A fat-soluble vitamin from leafy greens (K1) and fermented foods (K2). The body needs it for blood clotting. Most studied for bones and arteries.
Ask a doctor or pharmacist first if you:
Vitamin K is a group of fat-soluble vitamins. Vitamin K1 (phylloquinone) comes mostly from green leafy vegetables and is the main form in food. Vitamin K2 (menaquinones, such as MK-4 and MK-7) comes from fermented foods, and in small amounts from meat, dairy foods and eggs. Bacteria in the gut also make it.[1]
The body needs vitamin K to make clotting proteins and some proteins of bone.[1] Supplements contain vitamin K1 or vitamin K2 as MK-4 or MK-7.[1]
The best-proven use is one injection of vitamin K for newborn babies: it prevents bleeding in the first week of life.[2] In adults, most trials studied bones and the heart and arteries, and the results are mixed:[3][4]
Yes, for most people. Neither the US nor the EU has set an upper limit for vitamin K.[1][9] US experts found no reports of harm from vitamin K in food or supplements.[1] A 2017 safety review found no serious risk from MK-7 supplements.[10] In pregnancy the US adequate intake is the same as for other adult women: 90 µg a day.[1]
The main risk is for people who take warfarin or a similar blood thinner. Vitamin K works against these medicines, so a sudden change in intake can make them work more or less strongly.[1] The authors of one study warned that 50 µg of MK-7 a day or more may change their effect.[11] Vitamin K3 (menadione) is no longer used in supplements, because it damaged liver cells in lab studies.[1]
Vitamin K turns on proteins that cannot work without it: clotting factors in the blood, osteocalcin in bone and matrix Gla protein in blood vessels.[1] The body stores little vitamin K and uses it up fast.[1]
Bile and enzymes from the pancreas mix vitamin K with the fat of a meal, and cells of the small intestine absorb it. From spinach, the body absorbs only 4% to 17% as much vitamin K1 as from a tablet.[1]
From the gut, vitamin K travels in the lymph to the liver. The liver sends it on in lipoproteins to other tissues, such as bone, heart and brain.[1]
Warfarin and similar blood thinners work by blocking vitamin K. This is why a sudden change in vitamin K from food or supplements can make them work more or less strongly.[1]
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Trials gave 180 to 720 µg of vitamin K2 as MK-7, 500 µg to 5 mg of vitamin K1, or 45 mg of MK-4 a day.[12][13][14][15][16] The US adequate intake for adults is 120 µg a day for men and 90 µg for women.[1]
In 244 healthy women after menopause, MK-7 was tested on bones and on arteries at 180 µg a day.[12][17] A trial on coronary calcium gave 360 µg a day,[5] and a trial on heart valves gave 720 µg with 25 µg of vitamin D.[13] Trials of vitamin K1 gave 500 µg to 5 mg a day.[14][15] For newborn babies, the American Academy of Pediatrics advises one dose of 0.5 to 1 mg of vitamin K1.[1]
The US adequate intake is 2.0 µg a day up to 6 months, 30 µg at 1 to 3 years, 60 µg at 9 to 13 years and 75 µg at 14 to 18 years. For adult men it is 120 µg and for adult women 90 µg, also in pregnancy and breastfeeding.[1] EFSA set 1 µg of vitamin K1 per kg of body weight, which is 70 µg a day for adults.[9]
Most studies gave 0.1 to 0.25 mg a day, for 1 to 3 months.
The counts are study groups. A study group is one set of people in a study who took the same dose. One study can have several groups. These are amounts from studies, not advice for you.
47 of 166 study groups took vitamin K with something else. This shows what studies gave together, not what works together.
What studies used when they gave vitamin K for one goal, or to a group of people.
Each protocol describes what studies used, for one goal or for one group of people. It is not advice for you: talk to a doctor before you start a supplement.
12 questions in 4 groups. Open one for the short answer; each has its own page with the full answer and sources.
