Other Nutrients

Vitamin K

By All The Nutrients Editorial Team Published: 8 min read

Vitamin K is a fat-soluble vitamin named for the Danish word "Koagulation" — a nod to its discovery as the essential factor in blood clotting. The term covers two main naturally occurring forms: phylloquinone (K1), found in green leafy vegetables and plant oils, and menaquinones (K2, MK-4 through MK-13), produced by gut bacteria and found in fermented foods and animal products. Phylloquinone is the predominant dietary form and the one measured in most nutrition databases. The richest dietary sources are leafy greens such as kale, spinach, collard greens, and Swiss chard.

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Spices, sage, ground

1715 ug

What Does Vitamin K Do?

  • Blood coagulation: Vitamin K is an obligatory cofactor for γ-carboxylation, a post-translational modification that activates at least four coagulation factors (II, VII, IX, X) and two anticoagulant proteins (C and S). Without K, these proteins cannot bind calcium at wound sites — bleeding cannot be controlled. This is the mechanism exploited by warfarin, which blocks vitamin K recycling and is used as an anticoagulant medication.
  • Bone mineralisation: Vitamin K activates osteocalcin, the most abundant non-collagen protein in bone. Undercarboxylated osteocalcin (ucOC) — produced when K is insufficient — cannot bind calcium into hydroxyapatite crystals. Observational studies link low vitamin K intake to higher rates of osteoporosis and hip fracture; clinical trials with MK-7 supplements (180–360 mcg/day) show improvements in bone stiffness and reduced vertebral fracture risk in postmenopausal women.
  • Vascular calcification prevention: Matrix Gla protein (MGP) — the most potent known inhibitor of vascular calcification — requires vitamin K for activation. Undercarboxylated MGP accumulates in arterial walls when K is insufficient and allows calcium to deposit in vessel walls. Population studies associate low K status (measured by high ucMGP) with coronary artery calcification, arterial stiffness, and cardiovascular mortality.
  • Glucose metabolism: Carboxylated osteocalcin (bone-derived) acts as a hormone that stimulates insulin secretion from pancreatic β-cells and improves insulin sensitivity in muscle and adipose tissue. Adequate vitamin K status, by fully carboxylating osteocalcin, may therefore support normal glucose regulation — an emerging field with growing but not yet conclusive evidence.
Horizontal bar chart of vitamin K1 (phylloquinone) per 100g: Swiss chard raw 830mcg, kale raw 817mcg, collard greens cooked 623mcg, spinach raw 483mcg, broccoli cooked 141mcg, Brussels sprouts cooked 140mcg, soybean oil 184mcg. FDA AI reference line at 120mcg.
Leafy greens dwarf every other vitamin K source. A single cup of raw kale provides more than four times the adequate daily intake.
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Signs of Vitamin K Deficiency

True vitamin K deficiency causing spontaneous bleeding is rare in healthy adults because the gut microbiome synthesises some menaquinones and dietary phylloquinone is widespread. However, subclinical insufficiency — enough K for clotting but insufficient for full activation of bone and vascular proteins — may be common. Clinical deficiency is seen primarily in:

  • Newborns — at very high risk because K crosses the placenta poorly, breast milk is low in K, and the infant gut microbiome is not yet established. Vitamin K deficiency bleeding (VKDB) in newborns can cause life-threatening intracranial or gastrointestinal haemorrhage; prophylactic K1 injection at birth is standard practice worldwide
  • Easy bruising and prolonged bleeding — even from minor cuts; PT (prothrombin time) is prolonged on blood tests
  • Heavy menstrual periods or nosebleeds that are difficult to stop
  • Blood in urine or stools — signs of gastrointestinal or urinary tract bleeding
  • Calcification of arteries and soft tissue — linked to chronically low MGP activation; associated with elevated ucMGP in plasma
  • Poor bone density / increased fracture risk — from undercarboxylated osteocalcin impairing mineralisation, particularly relevant in postmenopausal women and older adults
  • Fat malabsorption — people with Crohn's disease, ulcerative colitis, cystic fibrosis, or following gastric bypass surgery absorb dietary phylloquinone poorly and are at heightened risk

How to Get the Most from Vitamin K

Vitamin K1 (phylloquinone) is tightly bound within chloroplast membranes in plant leaves, making it relatively poorly absorbed compared to K2 forms. Several strategies improve bioavailability:

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Always eat leafy greens with fat

Phylloquinone is fat-soluble and absorbed via chylomicrons. Adding fat to leafy greens — olive oil on salad, butter on steamed kale, avocado with spinach — increases K1 absorption by 3- to 10-fold compared to fat-free preparation. This is one of the most evidence-based diet tips in nutrition science: a study by Gijsbers et al. found that phylloquinone absorption from spinach was 244% higher when 30 g of butter was added to the meal.

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Cooking improves availability from some sources

Light cooking (steaming, stir-frying) disrupts chloroplast membranes in leafy greens, liberating phylloquinone and improving absorption compared to raw leaves. However, prolonged boiling in water can leach some K into the water. Cooking oil-rich foods (kale chips, sautéed spinach) simultaneously heats and adds fat — double benefit.

