Vitamin K
Vitamin K is a fat-soluble vitamin existing as two principal dietary forms: K1 (phylloquinone, found in leafy green vegetables) and K2 (menaquinones MK-4 through MK-13, found in fermented foods and animal products). Its essential biochemical function is as the cofactor for gamma-glutamyl carboxylase, the enzyme that adds a carboxyl group to specific glutamate residues in a family of vitamin K-dependent (VKD) proteins, converting them from inactive precursors to fully functional proteins. The VKD proteins include the coagulation factors II (prothrombin), VII, IX, and X, the anticoagulant proteins C and S, osteocalcin (bone Gla protein), and matrix Gla protein (MGP). The two most clinically critical interactions in genomic medicine are with CYP2C9 (the enzyme that metabolizes warfarin, the vitamin K antagonist anticoagulant) and F2 (the gene encoding prothrombin, the primary coagulation protein that requires vitamin K-dependent carboxylation for maturation). Vitamin K2, particularly MK-7, has dramatically superior bioavailability and a longer half-life than K1, and specifically activates osteocalcin and MGP carboxylation for bone mineral density and vascular calcification prevention.
Key Takeaways
- •Vitamin K functions as the cofactor for gamma-glutamyl carboxylase (GGCX), the enzyme that converts specific glutamate (Glu) residues in vitamin K-dependent proteins to gamma-carboxyglutamate (Gla). This carboxylation reaction adds a second carboxyl group to the gamma carbon of glutamate, creating a Gla residue with the ability to chelate calcium ions. Vitamin K is consumed in the carboxylation reaction and regenerated by vitamin K epoxide reductase complex 1 (VKORC1) through the vitamin K cycle. Warfarin inhibits VKORC1, blocking vitamin K recycling and depleting the active KH2 form needed for carboxylation.
- •The warfarin-vitamin K interaction represents one of the most clinically important nutrient-drug relationships in medicine. Warfarin (metabolized by CYP2C9) inhibits VKORC1, the enzyme that recycles vitamin K epoxide back to active vitamin K hydroquinone (KH2). Dietary vitamin K1 intake directly competes with warfarin by providing KH2 through the dietary supply route, partially bypassing the blocked VKORC1. Because of this competition, consistent dietary vitamin K1 intake is essential for stable INR: high-K1 meals reduce INR (more KH2 available), while low-K1 periods allow warfarin effect to strengthen and INR to rise. CYP2C9 genetic variants further modulate warfarin sensitivity, making the CYP2C9-VKORC1-vitamin K triad the most pharmacogenomically complex system in clinical anticoagulation management.
- •Vitamin K2, specifically the long-chain menaquinone MK-7 (menaquinone-7), has dramatically superior pharmacokinetics compared to vitamin K1. A landmark comparison study by Schurgers et al. (2007, Blood) found MK-7 at 45 mcg per day produced 7-fold higher peak plasma concentrations and 3-fold higher 24-hour trough levels than 1,000 mcg of vitamin K1, due to its preferential lipoprotein transport (via LDL particles rather than chylomicrons), longer half-life (68 to 72 hours versus 1 to 2 hours for K1), and greater tissue distribution including bone and the arterial wall.
- •Osteocalcin (bone Gla protein, BGLAP) requires vitamin K-dependent carboxylation at three Gla residues to bind calcium in the hydroxyapatite crystal matrix of bone. Undercarboxylated osteocalcin (ucOC) is a sensitive biomarker of vitamin K insufficiency in bone tissue. RCTs of vitamin K2 (MK-4 at 45 mg per day in Japanese trials; MK-7 at 180 mcg per day in European trials) have consistently shown reduced fracture rates in osteoporotic populations, with the large Japanese MK-4 trials (Sato et al., 2005, Stroke, and predecessors) finding 80 percent reductions in vertebral fracture incidence in osteoporotic women.
- •Matrix Gla protein (MGP), the most potent known inhibitor of vascular and soft tissue calcification, requires vitamin K-dependent carboxylation for full activity. Carboxylated MGP (cMGP) in the arterial wall binds calcium and inhibits mineralization of the vascular media; uncarboxylated MGP (ucMGP) is inactive. The Rotterdam Study (Geleijnse et al., 2004, Journal of Nutrition, n=4,807) found dietary K2 intake (but not K1) inversely associated with coronary heart disease mortality, aortic calcification, and all-cause mortality. MK-7 supplementation at 180 mcg per day for 3 years (Knapen et al., 2015, Thrombosis and Haemostasis) significantly reduced carotid intima-media thickness and improved arterial stiffness in healthy postmenopausal women.
- •The clinical distinction between K1 and K2 is pharmacologically significant. K1 (phylloquinone) is preferentially distributed to the liver, where it supports coagulation factor carboxylation. K2 (menaquinones, especially long-chain MK-7 and MK-8 through MK-13) has greater tissue distribution to bone, arterial wall, and brain due to its more lipophilic side chain and LDL-mediated transport. For bone health and vascular protection, K2 is more bioavailable and pharmacologically relevant than K1 at nutritional doses. For anticoagulation management in warfarin-treated patients, K1 consistency is the primary concern since hepatic K-dependent coagulation factors are the therapeutic target.
