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The complete evidence-based guide to Vitamin K — blood clotting, bone health, K1 vs K2 explained, deficiency symptoms, daily dosage, best food sources, and critical warfarin interactions.
Short, evidence-based answers to the most common Vitamin K questions.
Vitamin K activates proteins required for blood clotting (coagulation cascade) and calcium regulation. Without it, clotting factors II, VII, IX, and X cannot function, leading to uncontrolled bleeding. It also activates osteocalcin (bone protein) and MGP (matrix Gla protein) — a calcification inhibitor critical for arterial health.
K1 (phylloquinone) is found in leafy green vegetables and is primarily used by the liver for clotting factor activation. K2 (menaquinone, MK-4 to MK-13) is found in fermented foods and animal products and is preferentially used by bones and blood vessels. K2 has a longer half-life and may be more effective for bone and cardiovascular health.
The Adequate Intake (AI) is 120 µg/day for adult men and 90 µg/day for adult women. Note that no RDA has been established — only an AI — because there is insufficient data to define a precise requirement. Importantly, no upper limit has been set because no adverse effects from food or supplement sources have been observed.
K1 is richest in dark leafy greens: kale (531 µg/half cup), spinach (444 µg/half cup), Swiss chard, and broccoli. K2 is found in natto (a fermented soy product — the richest source at ~1,000 µg per 100g), gouda cheese, hard cheese, butter, egg yolks, chicken liver, and fermented dairy.
Yes — critically. Warfarin (Coumadin) works by blocking Vitamin K recycling, reducing clotting factor activity. Changes in dietary Vitamin K intake can destabilise warfarin therapy and alter INR. People on warfarin should maintain consistent (not necessarily low) Vitamin K intake and discuss any supplementation with their prescribing physician.
Yes — Vitamin K2 in particular activates osteocalcin, which binds calcium into bone matrix. Low K2 status is associated with higher fracture risk. Some trials show K2 supplementation improves bone mineral density and reduces fractures, particularly in postmenopausal women. The effect of K1 on bone is less clear.
Vitamin K is a group of fat-soluble vitamins essential for carboxylation — a chemical modification that activates certain proteins by adding a carboxyl group to glutamic acid residues (gamma-carboxylation). This reaction is dependent on Vitamin K as a cofactor and enables target proteins to bind calcium, activating them.
There are two main natural forms: Vitamin K1 (phylloquinone), synthesised by plants and found in green vegetables, and Vitamin K2 (menaquinones, MK-4 to MK-13), produced by bacteria and found in fermented foods and animal products. K2 subtypes vary in their side chain length — MK-7 has the longest half-life and greatest tissue distribution.
Seventeen human proteins require Vitamin K-dependent gamma-carboxylation to function: four clotting factors (II, VII, IX, X) and two anticoagulant proteins (C and S) in blood, osteocalcin and MGP in bone and vessels, and growth arrest-specific protein 6 (Gas6) in cell survival signalling. This gives Vitamin K a broader role than simply 'clotting'.
Vitamin K (hydroquinone form) donates electrons to activate glutamic acid → gamma-carboxyglutamic acid (Gla)
Activated Gla proteins bind calcium: clotting factors activate in blood; osteocalcin binds calcium in bone
Vitamin K is oxidised to epoxide → recycled back by VKOR enzyme (vitamin K epoxide reductase)
Warfarin inhibits VKOR → depletes active Vitamin K → reduces clotting factor activity
Requires dietary fat for absorption. Stored primarily in the liver (K1) and extra-hepatic tissues (K2). Body stores are limited — deficiency can develop within weeks.
K1 preferentially goes to the liver (clotting). K2 preferentially distributes to bones, arteries, and kidneys. Both activate the same types of proteins but in different tissues.
Warfarin blocks the Vitamin K recycling enzyme (VKOR). Consistent dietary K intake is critical for stable anticoagulation therapy.
Vitamin K's role as a cofactor for gamma-carboxylation gives it critical functions in blood coagulation, bone mineralisation, and arterial calcification prevention.
Vitamin K activates four clotting factors (II, VII, IX, X) and two anticoagulant proteins (C and S) in the coagulation cascade. Without adequate Vitamin K, even minor injuries can result in prolonged and excessive bleeding. Newborns receive Vitamin K injections at birth precisely because placental transfer is limited.
Osteocalcin — the most abundant non-collagen protein in bone — requires Vitamin K2 carboxylation to bind calcium and incorporate it into bone matrix. Undercarboxylated osteocalcin indicates functional K2 deficiency even when K1 levels appear normal. K2 supplementation (MK-7, 90–180 µg/day) has been shown to improve bone mineral density and reduce fracture rates.
Matrix Gla protein (MGP) — the body's most potent inhibitor of vascular calcification — requires Vitamin K2 to function. Without activation, MGP cannot prevent calcium from depositing in arterial walls. Population studies consistently link higher dietary K2 intake with lower rates of coronary artery calcification, arterial stiffness, and cardiovascular mortality.
