Vitamin K
A fat-soluble vitamin the body needs to make proteins for blood clotting and bone. It comes mainly from green leafy vegetables, and gut bacteria make some too.
90 mcg women
How much you need
The official Adequate Intake for adults, per day:
| Age | Men | Women | Pregnancy | Breastfeeding |
|---|---|---|---|---|
| 19–50 years | 120 mcg | 90 mcg | 90 mcg | 90 mcg |
| 51–70 years | 120 mcg | 90 mcg | — | — |
| Over 70 | 120 mcg | 90 mcg | — | — |
These are Adequate Intakes, not Recommended Dietary Allowances — there was not enough evidence to set a requirement, so they are based on how much vitamin K healthy people eat.
People aged 18 fall in the official 14–18 group — 75 mcg a day for males and females, including in pregnancy and when breastfeeding. There is no upper limit at any age. Pregnancy and breastfeeding change what you need — confirm yours with your midwife, obstetrician or doctor.
Upper limit
No upper limit has been set for vitamin K, because its potential for toxicity is low — no adverse effects from vitamin K in food or supplements have been reported in humans or animals. The interaction with blood thinners is described under Medicines that interact, below.
What it does
Vitamin K is a family of compounds: phylloquinone (vitamin K1) and the menaquinones (vitamin K2), numbered MK-4 to MK-13 [1]. It is a coenzyme for an enzyme needed to make the proteins involved in blood clotting — including prothrombin — and in bone metabolism [1]. Warfarin (Coumadin) works by blocking vitamin K, which is why people taking it need a consistent vitamin K intake [1]. Compared with the other fat-soluble vitamins, very little circulates in the blood, because it is quickly used and excreted [1].
Forms
Phylloquinone, the main form in the diet, is found mainly in green leafy vegetables [1]. Menaquinones come mostly from bacteria: natto (fermented soybeans) is rich in them, cheese and other fermented foods contain some, and meat, dairy foods and eggs contain modest amounts [1]. Bacteria in the gut make menaquinones too, which experts believe meet at least some of the body's need [1]. The body also makes MK-4 from phylloquinone [1]. Supplements contain phylloquinone, phytonadione (a synthetic vitamin K1), MK-4 or MK-7; menadione, sometimes called vitamin K3, is no longer used in supplements or fortified foods because it damaged liver cells in laboratory studies [1].
Where it comes from
- NattoK2 (MK-7)3 ounces850 mcg
- Collards, frozen, boiledK1½ cup530 mcg
- Turnip greens, frozen, boiledK1½ cup426 mcg
- Spinach, rawK11 cup145 mcg
- Kale, rawK11 cup113 mcg
How these five were chosen: The five foods with the most vitamin K per serving in the NIH fact sheet's food table. Fortified foods are marked.
| Item | Percent of the adult daily amount (120 mcg) |
|---|---|
| Natto | 708% |
| Collards, boiled | 442% |
| Turnip greens, boiled | 355% |
| Spinach, raw | 121% |
| Kale, raw | 94% |
Getting the most from it
Free phylloquinone is about 80% absorbed, but much less is absorbed from food, because in plants it is tightly bound — the body absorbs only 4% to 17% as much from spinach as from a tablet [1]. Eating vegetables with some fat improves absorption, although it stays lower than from oils [1].
In the United States
Most U.S. diets contain enough vitamin K [1]. In national survey data from 2011–2012, average intake from food for adults aged 20 and older was 138 mcg a day for men and 122 mcg for women, rising to 182 mcg and 164 mcg with supplements [1]. Analyses of earlier surveys found only about a third of people above the Adequate Intake, but the fact sheet notes that the AI is only an estimate of need, deficiency in adults is very rare, and food databases count mainly phylloquinone, missing menaquinones from food and gut bacteria [1].
Who is most likely to run low
- People with malabsorption syndromes and other digestive disorders, such as cystic fibrosis, celiac disease, ulcerative colitis and short bowel syndrome [1]
- People who have had bariatric surgery, whose vitamin K status can be low even without clinical signs [1]
What deficiency looks like
Bleeding and hemorrhage are the classic signs, but only in severe deficiency, which is judged by a significantly longer prothrombin time — how long blood takes to clot [1]. Deficiency could also reduce bone mineralisation and contribute to osteoporosis [1]. In adults, clinically significant deficiency is very rare and usually limited to people with malabsorption disorders or taking drugs that interfere with vitamin K; in healthy people eating a varied diet, an intake low enough to affect blood clotting is almost impossible [1]. Newborns are the exception: vitamin K crosses the placenta poorly and breast milk contains little, so the American Academy of Pediatrics recommends a single injection of vitamin K1 at birth [1].