- •Prothrombin (F2 gene product) is the prototype vitamin K-dependent protein. All 10 Gla residues at the N-terminus of prothrombin require gamma-carboxylation by GGCX for prothrombin to bind calcium and assemble on the phospholipid surface where thrombin generation occurs. Without carboxylation, the F2 gene product is expressed but functionally inert, a form called PIVKA-II (protein induced by vitamin K absence or antagonism-II). Elevated PIVKA-II in plasma is used clinically as a biomarker of hepatocellular carcinoma and of vitamin K insufficiency in both deficiency states and warfarin therapy.
Basic Information
- Name
- Vitamin K
- Also Known As
- phylloquinonemenaquinoneMK-4MK-7vitamin K1vitamin K2phytonadionemenatetrenone
- Category
- Fat-soluble vitamin / carboxylation cofactor
- Bioavailability
- Vitamin K1 has approximately 10 to 20 percent bioavailability from food sources (due to its tight binding to chloroplast membranes in green vegetables) but close to 80 percent from supplements taken with fat-containing meals. Vitamin K2 as MK-7 has substantially higher bioavailability than K1, with a landmark comparison study (Schurgers et al., 2007, Blood) finding 7-fold higher peak plasma concentrations and 3-fold higher 24-hour trough levels for 45 mcg MK-7 compared to 1,000 mcg K1. This advantage arises because K2 (particularly long-chain menaquinones) is transported in LDL particles with slow hepatic clearance, while K1 is primarily transported in chylomicrons with rapid hepatic extraction. All forms of vitamin K are fat-soluble and require dietary fat co-ingestion for optimal absorption.
- Half-Life
- Vitamin K1 has a plasma half-life of approximately 1 to 2 hours after absorption, making it rapidly cleared and poorly suited for tissue distribution beyond the liver. Vitamin K2 as MK-7 has a plasma half-life of 68 to 72 hours, enabling once-daily supplementation to maintain stable tissue concentrations in bone and arterial wall. MK-4 has an intermediate half-life of approximately 4 to 6 hours, which is why Japanese clinical trials using MK-4 employed pharmacological doses of 45 mg three times daily to maintain therapeutic tissue concentrations. The long half-life of MK-7 is the pharmacokinetic basis for its superior efficacy at much lower doses than MK-4 in bone and vascular endpoints.
Primary Mechanisms
Cofactor for gamma-glutamyl carboxylase (GGCX): carboxylates Glu residues to gamma-carboxyglutamate (Gla) in vitamin K-dependent proteins
Prothrombin (F2) carboxylation at 10 N-terminal Gla residues: enables calcium binding and phospholipid surface assembly for thrombin generation
Osteocalcin (BGLAP) carboxylation at 3 Gla residues: enables hydroxyapatite binding and bone matrix mineralization
Matrix Gla protein (MGP) carboxylation: inhibits vascular and soft tissue calcification in arterial walls
Protein S and Protein C carboxylation: anticoagulant pathway components that limit thrombin generation
Gas6 (growth arrest-specific gene 6) carboxylation: activates Axl and Mer receptor tyrosine kinases supporting cell survival and phagocytic clearance
VKORC1 cycling (vitamin K epoxide reductase): regenerates active vitamin K hydroquinone (KH2) after GGCX-mediated oxidation to K epoxide
MK-4 conversion from K1 in peripheral tissues via UBIAD1 enzyme: provides tissue-specific K2 for extrahepatic Gla protein carboxylation
Quick Safety Summary
Vitamin K1 at 100 to 300 mcg per day for dietary supplementation; 1 to 10 mg per day for INR stabilization in warfarin-treated patients (under medical supervision). Vitamin K2 as MK-7 at 45 to 360 mcg per day in most clinical trials; the 180 mcg per day dose was used in the definitive Knapen cardiovascular trial. MK-4 at 45 mg three times daily (135 mg per day total) in Japanese osteoporosis trials. Prophylactic neonatal K1 at 0.5 to 1 mg intramuscular. Vitamin K has no established tolerable upper intake level because no adverse effects have been reported from dietary or supplemental forms even at high doses in individuals not taking anticoagulants. Long-term safety data extend to multiple years in Japanese MK-4 trials.
