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Vitamin K

Evidence Level
Moderate
6 Clinical Trials
3 Documented Benefits
3/5 Evidence Score

Vitamin K is a fat-soluble vitamin the body needs to make blood clotting proteins and to activate proteins in bone, such as osteocalcin. It comes as K1 (phylloquinone), mainly from green leafy vegetables, and K2 (menaquinones such as MK-4 and MK-7), from fermented foods like natto, animal foods and gut bacteria. Supplements are sold for bone and heart support, but the evidence is mixed. Japanese trials using a drug-level dose of MK-4 (45 mg a day) reported fewer fractures, while later Western trials of K1 and MK-7 mostly found no effect on bone density, and an updated meta-analysis judged the fracture evidence insufficient. One 3-year MK-7 trial reported less arterial stiffening, but heart outcomes have not been tested. Vitamin K has no known toxicity from food or supplements, but people taking warfarin must keep their intake steady and talk to their doctor before using it.

Studied Dose Adequate intake: 120 mcg a day for men and 90 mcg for women. Trials used 180 or 360 mcg a day of MK-7, 5 mg a day of K1, and 45 mg a day of MK-4 (a prescription dose in Japan).
Active Compound Vitamin K1 (phylloquinone) or vitamin K2 (menaquinones, mainly MK-7 or MK-4).
Deficiency information View details

Vitamin K deficiency is uncommon in healthy adults and is judged by a clearly longer clotting time. The main concern is vitamin K deficiency bleeding in newborns, because little vitamin K crosses the placenta and breast milk is low in it; US and Canadian pediatric societies recommend a vitamin K shot at birth to prevent it. In adults, deficiency mainly affects people with fat malabsorption or after bariatric surgery.

Common symptoms

  • Easy bruising and prolonged bleeding from minor cuts
  • Bleeding gums or frequent nosebleeds
  • Heavy menstrual bleeding
  • Blood in urine or stool (dark, tarry stools)
  • Internal bleeding, especially in the gut
  • In newborns: bleeding from the umbilical stump, skin, nose or gut, or into the brain
  • Signs of brain bleeding in infants, such as extreme sleepiness, pale skin, a bulging soft spot, vomiting or seizures, need emergency care
  • Long-term low status may reduce bone mineralization

At-risk groups

  • Newborns not given vitamin K at birth
  • Exclusively breastfed infants (breast milk is low in vitamin K)
  • People with fat malabsorption, such as cystic fibrosis, celiac disease, ulcerative colitis or short bowel syndrome
  • People who have had bariatric surgery
  • People on long courses of antibiotics with low vitamin K intake
  • People taking bile acid sequestrants or orlistat for long periods
When to see a doctor: Any unexplained bleeding in a newborn, such as bleeding from the umbilical stump, blood in stool, vomiting blood, or signs of brain bleeding like extreme sleepiness or seizures, is an emergency and needs immediate care. In adults, unexplained easy bruising or bleeding is a reason to ask for a clotting test (PT/INR). If you take warfarin, do not start vitamin K supplements without your doctor's approval, because they change how the medicine works.
Explore related deficiencies →
Sources: NIH ODS: Vitamin K Health Professional Fact Sheet · Hemorrhagic Disease of Newborn: StatPearls (NBK558994)

Benefits

Bone health in postmenopausal women

Vitamin K activates osteocalcin, a bone protein. In a 3-year trial in 244 women, 180 mcg a day of MK-7 slowed bone loss at the spine and femoral neck. Other trials show mixed results: 360 mcg of MK-7 and 5 mg of K1 did not slow bone loss, and an updated meta-analysis found no clear effect on bone density and too little evidence to confirm fewer fractures. The strongest fracture data used a 45 mg MK-4 drug dose in Japan.

Supported by Trial 1, Trial 4

Normal blood clotting

Vitamin K is needed to make prothrombin and other clotting proteins in the liver. Deficiency is rare in healthy adults and shows up as easy bleeding; newborns are at higher risk, which is why US and Canadian pediatric societies recommend a vitamin K shot at birth.

Supported by Trial 4, Trial 1

Arterial stiffness in postmenopausal women

In a 3-year placebo-controlled trial in 244 healthy postmenopausal women, 180 mcg a day of MK-7 lowered a measure of aortic stiffness (pulse wave velocity), mostly in women who started with stiffer arteries. The researchers work at VitaK, a vitamin K research group at Maastricht University, the product was a branded MK-7, and no trial has tested heart attacks or strokes. A cohort study linked higher K2 intake from food with fewer coronary deaths.