MGP is also expressed in kidney tubules and helps prevent calcium deposits. Low Vitamin K status is associated with increased kidney stone risk. Adequate K2 may help regulate calcium metabolism in the kidneys, reducing urinary calcium deposition.
Gas6 (growth arrest-specific protein 6) — a K-dependent protein — plays roles in myelin formation, neuronal survival, and anti-inflammatory signalling in the brain. Emerging research suggests Vitamin K may influence cognitive ageing. Elderly people with low K status show accelerated cognitive decline in some observational studies.
Osteocalcin in its carboxylated form may stimulate insulin secretion from the pancreas and improve insulin sensitivity in muscle and adipose tissue. Some intervention trials show K supplementation improves insulin resistance markers, suggesting a role beyond skeletal health.
Classic Vitamin K deficiency is relatively rare in healthy adults eating varied diets — but subclinical K2 deficiency (affecting bone and arterial health) is widespread. The two forms have distinct deficiency profiles.
The most recognisable early symptom. Inadequate clotting factor activation means even minor capillary trauma causes visible bruising. May be the only symptom in mild deficiency.
Cuts, dental procedures, and minor injuries take significantly longer to stop bleeding. In severe deficiency, even superficial wounds bleed excessively and menstrual periods may become unusually heavy.
Dark urine or tarry/bloody stools indicate internal bleeding from the urinary or gastrointestinal tract. These are signs of significant deficiency requiring urgent medical attention.
Subclinical K2 deficiency — normal K1 but inadequate K2 — allows MGP to remain undercarboxylated. The result is unregulated calcium deposition in arterial walls, a risk factor for cardiovascular disease that develops silently over years.
Low K2 impairs osteocalcin carboxylation, reducing bone mineralisation efficiency. This manifests as reduced bone mineral density and elevated fracture risk — clinically indistinguishable from other causes of osteoporosis without specific testing.
Newborns are born with very low Vitamin K stores — placental transfer is minimal and breast milk is a poor source. Without prophylactic K injection at birth, vitamin K deficiency bleeding (VKDB) can cause severe intracranial and gastrointestinal haemorrhage in the first weeks of life.
Marginal Vitamin K status — often undetected before surgery — becomes clinically significant during and after operations requiring normal clotting function. Surgeons routinely check coagulation status before elective procedures.
The most sensitive functional marker of K2 deficiency — elevated ucOC (undercarboxylated osteocalcin) indicates inadequate K2 for full osteocalcin activation, even when serum K1 levels are normal. This is often the earliest detectable functional deficit.
Prolonged prothrombin time confirms insufficient clotting factor activation. Classic laboratory sign of clinically significant K deficiency.
Undercarboxylated osteocalcin >20% indicates inadequate K2 for bone protein activation. May coexist with normal serum K1 and normal clotting.
Dephospho-uncarboxylated MGP (dp-ucMGP) is the most sensitive marker of K2 status for vascular health. High levels predict arterial calcification risk.
Sufficient for clotting factor activation. Does not necessarily indicate adequate K2 for bone and vascular function.
Both osteocalcin and MGP fully carboxylated. Associated with best bone density and lowest arterial calcification scores.
Classic K1 deficiency (bleeding) is rare in healthy adults but common in specific at-risk groups. K2 insufficiency (bone/vascular) is far more widespread and largely unrecognised.
As a fat-soluble vitamin, K absorption requires bile and dietary fat. Crohn's disease, celiac disease, cystic fibrosis, short bowel syndrome, and chronic pancreatitis all impair fat-soluble vitamin absorption and can cause clinical K deficiency.
Broad-spectrum antibiotics reduce the gut bacteria that synthesise some Vitamin K2 (particularly MK-7). Prolonged antibiotic courses — especially in hospitalised patients with poor dietary intake — are a significant cause of deficiency.
Warfarin directly blocks Vitamin K recycling (VKOR inhibition), intentionally reducing clotting activity. Cholestyramine, orlistat, and some anticonvulsants also interfere with fat-soluble vitamin absorption and can deplete K stores.
Diets very low in both leafy green vegetables (K1) and fermented/animal foods (K2) provide insufficient Vitamin K. This is particularly common in elderly people with restricted diets, or those on low-fat or restrictive eating patterns.
The liver is the primary site of Vitamin K storage and clotting factor synthesis. Liver disease impairs both — resulting in coagulopathy even with adequate dietary intake. Vitamin K supplementation may not fully correct bleeding tendency in severe hepatic dysfunction.
Neonates are uniquely vulnerable: minimal placental K transfer, sterile gut at birth (no K2-producing bacteria), and low K content of breast milk combine to create a period of severe deficiency unless corrected by prophylactic injection.