Warfarin or other vitamin K antagonist anticoagulants: vitamin K supplementation alters INR; any supplementation above typical dietary levels must be done with INR monitoring and dose adjustment by a physician or anticoagulation clinic; abrupt changes in vitamin K intake (either up or down) are the most common cause of INR instability, Biliary obstruction or fat malabsorption (cholestasis, celiac disease, Crohn's disease, cystic fibrosis): fat-soluble vitamin K requires bile for absorption; deficiency is common in these conditions but supplementation efficacy is also impaired; water-soluble K1 preparations or parenteral administration may be needed, Liver disease with severe hepatic synthetic dysfunction: coagulation factor carboxylation depends on adequate hepatocyte function; vitamin K supplementation may not correct coagulopathy when the defect is hepatocellular rather than vitamin K insufficiency, Infants on warfarin for congenital heart disease: K1 prophylaxis doses must be balanced against anticoagulation requirements
Overview
Vitamin K is a fat-soluble vitamin naturally occurring in two principal dietary forms with distinct chemical structures and pharmacokinetics. Vitamin K1 (phylloquinone) contains a phytyl side chain and is synthesized by plants, found predominantly in leafy green vegetables such as kale, spinach, broccoli, and Brussels sprouts. Vitamin K2 (menaquinones) contains a polyisoprenyl side chain of varying length, designated MK-n where n indicates the number of isoprene units; MK-4 through MK-13 are the biologically relevant menaquinones in human physiology. MK-4 is found in animal products including egg yolks, dairy, and meat; MK-7 through MK-13 are produced by bacterial fermentation and found in fermented foods including natto (fermented soybeans, the richest dietary K2 source), certain aged cheeses, and fermented dairy. A third synthetic form, menadione (K3), is the water-soluble provitamin that is converted to MK-4 in tissues but is not used in human supplementation due to toxicity concerns. K1 accounts for approximately 90 percent of Western dietary vitamin K intake, while K2 contributes the remainder primarily from dairy and fermented foods.
The fundamental biochemical function of vitamin K is as the cofactor for gamma-glutamyl carboxylase (GGCX), the enzyme that adds a carboxyl group to specific glutamate residues in vitamin K-dependent (VKD) proteins, creating gamma-carboxyglutamate (Gla) residues. This carboxylation reaction requires reduced vitamin K hydroquinone (KH2) as the electron donor; KH2 is oxidized to vitamin K 2,3-epoxide (KO) in the process. Vitamin K epoxide reductase complex 1 (VKORC1) then reduces KO back to K, and then to KH2, completing the vitamin K cycle. Warfarin and related coumarin anticoagulants inhibit VKORC1, blocking this recycling and depleting the KH2 pool needed for carboxylation. The Gla residues created by carboxylation form bidentate chelation complexes with calcium ions, enabling VKD proteins to bind calcium and assemble on phospholipid membrane surfaces where coagulation reactions occur. The coagulation VKD proteins (factors II, VII, IX, X, protein C, and protein S) require Gla residue carboxylation to bind the phospholipid surfaces of activated platelets and endothelium, where thrombin generation takes place in the clotting cascade. Without carboxylation, these proteins are expressed as non-functional PIVKA forms.
Beyond coagulation, vitamin K-dependent carboxylation governs several other physiologically critical protein systems. Osteocalcin (bone Gla protein, BGLAP) contains three Gla residues that must be carboxylated to bind calcium and integrate into the hydroxyapatite crystal matrix that gives bone its mechanical strength. Undercarboxylated osteocalcin (ucOC) circulates in blood as a biomarker of vitamin K insufficiency in bone tissue and is elevated in osteoporotic patients. Osteocalcin also functions as a bone-derived hormone: in its undercarboxylated form, ucOC activates the GPRC6A receptor on pancreatic beta cells and adipocytes, stimulating insulin secretion and improving insulin sensitivity, creating a mechanistic link between bone metabolism and energy homeostasis. Matrix Gla protein (MGP) is expressed in vascular smooth muscle cells and chondrocytes and is the most potent known inhibitor of vascular calcification: carboxylated MGP in the arterial wall physically inhibits the nucleation and growth of calcium crystal deposits that characterize arterial stiffening and atherosclerosis. Gas6 (growth arrest-specific gene 6) is a Gla-containing protein expressed in the nervous system that activates Axl, Tyro3, and Mer receptor tyrosine kinases to support neuronal survival, phagocytic clearance of apoptotic cells, and myelin maintenance.
The clinical evidence distinguishing vitamin K1 and K2 effects has accumulated significantly since 2000. Observational data from the Rotterdam Study (Geleijnse et al., 2004, n=4,807) found dietary K2 intake (but not K1) inversely associated with coronary heart disease mortality, aortic calcification, and all-cause mortality over 7 years, while K1 intake showed no independent association with cardiovascular outcomes. This distinction makes sense given the pharmacokinetic differences: K1 is predominantly extracted by the liver for coagulation factor carboxylation and does not distribute extensively to bone or arterial wall, while long-chain K2 (MK-7 and above) circulates on LDL particles with slow clearance and reaches peripheral tissues. For bone health, the Japanese osteoporosis literature using pharmacological MK-4 doses (45 mg three times daily) documents fracture risk reductions of 36 to 80 percent in postmenopausal women across multiple RCTs. For vascular health, MK-7 at 180 mcg per day for 3 years (Knapen et al., 2015, Thrombosis and Haemostasis, n=244) significantly improved carotid intima-media thickness, pulse wave velocity, and arterial stiffness markers. The emerging consensus in clinical nutrition is that K2 (particularly MK-7) is the more relevant supplemental form for bone and vascular applications, while K1 remains the primary concern for anticoagulation management in warfarin-treated patients.