Supported by Trial 6, Trial 1

Mechanism of action

1

Blood clotting (coagulation)

Vitamin K is the cofactor for an enzyme (gamma-glutamyl carboxylase) that activates clotting factors such as prothrombin in the liver, letting them bind calcium and take part in clot formation. Warfarin works by opposing vitamin K's action.

2

Bone proteins

The same carboxylation step activates osteocalcin, a protein made by bone-building cells. Supplements reliably lower the share of inactive (undercarboxylated) osteocalcin in blood, but that has not consistently translated into higher bone density.

3

Matrix Gla protein in blood vessels

Vitamin K also activates matrix Gla protein, which may help limit calcium deposits in blood vessel walls. In the MK-7 trial, supplements halved the inactive form of this protein; whether this prevents heart disease is unproven.

Clinical trials

1
MK-7 and bone loss in postmenopausal women
PubMed

Randomized placebo-controlled trial of 180 mcg a day of MK-7 for 3 years (Knapen et al. 2013, Osteoporos Int)

244 healthy postmenopausal women in the Netherlands.

MK-7 improved vitamin K status and slowed the age-related decline in bone mineral density at the lumbar spine and femoral neck, but not the total hip. It also reduced loss of vertebral height in the lower thoracic spine.

2
MK-7 (natto capsules) and bone loss in early menopause
PubMed

Randomized double-blind placebo-controlled trial of 360 mcg a day of MK-7 for 1 year (Emaus et al. 2010, Osteoporos Int)

334 healthy Norwegian women aged 50 to 60, 1 to 5 years after menopause.

No difference in bone loss at the hip, femoral neck, spine or whole body after 12 months, although MK-7 lowered inactive osteocalcin.

3
ECKO trial: vitamin K1 in women with low bone density
PubMed

Randomized double-blind placebo-controlled trial of 5 mg a day of vitamin K1 for 2 years, extended up to 4 years (Cheung et al. 2008, PLoS Med)

440 postmenopausal women with osteopenia who were vitamin D replete.

No difference in bone density at the spine or hip over 2 to 4 years. Fewer women in the vitamin K group had clinical fractures (9 vs 20), but the trial was not designed to test fractures and the numbers were small. Supplements were well tolerated.

4
Vitamin K, bone density and fractures (updated meta-analysis)
PubMed

Systematic review and meta-analysis of 36 randomized trials of oral vitamin K in adults (Mott et al. 2019, Osteoporos Int)

Adults in 36 trials, mostly postmenopausal or osteoporotic women.

Vitamin K lowered the odds of any clinical fracture (OR 0.72), but not significantly when limited to trials at low risk of bias (OR 0.76, 95% CI 0.58 to 1.01), and did not reduce vertebral fractures. Spine density gains shrank and lost significance when high-risk-of-bias trials were removed. The authors found no evidence of a bone density effect.

5
Vitamin K and fractures (earlier meta-analysis)
PubMed

Systematic review and meta-analysis of randomized trials of oral vitamin K1 and K2 (Cockayne et al. 2006, Arch Intern Med)

13 trials with bone loss data; 7 with fracture data, all Japanese trials of MK-4.

MK-4 trials showed fewer vertebral (OR 0.40), hip (OR 0.23) and nonvertebral fractures (OR 0.19). The trials used 15 or 45 mg a day of MK-4, a drug dose, and the integrity of some included trials was later questioned.

6
MK-7 and arterial stiffness
PubMed

Randomized double-blind placebo-controlled trial of 180 mcg a day of MK-7 (MenaQ7) for 3 years (Knapen et al. 2015, Thromb Haemost)

244 healthy postmenopausal women (120 MK-7, 124 placebo).

Carotid-femoral pulse wave velocity and a stiffness index fell in the MK-7 group, with more improvement in women with stiffer arteries at the start. MK-7 halved inactive matrix Gla protein but did not change inflammation markers. Heart events were not measured.

Side effects and drug interactions

Common Potential side effects

No adverse effects from vitamin K in food or supplements have been reported, so no upper limit has been set.
In a 4-year trial, 5 mg a day of vitamin K1 was well tolerated, with no difference in side effects from placebo.
Vitamin K counteracts warfarin; people taking it must not start or stop supplements without their doctor (see interactions).

Important Drug interactions

Warfarin (Coumadin) and similar anticoagulants: these drugs work against vitamin K, so sudden changes in vitamin K intake from food or supplements can raise or lower their effect. Keep intake steady and talk to your doctor before taking a supplement.
Antibiotics: long courses can reduce vitamin K made by gut bacteria; cephalosporins such as cefoperazone may also block vitamin K action.
Bile acid sequestrants (cholestyramine, colestipol) and orlistat: they reduce absorption of fat-soluble vitamins, including vitamin K.

