supporting periodontal tissue integrity and reducing disease severity, as lower serum levels correlate with advanced periodontitis in case-control studies involving over 200 participants. Higher dietary vitamin K2 intake is associated with decreased probing depth and attachment loss in population-based analyses, potentially via enhanced osteogenic differentiation of periodontal ligament stem cells. In renal health, vitamin K2 contributes to reduction, particularly in (CKD), where supplementation decreases systemic markers like and interleukin-6 by inactivating signaling. Clinical reviews indicate that MK-7 at 360 μg daily for 12 weeks lowers pro-inflammatory cytokines in patients, alongside modest improvements in glomerular filtration rates. Regarding , a 2025 Frontiers review underscores vitamin K2's roles in mitigating -related challenges, including reduced undercarboxylation of to support metabolic balance and lower risks of postmenopausal deficiencies. Supplementation with MK-7 has shown potential in slowing age-related declines in turnover during , based on randomized trials in women over 50, though effects on remain underexplored in primary . In terms of metabolic effects and body composition, there is no scientific evidence that vitamin K2 supplementation causes weight gain. A 3-year randomized placebo-controlled trial in postmenopausal women demonstrated that daily supplementation with 180 μg MK-7 had no overall effect on body composition, but in subgroups exhibiting a strong increase in carboxylated osteocalcin ("good responders"), it was associated with significant reductions in abdominal fat mass and estimated visceral adipose tissue area compared to placebo. Animal studies, including models in rats and Caenorhabditis elegans, indicate that vitamin K2 may decrease total fat accumulation or enhance fat degradation and mitigate fat accumulation in high-fat diet conditions. Obesity in humans is associated with poorer circulating vitamin K status and higher accumulation of vitamin K in adipose tissue, although no causal relationship has been established whereby vitamin K2 intake leads to weight gain.

Deficiency and Toxicity

Vitamin K2 deficiency arises primarily from inadequate dietary intake, particularly in Western diets that are low in fermented foods and organ meats, the main sources of menaquinones. Modern has further reduced vitamin K2 availability since the mid-20th century, leaving approximately 30% of vitamin K-dependent proteins inactive, a figure that rises with age. Additionally, broad-spectrum antibiotics can disrupt responsible for endogenous vitamin K2 production, leading to significantly reduced hepatic menaquinone levels—down to 70 pmol/g from 423 pmol/g in controls. disorders, such as celiac disease, exacerbate this by impairing uptake of both dietary and bacterially produced forms; newly diagnosed celiac patients exhibit markedly lower levels of MK-4 and MK-7 (e.g., 27% deficiency rate for MK-7 versus 5% in controls). Unlike vitamin K1 deficiency, which classically presents with overt bleeding due to impaired coagulation factor synthesis, vitamin K2 deficiency rarely manifests such hemorrhagic signs in adults, as its roles are predominantly extrahepatic. Instead, symptoms are often subclinical and include increased risk of osteoporosis from disrupted osteocalcin carboxylation, leading to bone mineral loss, and vascular calcification due to inactive matrix Gla protein. Potential neurological effects, such as accelerated decline in cerebrovascular function, may also occur through impaired sphingolipid synthesis and vascular integrity, though these are less well-characterized. Deficiency status is commonly assessed using serum undercarboxylated (ucOC) levels, a sensitive reflecting insufficient γ-carboxylation; elevated ucOC correlates inversely with parameters like and broadband ultrasound attenuation. Subclinical vitamin K2 deficiency is prevalent in various populations, affecting up to 51% of hemodialysis patients based on ucOC markers and showing higher rates (e.g., over 30%) in those with per 2025 analyses. No established upper intake level exists for vitamin K2 due to its low potential, with no adverse effects reported from high oral doses up to 45 mg/day of MK-4 or 135 mg/day overall, and no risk of hypercoagulation even at these levels. Additionally, 400 mcg of vitamin K2 as MK-7 per day is fully safe; the Council for Responsible Nutrition (CRN) indicates an upper limit of 375 mcg/day for supplements as the highest observed intake, with no toxicity reported even at 1000 mcg/day or higher in studies.