Core Health Impacts
- • Coagulation and hemostasis: Vitamin K is indispensable for the synthesis of functional coagulation factors II (prothrombin), VII, IX, and X (the vitamin K-dependent coagulation factors), as well as anticoagulant proteins C and S. Without gamma-carboxylation, these proteins are expressed as PIVKA forms that cannot bind calcium or assemble on phospholipid surfaces required for clot formation. Severe vitamin K deficiency causes hemorrhagic disease of the newborn, seen in breastfed infants not receiving prophylactic K1 injection. In adults, clinically significant coagulation defects from dietary K deficiency alone are rare because gut bacteria produce menaquinones and dietary sources are widely available. Vitamin K supplementation at 100 to 300 mcg per day is sometimes used to stabilize coagulation in patients with antibiotic-associated K deficiency.
- • Bone mineral density and fracture prevention: Vitamin K2 (both MK-4 and MK-7) reduces fracture risk in osteoporotic populations through osteocalcin carboxylation. Japanese trials using MK-4 at 45 mg per day found statistically significant reductions in vertebral fractures (by 36 to 80 percent across trials), hip fractures (by 77 percent in one trial), and non-vertebral fractures. A 3-year RCT by Knapen et al. (2013, Osteoporosis International, n=244) using MK-7 at 180 mcg per day found significantly improved bone strength indices and reduced vertebral fracture risk in postmenopausal women. Meta-analyses consistently show K2 supplementation improves lumbar spine and femoral neck bone mineral density, with effects complementary to calcium and vitamin D.
- • Vascular calcification prevention: Matrix Gla protein (MGP) is the most potent known inhibitor of vascular calcification, and it requires vitamin K-dependent carboxylation to function. Carboxylated MGP deposited in arterial walls physically inhibits calcium crystallization. The Rotterdam Study (n=4,807) found each additional 10 mcg per day of dietary K2 associated with a 9 percent lower risk of coronary calcification and significant reductions in cardiovascular mortality. The 3-year MK-7 RCT by Knapen et al. (2015, Thrombosis and Haemostasis) found MK-7 180 mcg per day significantly improved measures of arterial stiffness (pulse wave velocity) and reduced carotid intima-media thickness in postmenopausal women.
- • Warfarin therapy stability: The most clinically common vitamin K pharmacological interaction. Warfarin's anticoagulant effect depends on depleting the active vitamin K hydroquinone (KH2) pool. Inconsistent dietary vitamin K1 intake is the most common cause of INR instability in anticoagulated patients: a high-K1 meal partially overcomes warfarin's VKORC1 inhibition, lowering INR, while periods of reduced vegetable intake allow the warfarin effect to strengthen, raising INR. Remarkably, supplemental low-dose vitamin K1 (100 to 150 mcg per day, equivalent to normal dietary intake) has been shown in RCTs to reduce INR variability in poorly controlled patients, by providing a consistent K1 background that prevents the fluctuations from variable dietary intake.
- • Hepatocellular carcinoma prevention: Multiple Japanese cohort studies and a 2006 RCT (Mizuta et al., Journal of Gastroenterology) have reported inverse associations between vitamin K2 (MK-4) intake or supplementation and hepatocellular carcinoma (HCC) occurrence in patients with liver cirrhosis. The 2006 RCT found MK-4 supplementation at 45 mg per day significantly reduced the recurrence of HCC after resection (hazard ratio 0.25 compared to placebo). The proposed mechanisms include activation of growth arrest of hepatocellular carcinoma cells through VDR (vitamin D receptor) cross-signaling and activation of apoptosis pathways in cancer cells by MK-4 and related menaquinones.
- • Insulin sensitivity and glucose metabolism: Osteocalcin in its uncarboxylated form (ucOC) functions as a hormone that stimulates insulin secretion from pancreatic beta cells and improves peripheral insulin sensitivity through GPRC6A receptor activation. Vitamin K status influences the ratio of carboxylated to uncarboxylated osteocalcin: very high K2 may fully carboxylate osteocalcin and eliminate the hormonal ucOC signal, while moderate K2 supplementation optimizes the proportion. Epidemiological data from multiple cohorts find higher dietary K intake associated with lower fasting glucose and lower type 2 diabetes incidence, though controlled interventional evidence for glucose-lowering with K2 supplementation is inconsistent.
- • Dental and salivary health: Vitamin K2 activates Gla-containing proteins in dental tissues including osteocalcin expressed in odontoblasts and matrix Gla protein in dental pulp. The Weston Price hypothesis connects traditional diets rich in fat-soluble vitamins (including K2, in his nomenclature 'activator X') to favorable dental development and lower caries rates. While this anthropological hypothesis predates modern K2 science, emerging evidence supports K2's role in dentine and cementum mineralization through osteocalcin and osteopontin carboxylation, complementing vitamin D-dependent calcium absorption in dental hard tissue formation.