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Frequently asked questions about Vitamin K

What is the difference between vitamin K1 and K2?

K1 (phylloquinone) comes mainly from green leafy vegetables and is the main dietary form. K2 (menaquinones, such as MK-4 and MK-7) comes from fermented foods like natto, animal foods and gut bacteria. Bone and heart supplements usually contain MK-7, but the evidence for those uses is mixed.

How much vitamin K should I take?

The adequate intake is 120 mcg a day for men and 90 mcg for women. MK-7 trials used 180 to 360 mcg a day.

Should I take vitamin K with vitamin D?

They are often sold together because both are involved in bone metabolism, but large trials showing that adding vitamin K to vitamin D prevents fractures are lacking. Either can be taken separately.

Who should be cautious with vitamin K?

Anyone taking the blood thinner warfarin, because vitamin K works against that drug. If you are on warfarin, do not start or change vitamin K supplements without your doctor's guidance, and keep the vitamin K in your diet steady.

What is Vitamin K?

Vitamin K is a fat-soluble vitamin the body needs to make blood clotting proteins and to activate proteins in bone, such as osteocalcin. It comes as K1 (phylloquinone), mainly from green leafy vegetables, and K2 (menaquinones such as MK-4 and MK-7), from fermented foods like natto, animal foods and gut bacteria.

What is Vitamin K used for?

Vitamin K is researched primarily for Bone Health and Cardiovascular. Vitamin K activates osteocalcin, a bone protein. In a 3-year trial in 244 women, 180 mcg a day of MK-7 slowed bone loss at the spine and femoral neck.

What are the signs of Vitamin K deficiency?

Vitamin K deficiency is uncommon in healthy adults and is judged by a clearly longer clotting time. The main concern is vitamin K deficiency bleeding in newborns, because little vitamin K crosses the placenta and breast milk is low in it; US and Canadian pediatric societies recommend a vitamin K shot at birth to preven…

Is Vitamin K safe, and does it have side effects?

Side effects and cautions reported for Vitamin K include: No adverse effects from vitamin K in food or supplements have been reported, so no upper limit has been set. In a 4-year trial, 5 mg a day of vitamin K1 was well tolerated, with no difference in side effects from placebo. It may also interact with some medications. Vitamin K is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Vitamin K interact with any medications?

Possible interactions include: Warfarin (Coumadin) and similar anticoagulants: these drugs work against vitamin K, so sudden changes in vitamin K intake from food or supplements can raise or lower their effect. Keep intake steady and talk to your doctor before taking a supplement. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Vitamin K?