Therapeutic Applications and Interactions

Supplements and Clinical Uses

Vitamin K2 supplements are available in two primary forms: menaquinone-4 (MK-4), which is typically synthetic and has a shorter of 6–8 hours, and menaquinone-7 (MK-7), derived from bacterial and characterized by higher and a longer of several days compared to MK-4. MK-7 is often preferred for supplementation due to its prolonged circulation, allowing better tissue distribution beyond the liver. These forms are commonly combined with vitamin D3 in over-the-counter products to enhance synergy. Vitamin D3 promotes intestinal calcium absorption, while vitamin K2 activates proteins such as osteocalcin and matrix Gla protein to direct calcium deposition into bones and teeth, thereby preventing its accumulation in soft tissues like arteries. Vitamin D and vitamin K2 interact synergistically: vitamin D promotes calcium absorption, while vitamin K2 (especially MK-7) directs calcium to bones and prevents arterial calcification, supporting bone and cardiovascular health. They are generally safe to take together at recommended doses, with no major adverse interactions reported. This combination supports bone health by increasing bone mineral density and cardiovascular health by reducing vascular calcification, as evidenced by clinical studies showing greater efficacy when used together compared to individually. Systematic reviews and studies indicate that combining vitamin D3 and K2 improves bone mineral density, especially in postmenopausal women, reduces bone loss, and may benefit cardiovascular health by decreasing arterial stiffness or calcification. However, there is no conclusive evidence that moderate vitamin D3 alone is harmful without K2, and some reviews find insufficient proof for routine combination supplementation in all populations. For instance, a randomized controlled trial in postmenopausal women with osteoporosis demonstrated significantly greater increases in lumbar spine bone mineral density with combined supplementation of 0.75 μg/day vitamin D3 and 45 mg/day MK-4 compared to either vitamin alone or calcium supplementation. Typical dosages in modern combined supplements often include 1,000 to 5,000 IU of vitamin D3 with 100 to 200 μg of MK-7 per serving, consistent with safety guidelines for both vitamins. There are no official combined dosage guidelines from bodies like the NIH or FDA specific to 2024-2026. Standard recommended intakes apply: vitamin D 600-800 IU/day for adults (higher if deficient, upper limit 4,000 IU); total vitamin K 90-120 mcg/day (no separate RDA for K2). Expert and review recommendations from 2024-2025 include vitamin K2 (MK-7) 100-300 mcg/day generally; 100 mcg/day (or 800 mcg/week) when taking high-dose vitamin D (>7,000 IU/day or serum 25(OH)D ≥80 ng/ml). Higher K2 doses (180-360 mcg/day MK-7) have been studied for heart benefits. Consult a healthcare provider for personalized dosing, especially with medications or conditions. However, in therapeutic contexts involving high-dose vitamin D3 supplementation, vitamin K2 dosages may require adjustment to support vascular health and prevent arterial calcification associated with elevated calcium absorption. There is no single official recommended dosage for vitamin K2 when co-supplementing with high-dose vitamin D3, as it varies by protocol, individual factors, and monitoring of calcium levels. For example, in the Coimbra protocol for autoimmune diseases, which utilizes high-dose vitamin D3 (mean approximately 35,000 IU/day, with doses up to over 150,000 IU/day), vitamin K2 is supplemented at 100–800 µg/day, individualized based on serum and urinary calcium levels to mitigate calcification risks. Some experts recommend 100 mcg/day MK-7 (or 800 mcg/week) specifically for vitamin D doses exceeding 7,000 IU/day or when serum 25(OH)D levels reach or exceed 80 ng/ml. Other studies suggest 100–200 µg/day MK-7 for co-supplementation with vitamin D in at-risk populations to promote vascular health, while certain clinical trials have employed higher doses such as 720 µg/day MK-7 combined with lower vitamin D intakes. Recommended dosages for general supplementation with MK-7 range from 100 to 400 μg per day, considered safe based on recent CRN guidance establishing a highest observed intake of 375 μg/day for supplements and clinical studies showing no toxicity reported even at 1000 μg/day or higher. Studies support 180 μg of MK-7 for and cardiovascular benefits. Due to its shorter half-life, MK-4 requires higher or more frequent doses for similar effects; for therapeutic purposes, such as treatment, doses of 45 mg per day of MK-4 have been approved in , where it is used as an adjunct therapy to improve mineral density (BMD). In the United States, MK-7 has received GRAS status from the FDA for use as a in certain foods, such as products, at levels up to 50 μg per day (GRN 245). Clinically, Vitamin K2 serves as an adjunct for preventing vascular calcification in (CKD) patients, where supplementation has shown promise in slowing progression by activating . It is also applied in managing postmenopausal loss, with s (RCTs) demonstrating improvements in BMD, such as a 2.17% increase in lumbar spine density and reductions in undercarboxylated levels. An ongoing 2025 is exploring the effects of vitamin K2 combined with vitamin D3 on gut and blood levels (NCT07199829). A 2024 demonstrated enhanced BMD gains with vitamin K2 combined with recombinant human in postmenopausal women with . Efficacy in RCTs supports modest BMD increases of 1-3% over 1-2 years, particularly when combined with calcium or exercise, though results vary by dosage and population.