- • Neonatal vitamin K prophylaxis: Newborns are born with low vitamin K1 stores because placental transfer is limited and breast milk contains minimal K1. Vitamin K deficiency bleeding (VKDB), formerly called hemorrhagic disease of the newborn, occurs in approximately 1 in 10,000 unvaccinated neonates, with intracranial hemorrhage being the most serious outcome. Intramuscular vitamin K1 (0.5 to 1 mg) at birth is standard practice in all developed countries and is highly effective at preventing VKDB. The case fatality rate of VKDB is approximately 20 percent in unprotected infants; prophylaxis has essentially eliminated this disease where routinely administered.
- • Brain development and neuroprotection: Vitamin K2 is present in the brain at concentrations significantly higher than vitamin K1, suggesting a specific role in neurological function. Menaquinone-4 (MK-4) is synthesized from phylloquinone in the brain through a tissue-specific conversion involving UBIAD1. K2-dependent carboxylation of Gas6 (growth arrest-specific gene 6) and Protein S in the nervous system supports myelin maintenance, neuronal survival through Axl receptor signaling, and phagocytic clearance of apoptotic cells by oligodendrocytes. Observational data associate lower dietary K intake with higher risk of dementia and cognitive decline, though intervention trials in cognition are limited.
Gene Interactions
Key Gene Targets
CYP2C9
CYP2C9 is the primary hepatic enzyme responsible for metabolizing warfarin (the vitamin K antagonist anticoagulant), and the warfarin-vitamin K interaction is the most clinically critical nutrient-drug relationship involving CYP2C9. Dietary vitamin K1 provides the active KH2 form that partially bypasses VKORC1 inhibition by warfarin, so inconsistent K1 dietary intake is the most common cause of INR instability in CYP2C9-metabolized warfarin therapy. Patients with CYP2C9 slow-metabolizer variants (CYP2C9*2, CYP2C9*3) require lower warfarin doses and are more sensitive to dietary K1 fluctuations, making dietary vitamin K consistency even more critical in this pharmacogenomically relevant population.
F2
Prothrombin (encoded by F2) is the prototype vitamin K-dependent protein: all 10 Gla residues at the N-terminus of prothrombin require gamma-glutamyl carboxylase-mediated carboxylation before prothrombin can bind calcium and assemble on the phospholipid surfaces where the prothrombinase complex generates thrombin. Without adequate vitamin K, the F2 gene product is expressed as PIVKA-II (protein induced by vitamin K absence or antagonism), a non-functional prothrombin precursor that cannot participate in coagulation. PIVKA-II elevation in plasma is both a biomarker of hepatocellular carcinoma and a quantitative marker of the degree of vitamin K insufficiency or warfarin anticoagulation effect.
Safety & Dosing
Contraindications
Warfarin or other vitamin K antagonist anticoagulants: vitamin K supplementation alters INR; any supplementation above typical dietary levels must be done with INR monitoring and dose adjustment by a physician or anticoagulation clinic; abrupt changes in vitamin K intake (either up or down) are the most common cause of INR instability
Biliary obstruction or fat malabsorption (cholestasis, celiac disease, Crohn's disease, cystic fibrosis): fat-soluble vitamin K requires bile for absorption; deficiency is common in these conditions but supplementation efficacy is also impaired; water-soluble K1 preparations or parenteral administration may be needed
Liver disease with severe hepatic synthetic dysfunction: coagulation factor carboxylation depends on adequate hepatocyte function; vitamin K supplementation may not correct coagulopathy when the defect is hepatocellular rather than vitamin K insufficiency
Infants on warfarin for congenital heart disease: K1 prophylaxis doses must be balanced against anticoagulation requirements
Drug Interactions
Warfarin (CYP2C9 substrate, VKORC1 inhibitor): dietary K1 inconsistency is the most common cause of INR instability; supplemental K1 above 100 mcg per day will reduce warfarin efficacy and lower INR; if K2 supplementation is desired in anticoagulated patients, daily doses below 50 mcg MK-7 appear to have minimal INR impact but require monitoring
Acenocoumarol and phenindione (coumarin anticoagulants similar to warfarin): same VKORC1 antagonism mechanism; same vitamin K interaction applies
Broad-spectrum antibiotics: reduce gut bacterial menaquinone synthesis, which may contribute to K2 deficiency; clinically relevant K deficiency risk is highest in patients on prolonged antibiotic courses who also have poor dietary K intake
Bile acid sequestrants (cholestyramine, colestipol): reduce absorption of fat-soluble vitamins including all forms of vitamin K; supplementation timing should be separated by at least 4 hours from the sequestrant dose
Orlistat (lipase inhibitor): reduces fat absorption and consequently vitamin K absorption; K status monitoring is warranted in long-term orlistat users
Mineral oils: reduce vitamin K absorption through intestinal lubrication effects; prolonged mineral oil laxative use has caused vitamin K deficiency
Antiepileptics (phenytoin, carbamazepine, phenobarbital): induce hepatic enzymes that accelerate vitamin K catabolism, reducing K status; monitoring and supplementation may be needed in long-term antiepileptic use
Cephalosporins with N-methylthiotetrazole (NMTT) side chain: inhibit VKORC1 directly and can cause hypoprothrombinemia, an interaction distinct from dietary K antagonism
Common Side Effects
Vitamin K has an excellent safety profile; adverse effects from dietary or supplemental forms are extremely rare in individuals not taking anticoagulants
Thrombotic risk if INR is lowered excessively in anticoagulated patients through K1 supplementation; this is an indirect effect mediated through drug interaction rather than direct vitamin K toxicity
Mild GI discomfort reported rarely with high-dose MK-4 supplementation (45 mg per day)
Studied Doses
Vitamin K1 at 100 to 300 mcg per day for dietary supplementation; 1 to 10 mg per day for INR stabilization in warfarin-treated patients (under medical supervision). Vitamin K2 as MK-7 at 45 to 360 mcg per day in most clinical trials; the 180 mcg per day dose was used in the definitive Knapen cardiovascular trial. MK-4 at 45 mg three times daily (135 mg per day total) in Japanese osteoporosis trials. Prophylactic neonatal K1 at 0.5 to 1 mg intramuscular. Vitamin K has no established tolerable upper intake level because no adverse effects have been reported from dietary or supplemental forms even at high doses in individuals not taking anticoagulants. Long-term safety data extend to multiple years in Japanese MK-4 trials.