Green leafy vegetables have the most vitamin K1. Half a cup of boiled collards has 530 µg, 1 cup of raw spinach 145 µg and half a cup of boiled broccoli 110 µg.[1] Vegetable oils add some: 1 tablespoon of soybean oil has 25 µg.[1]
Natto, fermented soybeans from Japan, is the richest source of vitamin K2: 3 ounces have 850 µg of MK-7.[1] Meat, cheese and eggs have small amounts of MK-4, about 2 to 13 µg per serving.[1] The body absorbs much less vitamin K1 from vegetables than from oils or tablets, and eating the vegetables with some fat helps.[1]
Deficiency is very rare in healthy adults. With a varied diet, it is almost impossible to eat so little vitamin K that blood clotting changes.[1] US adults get 122 to 138 µg a day from food on average.[1]
The groups at risk are:
The main sign of deficiency is bleeding, because blood takes longer to clot.[1]
To prevent bleeding. Newborn babies have little vitamin K: little crosses the placenta, and breast milk contains little.[1] Without it, some babies bleed in the first weeks of life, sometimes inside the head, which can be fatal.[1]
A Cochrane review found that one injection of 1 mg of vitamin K after birth prevents bleeding in the first week of life.[2] The American Academy of Pediatrics advises one injection of 0.5 to 1 mg at birth.[1] Doses by mouth improved blood clotting tests, but the review found no randomized trial that tested them against bleeding.[2]
Not clearly. A 2019 review of 36 trials found no evidence of an effect on bone density or spine fractures in women after menopause or people with osteoporosis.[3] The odds of any clinical fracture were 28% lower with vitamin K, but in the trials with a low risk of bias this result could be due to chance.[3] One trial in healthy women after menopause found slower bone loss with 180 µg of MK-7 a day for 3 years.[12]
Not consistently. A 2020 review of 9 trials found that vitamin K did not reliably slow calcium build-up, plaque or stiffening of the arteries.[4] A 2023 review found no slowing in 5 trials of vitamin K in people without kidney disease.[19] A 2026 trial found slower build-up with 360 µg of MK-7 a day for 2 years in people with coronary artery disease.[5]
One trial found that it does, in older adults. In China, 199 adults aged 65 and older with night-time leg cramps took 180 µg of MK-7 or a placebo every day for 8 weeks. With MK-7, cramps fell from 2.6 to 0.96 a week. With placebo, they went from 2.71 to 3.63 a week. Cramps were also shorter and less painful with MK-7, and no side effects of MK-7 were found.[6]
In 2025 the journal retracted the first version of this paper and replaced it with a corrected version.[6] A small crossover trial in people on dialysis (39 completed it) also found fewer, shorter and milder cramps with 360 µg of vitamin K2 a day.[20]
Only a little, in the trials so far. A 2023 meta-analysis of 7 trials with 813 people found small drops in fasting blood sugar and in insulin resistance, but not in fasting insulin.[7] A 2024 meta-analysis of 17 trials found lower insulin resistance (HOMA-IR), but no effect on blood sugar, blood fats, weight or CRP, a marker of inflammation.[8]
In studies that followed 105,798 people, those with more vitamin K had a 21% lower risk of type 2 diabetes.[7] Studies of this kind show a link, not that vitamin K is the cause.[7]
Only after talking to the doctor who manages the warfarin. Warfarin works by blocking vitamin K, so a sudden change in vitamin K from food or supplements can make it work more or less strongly.[1] People who take warfarin need to keep their vitamin K intake steady.[1] The authors of one study warned that 50 µg of MK-7 a day or more may change the effect of warfarin.[11]
Antibiotics, some cholesterol medicines and the weight-loss medicine orlistat can lower vitamin K in the body.[1]
People who take these medicines for a long time should ask a doctor or pharmacist about their vitamin K status.[1]
It is not proven. MK-7, a form of vitamin K2, stays in the blood longer than vitamin K1. In healthy volunteers, it built up to higher levels with daily intake and activated more osteocalcin.[11] But two trials that compared vitamin K1 with MK-4 found that neither changed bone density more than placebo.[16][21]
Some supplements combine them, and a 2020 meta-analysis found a small gain in bone density from the pair.[1][22] It pooled 8 trials with 971 people. Vitamin K with vitamin D raised total bone density a little.[22] When only the vitamin K group also gets vitamin D or calcium, part of the effect may come from them.[1]
In bone trials that gave calcium and vitamin D to everyone, vitamin K added no bone density.[16][14] A 2-year trial included 365 men with calcium in the aortic valve. In it, 720 µg of MK-7 with 25 µg of vitamin D a day did not slow calcium build-up in the valve.[13]
Yes, a meal with some fat helps. Vitamin K1 is absorbed together with fat from food.[9] In the gut, bile and enzymes from the pancreas mix vitamin K with fat before the small intestine absorbs it.[1]
The body absorbs about 80% of vitamin K1 in its free form, but much less from food. From spinach it absorbs only 4% to 17% as much as from a tablet. Eating vegetables with some fat raises this amount.[1]