What the evidence supports, and what it doesn't
Established. Blood clotting. Vitamin K is needed to make prothrombin and other clotting proteins, and the only clinically significant measure of vitamin K status is prothrombin time [1]. To prevent vitamin K deficiency bleeding in newborns, the American Academy of Pediatrics recommends a single dose at birth [1].
Not supported by trials. Bone density in older adults: trials since 2006 have found that vitamin K supplements did not change bone density in elderly men or women. In one, 381 postmenopausal women took 1 mg phylloquinone, 45 mg MK-4 or a placebo a day for 12 months, all with calcium and vitamin D; both forms lowered undercarboxylated osteocalcin, but bone density at the spine and hip did not differ [1].
Mixed. Fractures: a 2006 review of 13 trials, mostly in postmenopausal women in Japan, found that 12 improved bone density, and in the seven with fracture data — all using MK-4 at pharmacological doses of 15 or 45 mg a day — fractures of the hip, spine and other bones were reduced [1]. A later trial found that 180 mcg of MK-7 a day for 3 years improved bone strength in postmenopausal women [1]. Whether any form reduces the risk of osteoporosis is unclear; giving vitamin D or calcium alongside vitamin K may explain some positive results [1]. Heart disease: in Dutch observational studies, higher menaquinone intake — but not phylloquinone — was linked to less arterial calcification, and in 4,807 adults aged 55 and older, to 57% lower coronary heart disease mortality in the highest third of intake [1]. In a trial of 388 healthy adults aged 60–80 given 500 mcg phylloquinone a day for 3 years, coronary calcification did not differ overall, but progressed less among the 295 who took the supplements as directed; the authors called for larger studies [1].
Where official guidance disagrees
The European Food Safety Authority has approved a health claim that "a cause and effect relationship has been established between the dietary intake of vitamin K and the maintenance of normal bone" [1]. The FDA has not authorised a health claim for vitamin K in the United States [1]. In Japan and other parts of Asia, a pharmacological dose of MK-4, 45 mg, is used to treat osteoporosis [1].
Too much
No adverse effects from vitamin K in food or supplements have been reported in humans or animals, so the Food and Nutrition Board set no upper limit [1]. The risk that matters is interaction, not toxicity: for anyone taking warfarin or a similar blood thinner, a sudden change in vitamin K intake — up or down — can strengthen or weaken the drug's effect [1].
Medicines that interact
Warfarin (Coumadin), and similar anticoagulants used in some European countries — phenprocoumon, acenocoumarol and tioclomarol — work against vitamin K, and the interaction can be serious and dangerous: people taking them need to keep their vitamin K intake from food and supplements consistent, because sudden changes can increase or decrease the drug's effect [1]. Antibiotics can kill the gut bacteria that make vitamin K, especially cephalosporins such as cefoperazone, although supplements are usually needed only when antibiotic use is prolonged and intake is poor [1]. Bile acid sequestrants such as cholestyramine (Questran) and colestipol (Colestid) can reduce vitamin K absorption [1]. Orlistat (Alli, Xenical) reduces the absorption of fat-soluble vitamins, and combined with warfarin can significantly lengthen prothrombin time [1]. If you take a blood thinner, talk to your doctor before changing how much vitamin K you eat or taking any supplement that contains it; for the other medicines, ask your doctor or pharmacist.
References
- NIH Office of Dietary Supplements. Vitamin K — Fact Sheet for Health Professionals. Updated 29 March 2021. ods.od.nih.gov
How we reviewed this page
Every value on this page comes from the NIH Office of Dietary Supplements — Vitamin K, Health Professional Fact Sheet, which presents the official U.S. intake recommendations set by the National Academies. The source was last updated on March 29, 2021, and this page was checked against it on October 18, 2026. Numbered markers link to the source for each claim, so you can check the original rather than take our word for it.
Drafted with AI assistance, then edited and fact-checked by a human. No value is published on the strength of a model's recall.
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General information only. This is not medical advice.