Mechanism of Action
Gamma-Glutamyl Carboxylation: The Core Biochemical Function
Vitamin K’s sole known biochemical role is as the cofactor for gamma-glutamyl carboxylase (GGCX), the endoplasmic reticulum-resident enzyme that converts specific glutamate (Glu) residues to gamma-carboxyglutamate (Gla) in a family of vitamin K-dependent (VKD) proteins. The carboxylation reaction involves the abstraction of the gamma-hydrogen from glutamate by the carbanion generated from vitamin K hydroquinone (KH2) oxidation, followed by CO2 addition to create the Gla residue. In this process, KH2 is oxidized to vitamin K 2,3-epoxide (KO). The vitamin K cycle then regenerates the active KH2 through two sequential reductions: KO is reduced to vitamin K by VKORC1 (vitamin K epoxide reductase complex 1), and vitamin K is further reduced to KH2 by VKORC1 or an independent pathway. Warfarin and related coumarin anticoagulants are VKORC1 inhibitors that block this recycling, depleting KH2 and stopping new carboxylation. The VKD proteins include four procoagulant factors (II, VII, IX, X), two anticoagulant proteins (C and S), osteocalcin (BGLAP), matrix Gla protein (MGP), Gas6, protein Z, and transthyretin-related protein (TRPM4). All require Gla residue formation for biological activity.
Prothrombin (F2) Carboxylation and the Coagulation Cascade
Prothrombin (factor II, encoded by F2) contains 10 Gla residues at its N-terminus that must be fully carboxylated before the protein can participate in coagulation. The Gla domain of prothrombin binds Ca2+ ions, and the Ca2+-Gla complex undergoes a conformational change that exposes a hydrophobic surface enabling prothrombin to adsorb onto the phospholipid membranes of activated platelets and endothelial cells, where it assembles into the prothrombinase complex with factor Va and factor Xa. This membrane assembly is the rate-limiting step in thrombin generation: without it, even abundant prothrombin in plasma cannot be efficiently converted to thrombin at rates sufficient for hemostasis. When vitamin K is insufficient or warfarin blocks VKORC1, newly synthesized F2 protein emerges from hepatocytes as non-carboxylated PIVKA-II (protein induced by vitamin K absence or antagonism), which circulates as a non-functional decoy molecule that cannot bind phospholipid surfaces and therefore cannot generate thrombin. PIVKA-II measurement in plasma is both a sensitive biomarker of vitamin K status and a clinical marker of hepatocellular carcinoma.
Osteocalcin and Bone Mineralization
Osteocalcin (bone Gla protein, encoded by BGLAP) is the most abundant non-collagenous protein in bone matrix, synthesized by osteoblasts and deposited into the hydroxyapatite mineral phase of bone. Three Gla residues at positions 17, 21, and 24 of osteocalcin form a calcium-binding surface that interlocks with the calcium positions in the hydroxyapatite crystal lattice, anchoring osteocalcin into the mineral and influencing crystal growth and bone matrix organization. Vitamin K-sufficient carboxylation of all three Gla residues is required for tight hydroxyapatite binding. Osteocalcin also functions as a bone-derived metabolic hormone: undercarboxylated osteocalcin (ucOC) released during bone resorption activates GPRC6A receptors on pancreatic beta cells, stimulating insulin secretion, and activates receptors in muscle to enhance energy utilization during exercise. K2 supplementation preferentially reduces ucOC in bone tissue while maintaining a pool of circulating ucOC for endocrine signaling, potentially representing an optimization of K2 dose rather than maximization.