NutraSmarts rates the evidence for Vitamin K as Moderate (3 out of 5). It is backed by 6 study summaries and 9 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(9 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Violi F, Lip GY, Pignatelli P, Pastori D. Interaction Between Dietary Vitamin K Intake and Anticoagulation by Vitamin K Antagonists: Is It Really True?: A Systematic Review. Medicine (Baltimore). 2016;95(10):e2895. doi: 10.1097/MD.0000000000002895.PubMedUsed to support: Systematic review of 2 dietary trials and 9 observational studies in people taking vitamin K antagonists (supplement-only studies excluded): conflicting evidence on how dietary vitamin K affects INR; an effect may appear only above about 150 mcg a day; the authors advise keeping vitamin K intake stable rather than restricting it.
  2. Knapen MH, Drummen NE, Smit E, Vermeer C, Theuwissen E. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporos Int. 2013;24(9):2499-507. doi: 10.1007/s00198-013-2325-6.PubMedUsed to support: 3-year RCT in 244 healthy postmenopausal women: 180 mcg a day of MK-7 improved vitamin K status and slowed the decline in bone mineral density at the lumbar spine and femoral neck (not the total hip), improved bone strength indices and reduced loss of vertebral height in the lower thoracic spine.
  3. Geleijnse JM, Vermeer C, Grobbee DE, Schurgers LJ, Knapen MH, van der Meer IM, Hofman A, Witteman JC. Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. J Nutr. 2004;134(11):3100-5. doi: 10.1093/jn/134.11.3100.PubMedUsed to support: Rotterdam Study cohort (4,807 adults followed to 2000): higher dietary menaquinone (K2) intake was associated with lower coronary heart disease mortality (RR 0.43 in the top vs bottom third), lower all-cause mortality and less severe aortic calcification; phylloquinone (K1) was not related to any outcome. Observational, so it cannot show cause and effect.
  4. Cockayne S, Adamson J, Lanham-New S, Shearer MJ, Gilbody S, Torgerson DJ. Vitamin K and the prevention of fractures: systematic review and meta-analysis of randomized controlled trials. Arch Intern Med. 2006;166(12):1256-61. doi: 10.1001/archinte.166.12.1256.PubMedUsed to support: Meta-analysis of 13 trials with bone loss data, all but one favoring vitamin K; the 7 fracture trials were all Japanese trials of MK-4 and showed fewer vertebral (OR 0.40), hip (OR 0.23) and nonvertebral fractures (OR 0.19). An erratum was published in 2018, and a later update noted that the integrity of some earlier evidence had been questioned.
  5. Mott A, Bradley T, Wright K, Cockayne ES, Shearer MJ, Adamson J, Lanham-New SA, Torgerson DJ. Effect of vitamin K on bone mineral density and fractures in adults: an updated systematic review and meta-analysis of randomised controlled trials. Osteoporos Int. 2019;30(8):1543-1559. doi: 10.1007/s00198-019-04949-0.PubMedUsed to support: Updated meta-analysis of 36 RCTs of oral vitamin K: in postmenopausal or osteoporotic patients, odds of any clinical fracture were lower (OR 0.72) but not significantly in low risk-of-bias trials (OR 0.76, 95% CI 0.58 to 1.01); no difference in vertebral fractures; spine density differences shrank and lost significance without high risk-of-bias trials. Concluded there is no evidence vitamin K affects bone density and insufficient evidence on fractures.
  6. Cheung AM, Tile L, Lee Y, Tomlinson G, Hawker G, Scher J, Hu H, Vieth R, Thompson L, Jamal S, Josse R. Vitamin K supplementation in postmenopausal women with osteopenia (ECKO trial): a randomized controlled trial. PLoS Med. 2008;5(10):e196. doi: 10.1371/journal.pmed.0050196.PubMedUsed to support: ECKO RCT in 440 postmenopausal women with osteopenia: 5 mg a day of vitamin K1 for 2 to 4 years did not slow bone density loss at the spine or hip; fewer clinical fractures (9 vs 20) in a trial not powered for fractures; well tolerated.
  7. Emaus N, Gjesdal CG, Almås B, Christensen M, Grimsgaard AS, Berntsen GK, Salomonsen L, Fønnebø V. Vitamin K2 supplementation does not influence bone loss in early menopausal women: a randomised double-blind placebo-controlled trial. Osteoporos Int. 2010;21(10):1731-40. doi: 10.1007/s00198-009-1126-4.PubMedUsed to support: RCT in 334 Norwegian women 1 to 5 years after menopause: 360 mcg a day of MK-7 as natto capsules for 1 year lowered inactive osteocalcin but did not change bone loss at any site.
  8. Knapen MH, Braam LA, Drummen NE, Bekers O, Hoeks AP, Vermeer C. Menaquinone-7 supplementation improves arterial stiffness in healthy postmenopausal women. A double-blind randomised clinical trial. Thromb Haemost. 2015;113(5):1135-44. doi: 10.1160/TH14-08-0675.PubMedUsed to support: 3-year RCT in 244 healthy postmenopausal women: 180 mcg a day of MK-7 (MenaQ7) lowered carotid-femoral pulse wave velocity and a stiffness index, especially in women with high baseline stiffness, and halved inactive matrix Gla protein, without changing inflammation or endothelial markers.
  9. Office of Dietary Supplements, National Institutes of Health. Vitamin K: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. 2021;Updated March 29, 2021..SourceUsed to support: Not PubMed-indexed (official NIH fact sheet). Vitamin K (K1 phylloquinone from leafy greens; K2 menaquinones such as MK-4 and MK-7 from fermented and animal foods and gut bacteria) is a cofactor for proteins in blood clotting and bone metabolism, including prothrombin, osteocalcin and matrix Gla protein. Adequate intakes are 120 mcg (men) and 90 mcg (women). It is unclear whether supplementation reduces osteoporosis risk; Japanese fracture trials used 45 mg MK-4, a drug dose in Japan; later trials found no bone density effect. The role in arterial calcification is unclear. No upper limit was set because no adverse effects from food or supplements have been reported. People on warfarin need consistent vitamin K intake; prolonged antibiotics, bile acid sequestrants and orlistat can lower vitamin K status. Newborns are at risk of vitamin K deficiency bleeding.