Interactions with Medications

Vitamin K2 interacts significantly with medications, particularly vitamin K antagonists (VKAs) such as , which inhibit (VKOR), an essential for recycling vitamin K2 in the of factors. This inhibition reduces the availability of active vitamin K2, enhancing the effect, but supplementation with vitamin K2 can antagonize this by restoring , potentially leading to decreased international normalized ratio (INR) and increased risk. For patients on , maintaining a stable daily intake of vitamin K2 from supplements or diet is recommended to avoid fluctuations in anticoagulation stability, with dosage adjustments often required upon initiation of supplementation. Beyond VKAs, statins may indirectly lower vitamin K2 levels by inhibiting the , which is involved in menaquinone (vitamin K2) biosynthesis, potentially exacerbating vascular calcification in long-term users. Broad-spectrum antibiotics, such as cephalosporins, can reduce vitamin K2 production by disrupting that synthesize menaquinones, leading to a potential decrease in vitamin K status and increased bleeding risk when combined with anticoagulants. However, caution is advised in patients with hemophilia or other bleeding disorders, where enhanced coagulation from vitamin K2 could complicate management of factor deficiencies, though direct interactions remain understudied. Recent 2025 studies emphasize that direct oral anticoagulants (DOACs) offer advantages over VKAs due to minimal interactions with vitamin K2, reducing the need for dietary monitoring and improving safety in patients with atrial fibrillation or venous thromboembolism, while VKAs continue to require vigilant vitamin K management.

Role in Other Organisms

In Animals and Humans

Vitamin K2 functions through conserved vitamin K-dependent proteins (VKDPs) across mammals, including coagulation factors such as prothrombin and regulatory proteins like (MGP), which inhibit vascular calcification in both humans and other species. These proteins undergo gamma-carboxylation, a facilitated by vitamin K2 forms like menaquinone-4 (MK-4), ensuring similar physiological roles in blood clotting and tissue mineralization. For instance, cows convert dietary vitamin K into MK-4, which is secreted into , providing a bioavailable source for nursing offspring and highlighting mammalian conservation of this pathway. Despite these similarities, sourcing of vitamin K2 differs between humans and herbivores. Herbivores, such as ruminants, benefit from substantial endogenous production by , with absorption enhanced by coprophagy or rumination, reducing reliance on external sources. In contrast, humans depend more on dietary intake from fermented foods and animal products, as hindgut bacterial synthesis yields limited absorbable vitamin K2. Deficiency appears rarer in wild animals, who access diverse natural diets rich in , whereas captive animals like giant anteaters may develop clotting disorders without supplementation. In veterinary practice, vitamin K supplementation (such as , convertible to menaquinone-4) supports bone health in ; elevated dietary levels improve bone quality by enhancing mineralization and reducing breakage susceptibility. Toxicity studies in , including mice and rats, demonstrate high safety margins, with no adverse effects observed even at doses up to 5000 mg/kg body weight for MK-7. Evolutionarily, bacterial symbiosis in the gut has been crucial for provision across animal lineages, enabling VKDP function in nutrient-scarce environments. However, in humans, Western diets—characterized by low fermented food consumption and high processed items—disrupt this , often resulting in suboptimal vitamin K status compared to ancestral patterns.

In Microorganisms

Vitamin K2, also known as menaquinone (MK), is primarily synthesized by and through a dedicated biosynthetic pathway that converts chorismate or isochorismate precursors into the core, followed by to form various MK homologs such as MK-7 or MK-8. This process is crucial for prokaryotic metabolism, particularly in electron transport chains where MK serves as a mobile carrier in the cytoplasmic , facilitating the transfer of electrons during respiration. In anaerobic or microaerophilic conditions, MK shuttles electrons between membrane-bound complexes, enabling energy generation via . A prominent example is , a facultative anaerobe that predominantly produces MK-8 as its primary menaquinone isoform under anaerobic conditions, where it replaces ubiquinone in the respiratory chain to support fumarate reduction and other terminal acceptors. In this bacterium, MK-8 involves enzymes like MenA (1,4-dihydroxy-2-naphthoate polyprenyltransferase) and is tightly regulated to meet respiratory demands, with efforts enhancing MK-8 yields by overexpressing pathway genes and modulating precursor pools. Similarly, in many such as , MK acts as the sole in the system, underscoring its indispensability for aerobic and anaerobic respiration in these organisms. Menaquinones are essential for the majority of and most anaerobic species, comprising the primary or exclusive mediators in their membranes and supporting survival in diverse environments. This quinone-dependent activity is vital for approximately 30% of bacterial taxa that rely on , highlighting MK's ecological role in microbial communities such as soils and sediments. In human contexts, gut microbiota produce MK variants that may contribute an estimated 10-50% of daily vitamin K requirements, though the exact amount absorbed remains uncertain, with species like Lactobacillus and Bifidobacterium generating long-chain forms absorbed in the colon. Fermented foods further link bacterial MK production to human nutrition; for instance, Bacillus subtilis natto in Japanese natto fermentation yields high levels of MK-7 (up to 1,000 μg/100g), enhancing bioavailability through static culture conditions that optimize biosynthesis. Recent 2025 research has advanced probiotic engineering for elevated MK yields, with metabolic rewiring of Lactococcus lactis via pathway overexpression achieving up to 10-fold increases in MK-9 production, paving the way for fortified foods and therapeutics. Additionally, menaquinone biosynthesis pathways are emerging as targets for combating antimicrobial resistance, as inhibitors disrupting MK-dependent electron transport selectively impair Gram-positive pathogens like Staphylococcus aureus without affecting mammalian hosts.

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