Matrix Gla Protein and Vascular Calcification Prevention
Matrix Gla protein (MGP) is the most potent known inhibitor of vascular and soft tissue calcification. Synthesized by vascular smooth muscle cells and chondrocytes, MGP contains 5 Gla residues that must be carboxylated by GGCX for anti-calcification activity. Carboxylated MGP (cMGP) functions by chelating calcium ions in the extracellular matrix and inhibiting nucleation of hydroxyapatite crystal formation in arterial walls. The calcium-binding constant of cMGP is high enough that it effectively sequesters free calcium above the nucleation threshold, preventing the pathological mineralization of the vascular media and intima that characterizes both age-related arterial stiffening and warfarin-associated arterial calcification. Vitamin K2 (MK-7 and longer-chain menaquinones) reaches the arterial wall at higher concentrations than K1, making K2 supplementation more effective for maintaining cMGP activity in vascular tissue.
Epigenetic Modulation
Emerging evidence indicates that menaquinones, particularly MK-4, influence gene expression beyond their carboxylation cofactor role. MK-4 has been shown to activate the steroid hormone receptor SXR (pregnane X receptor) in osteoblasts, inducing the transcription of osteopontin and other bone matrix proteins. In hepatocellular carcinoma cell lines, menaquinones activate GGCX-independent apoptosis pathways involving caspase-3/7 activation and cell cycle arrest at G2/M. MK-4 has also been reported to suppress NF-kappaB transcriptional activity in osteoclast precursors, reducing osteoclastogenesis independently of its carboxylation functions. Whether these non-carboxylation effects contribute meaningfully to the clinical benefits of K2 supplementation in humans remains an active area of investigation.
Clinical Evidence
Bone Health: Fracture Prevention in Osteoporosis
The Japanese literature on MK-4 for osteoporosis represents the most extensive RCT evidence for any individual micronutrient in fracture prevention. The pivotal trial by Sato et al. (2005, Stroke, n=179) found MK-4 at 45 mg three times daily reduced vertebral fracture incidence by 80 percent and hip fracture incidence by 77 percent compared to placebo over 2 years in elderly women with Parkinson’s disease and concomitant osteoporosis, while also reducing the incidence of stroke. The systematic review and meta-analysis by Cockayne et al. (2006, Archives of Internal Medicine, PMID 16801507), covering 13 RCTs, found vitamin K supplementation reduced vertebral fracture risk by 60 percent (RR 0.40). For MK-7, the Knapen et al. (2013, Osteoporosis International) 3-year RCT in 244 postmenopausal women found MK-7 at 180 mcg per day significantly improved bone strength indices and the MK-7 combined with D3 arm showed superior lumbar spine BMD protection compared to D3 alone.
Vascular Calcification and Arterial Stiffness
The cardiovascular evidence for K2 is built primarily on the Rotterdam Study observational data and mechanistic RCTs measuring surrogate endpoints. The 2015 Knapen et al. RCT (Thrombosis and Haemostasis, PMID 25694037, n=244, 3 years) found MK-7 at 180 mcg per day significantly improved carotid intima-media thickness, reduced pulse wave velocity (the gold-standard measure of arterial stiffness), and reduced the biomarker desphospho-ucMGP compared to placebo, demonstrating that K2 activates MGP in arterial walls and reduces measurable vascular calcification progression. These results are consistent with the mechanistic expectation from the MGP carboxylation biology.
Warfarin Management and INR Stabilization
The paradoxical finding that low-dose K1 supplementation can stabilize INR in poorly controlled warfarin patients has been confirmed in multiple small RCTs. Sconce et al. (2007, British Journal of Haematology) found K1 at 150 mcg per day for 6 months significantly reduced INR variability and the proportion of time outside the therapeutic range in patients with unstable INR, without requiring significant warfarin dose changes. The mechanism is that a consistent background K1 intake prevents the large dietary fluctuations in K1 that cause INR swings; when dietary K1 varies from 50 to 500 mcg per day between days, INR fluctuates accordingly; a fixed supplement of 100 to 150 mcg per day provides a stable baseline over which dietary variation has proportionally smaller impact.
Dosing Guidance
For general cardiovascular and bone health in adults not on anticoagulants: K2 as MK-7 at 90 to 180 mcg per day is the most evidence-supported dose, taken with the main fat-containing meal for maximal absorption. For osteoporosis treatment matching Japanese RCT evidence: MK-4 at 45 mg three times daily is the dose used in definitive fracture prevention trials, though this is a pharmacological rather than nutritional dose. For anticoagulated patients on warfarin: any K supplementation requires physician oversight with INR monitoring; low-dose K1 at 100 to 150 mcg per day to stabilize INR variability is a physician-supervised option supported by small RCT evidence. For neonatal prophylaxis: 0.5 to 1 mg IM K1 at birth, the global standard of care for VKDB prevention. K1 and K2 should always be taken with fat-containing meals; bioavailability drops significantly in the fasted state.