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Warfarin users: maintain consistent K intake, do not avoid greens

Warfarin works by blocking vitamin K recycling. Dramatically changing vitamin K intake — whether eating a large volume of leafy greens one day and none the next — destabilises INR (anticoagulation level) and increases both clotting and bleeding risk. The evidence-based recommendation is to eat a consistent, moderate amount of vitamin K-rich foods every day, not to avoid them. Dose adjustments to warfarin can compensate for any intake level. Discuss dietary patterns with the prescribing clinician.

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Consider K2 (menaquinone) sources for bone and vascular benefits

K2 forms — particularly MK-7 found in nattō (fermented soybeans), and MK-4 in fermented cheeses and egg yolks — have substantially longer half-lives than K1 (days vs. hours) and distribute more broadly to bone and arterial tissue. K1 is adequate for coagulation but K2 may be more relevant for bone and vascular proteins (osteocalcin, MGP). Nattō is the richest MK-7 source (~900 mcg per 100 g); European-style aged cheese and egg yolks provide modest MK-4.

Common Vitamin K Questions

How much vitamin K do I need per day?

The FDA Daily Value for vitamin K is 120 mcg/day for adults. The National Academies set an Adequate Intake (AI) — not an RDA, because data for an EAR are insufficient — of 120 mcg/day for adult men and 90 mcg/day for adult women. These values are based primarily on intakes associated with normal coagulation; intakes needed for full activation of bone and vascular proteins may be higher, which is an active research area. There is no established Tolerable Upper Intake Level for vitamin K from food or phylloquinone supplements because no adverse effects have been documented at high dietary intakes (the UL for synthetic K3 / menadione is a separate matter — it is not used as a supplement in humans).

What foods are highest in vitamin K?

Vitamin K1 (phylloquinone) is concentrated in green leafy vegetables. Per 100 g: kale raw (817 mcg), spinach raw (483 mcg), collard greens cooked (623 mcg), Swiss chard raw (830 mcg), broccoli cooked (141 mcg), Brussels sprouts cooked (140 mcg), parsley fresh (1,640 mcg — extremely high but used in small amounts), soybean oil (184 mcg per 100 mL), and canola oil (71 mcg per 100 mL). One cup of raw kale (67 g) provides approximately 547 mcg — over four times the daily adequate intake. Vitamin K2 sources include nattō (~900 mcg MK-7 per 100 g), aged hard cheeses (10–50 mcg MK-4 per 100 g), chicken liver, and egg yolks.

Can I take vitamin K if I am on warfarin?

Yes — but with important caveats. Warfarin acts by blocking vitamin K recycling, and its dose is calibrated against a patient's usual vitamin K intake. The key is consistency: if you eat similar amounts of vitamin K-rich foods daily, your INR stays stable and the warfarin dose can be adjusted accordingly. Sudden large increases (e.g., adding a daily kale salad when you previously ate almost no leafy greens) will temporarily decrease warfarin's effect and lower your INR. Similarly, cutting out all vitamin K sources when you normally eat leafy greens will raise INR. Some clinicians now use low-dose vitamin K supplements (100–200 mcg/day) to stabilise INR in highly variable patients. Always discuss dietary and supplement changes with the clinician managing your anticoagulation.

What is the difference between vitamin K1 and K2?

Vitamin K1 (phylloquinone) is the plant-derived form, found predominantly in green leafy vegetables and plant oils. It is absorbed in the gut and taken up rapidly by the liver, where it is preferentially used for coagulation factor carboxylation. Its serum half-life is roughly 1–2 hours. Vitamin K2 (menaquinones, MK-4 through MK-13) are longer-chain forms: MK-4 is produced by tissue conversion from K1 and found in animal products; MK-7 through MK-13 are synthesised by gut bacteria and found in fermented foods (nattō, cheese, sauerkraut). K2 forms have much longer half-lives (MK-7 is ~72 hours), distribute more broadly to extrahepatic tissues — particularly bone and arterial walls — and are thought to be more relevant for osteocalcin and MGP carboxylation, making them potentially important for bone and cardiovascular health beyond their clotting role.

Does vitamin K help with osteoporosis?

Evidence is accumulating but not yet conclusive. Observational studies consistently associate higher vitamin K intake with lower hip-fracture risk and better bone mineral density, particularly in older adults. Randomised trials with MK-7 supplementation (180–360 mcg/day for 1–3 years) show measurable improvements in bone stiffness (measured by quantitative ultrasound) and in carboxylated osteocalcin levels, with one trial in Dutch postmenopausal women showing reduced vertebral fracture risk. Trials using K1 supplements at pharmacological doses (1–5 mg/day) in Japan also show fracture-risk reductions. However, the evidence is not yet strong enough for K supplementation to be recommended as a standalone osteoporosis treatment in guidelines. The practical implication is that consistently eating vitamin K-rich foods is part of a bone-healthy diet alongside calcium and vitamin D.