No RDA has been established — only an Adequate Intake (AI). No upper limit is set due to absence of toxicity. K1 and K2 intakes are often tracked separately as they serve different physiological roles.
K2 research suggests higher targets: The AI is based on K1 and clotting function only. For optimal bone mineralisation and arterial calcification prevention, research suggests 100–200 µg/day of K2 (MK-7) — far above typical Western dietary intake. This gap is why K2 supplementation is increasingly recommended for bone and cardiovascular health.
Select foods you regularly eat to estimate your K1 and K2 intake separately — and see whether you're getting enough of each form for clotting, bone, and vascular health.
⚠️ On warfarin? Vitamin K significantly affects anticoagulation. Keep intake consistent and discuss changes with your doctor before adjusting diet or taking supplements.
Select foods above to see your Vitamin K intake.
K1 and K2 come from completely different food categories. K1 dominates in green vegetables; K2 is found in fermented foods, aged cheeses, and animal products — foods many people consume infrequently.
Values in µg per serving. K1 and K2 are tracked separately as they serve different physiological roles.
| Food Source | Serving | K (µg) |
|---|---|---|
| 🥬Kale (cooked) | ½ cup | 531 |
| 🌿Swiss chard (cooked) | ½ cup | 573 |
| 🌿Spinach (cooked) | ½ cup | 444 |
| 🥦Broccoli (cooked) | ½ cup | 110 |
| 🫛Brussels sprouts (cooked) | ½ cup | 109 |
| 🥗Romaine lettuce (raw) | 1 cup | 57 |
| Food Source | Serving | K (µg) |
|---|---|---|
| 🫙Natto (fermented soy) | 85g | 850 |
| 🧀Gouda / Edam cheese | 28g | 75 |
| 🧀Hard aged cheese | 28g | 50 |
| 🥚Egg yolk | 1 large | 32 |
| 🧈Butter (grass-fed) | 1 tbsp | 15 |
| 🫀Chicken liver (cooked) | 85g | 13 |
Supplementation is most warranted for K2, which is difficult to obtain from typical Western diets. K1 sufficiency is easier to achieve through diet. The choice of K2 subtype matters — MK-7 has the best evidence.
Perhaps the most important Vitamin K insight: high-dose Vitamin D3 supplementation without adequate K2 may increase arterial calcification risk. Vitamin D increases calcium absorption; K2 (via MGP activation) is needed to direct that calcium to bone rather than arterial walls. Co-supplementation of D3 with K2 MK-7 is widely recommended by nutrition researchers.
Effective for correcting clotting-related deficiency. Short half-life — needs daily intake. Found in most multivitamins. Less evidence for bone/vascular benefits compared to K2.
Short half-life — requires multiple daily doses (1,500 µg 3× daily used in Japanese osteoporosis trials). Effective at high doses for bone density. The form found in animal foods and produced by BCO1 conversion from K1.
Long half-life (3 days) — effective at once-daily dosing of 90–180 µg. Derived from natto or synthetic production. Best evidence for bone mineral density and arterial calcification reduction. Preferred form for supplementation.
Combined formula providing both short-acting (MK-4) and long-acting (MK-7) forms. May provide broader tissue distribution. Less studied than MK-7 alone.
Warfarin users: do NOT supplement Vitamin K without medical supervision. Even MK-7 at 90 µg/day can affect INR. If supplementation is needed, it must be done with close INR monitoring and dose adjustments.
Vitamin K from food and most supplement doses is exceptionally safe — no upper limit has been established. The primary concern is its interaction with anticoagulant medications.
The most clinically significant interaction. Warfarin works by blocking Vitamin K recycling — changes in K intake directly affect anticoagulation intensity (INR). Sudden increases in dietary K can cause under-anticoagulation (clot risk); sudden decreases can cause over-anticoagulation (bleeding risk). Consistent K intake is essential.
MK-7's long half-life makes it particularly potent at affecting INR even at doses as low as 90 µg/day. People on warfarin must avoid K2 MK-7 supplements unless under strict medical monitoring. MK-4 at low doses has less impact but should still be discussed with a prescriber.
Orlistat (weight-loss medication), cholestyramine (bile acid sequestrant), and certain anticonvulsants (phenytoin, carbamazepine) reduce fat-soluble vitamin absorption and can deplete Vitamin K over time. Long-term users should monitor K status.
Conditions causing fat malabsorption (IBD, cystic fibrosis, celiac, chronic pancreatitis, post-bariatric surgery) can cause clinical Vitamin K deficiency. Supplementation or parenteral administration may be necessary.
No upper limit has been established for Vitamin K because no adverse effects — including from very high intakes of K1 from greens or K2 from natto — have been observed. The only safety concern is the warfarin interaction, not toxicity from Vitamin K itself.
CleverHabits Editorial Team provides research-based educational content about nutrition, vitamins, healthy habits, and dietary supplements. Our articles are created using publicly available scientific research, nutritional guidelines, and reputable health sources.
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