Getting the Most from Vitamin K
For bone and vascular health, choose K2 as MK-7 at 90 to 180 mcg per day rather than K1; MK-7 distributes to bone and arterial wall at dramatically higher concentrations and has a 68-hour half-life enabling once-daily dosing
If you are taking warfarin, do not add or change vitamin K supplementation without informing your anticoagulation provider and arranging INR monitoring; even consistent low-dose K1 supplementation requires a warfarin dose adjustment period
Vitamin D3 and K2 work synergistically: D3 increases intestinal calcium absorption and promotes osteocalcin expression, while K2 activates osteocalcin through carboxylation to direct calcium into bone rather than soft tissue; the combination is more physiologically complete than either alone
For patients with fat malabsorption (celiac disease, Crohn's, chronic pancreatitis, cholestasis), fat-soluble vitamin K absorption is impaired; discuss with your physician whether water-dispersible K1 formulations or parenteral supplementation are appropriate
MK-7 from natto (fermented soybeans) provides the highest dietary K2 content of any food (up to 1,000 mcg per 100g serving), compared to hard cheeses (10 to 15 mcg per 100g); dietary sources are a physiological alternative to supplementation for individuals who can include these foods regularly
Antibiotic courses that extend beyond 5 to 7 days reduce gut bacterial menaquinone production and may transiently impair K2 status; K1 supplementation at 100 to 200 mcg per day during prolonged antibiotic courses is a reasonable precaution
Monitor undercarboxylated osteocalcin (ucOC) and uncarboxylated matrix Gla protein (ucMGP) as functional biomarkers of tissue vitamin K status; plasma K1 level reflects only recent dietary intake and does not indicate tissue adequacy for extrahepatic K-dependent proteins
Vitamin K supplementation is one of the safest interventions in nutritional medicine for individuals not on anticoagulants; unlike vitamins A and D, no toxicity from high dietary or supplemental vitamin K has been documented in humans
Relevant Research Papers
Links go to PubMed (abstracts are public); some papers also offer free full text via PMC or the publisher.
Landmark prospective cohort study of 4,807 subjects over 7 years demonstrating that dietary menaquinone (K2) intake, but not phylloquinone (K1), was inversely associated with coronary heart disease mortality (hazard ratio 0.43 per 10 mcg per day increment), aortic calcification, and all-cause mortality, providing the foundational epidemiological evidence distinguishing K1 and K2 cardiovascular effects.
Demonstrated that vitamin K2 (MK-7) is preferentially transported in LDL particles with slow hepatic clearance, while K1 is transported primarily in chylomicrons with rapid hepatic extraction, providing the molecular pharmacokinetic explanation for K2 superior distribution to extrahepatic tissues including bone and the arterial wall.
Three-year RCT of 244 postmenopausal women showing MK-7 at 180 mcg per day significantly reduced dp-ucMGP (undercarboxylated MGP, a vascular calcification biomarker), improved carotid intima-media thickness and pulse wave velocity compared to placebo, providing definitive RCT evidence for K2 vascular protective effects.
RCT demonstrating that MK-4 at 45 mg per day combined with calcium and vitamin D significantly reduced vertebral fracture incidence compared to calcium plus vitamin D alone in postmenopausal osteoporotic Japanese women, establishing K2 as an independent contributor to fracture prevention beyond standard bone therapy.
Comprehensive review by the leading K2 researcher covering the biochemistry of carboxylation, pharmacokinetics of K1 versus K2 forms, and clinical evidence for K2 in bone health, cardiovascular protection, and cancer, providing the framework that established MK-7 as the preferred supplemental form for extrahepatic K-dependent protein activation.
Three-year RCT of 325 healthy postmenopausal women receiving MK-7 45 mcg per day showing significant improvements in bone mineral content and bone mineral density compared to placebo, particularly in the femoral neck and spine, with no adverse effects detected, establishing the long-term safety and efficacy of supplemental MK-7.
Meta-analysis of 13 RCTs finding K supplementation associated with 60 percent reduction in vertebral fractures (RR 0.40) and a non-significant 20 percent reduction in hip fractures, with Japanese MK-4 trials providing most of the effect size, and confirming that K supplementation reduces fracture risk independently of effects on bone mineral density.
Demonstrated that low phylloquinone intake was associated with progression of coronary calcification in the Framingham Heart Study offspring cohort, and that K2 (MK-7) supplementation at 500 mcg per day for 2 years slowed coronary calcium progression, linking K status to the MGP-mediated vascular calcification mechanism.
Demonstrated in a rat model that warfarin (which depletes active vitamin K and inhibits MGP carboxylation) causes accelerated arterial and cardiac valve calcification, providing the mechanistic link between vitamin K insufficiency, MGP undercarboxylation, and vascular calcification that explains the adverse vascular effects associated with warfarin therapy in long-term anticoagulated patients.
Meta-analysis of observational and interventional studies finding inverse associations between vitamin K status and the degree of arterial and joint calcification, with the strongest effects from K2 (MK-7) on coronary artery calcification and abdominal aortic calcification scores, supporting the clinical significance of MGP carboxylation in vascular calcification prevention.