Vitamin K: Benefits, Dosage, and Safety

What vitamin k is

Vitamin K is the generic name for a family of fat-soluble compounds sharing a 2-methyl-1,4-naphthoquinone structure. The two natural groups are phylloquinone (vitamin K1), the main dietary form found mostly in green leafy vegetables, and the menaquinones (vitamin K2, designated MK-4 through MK-13), which are largely of bacterial origin and present in some animal-based and fermented foods. Phytonadione is a synthetic form of K1, and menadione (K3) is a synthetic form no longer used in supplements or fortified foods because laboratory studies in the 1980s and 1990s showed it can damage hepatic cells. Almost all long-chain menaquinones are also produced by bacteria in the human gut, and MK-4 is produced by the body from phylloquinone.

What it does

• Acts as a coenzyme for vitamin K-dependent carboxylase, the enzyme required to synthesize proteins involved in hemostasis (blood clotting)
• Supports synthesis of prothrombin (clotting factor II), a vitamin K-dependent plasma protein directly involved in blood clotting
• Supports carboxylation of osteocalcin, a vitamin K-dependent protein in bone that may be involved in bone mineralization or turnover
• Supports matrix Gla-protein (MGP), a vitamin K-dependent protein in vascular smooth muscle, bone, and cartilage that is under study for a possible role in reducing abnormal calcification

How much vitamin k per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Vitamin K deficiency is only considered clinically relevant when prothrombin time increases significantly due to a decrease in the prothrombin activity of blood. Bleeding and hemorrhage are the classic signs and occur only in severe cases. In newborns, deficiency can cause vitamin K deficiency bleeding (VKDB), with bleeding in the umbilicus, gastrointestinal tract, skin, nose, or other sites, and in some cases sudden intracranial bleeding with a high mortality rate. Because vitamin K is required for carboxylation of osteocalcin, deficiency could also reduce bone mineralization. Clinically significant deficiency in adults is very rare.

Who may need more

• Newborns, who are recommended to receive a single intramuscular dose of 0.5 to 1 mg vitamin K1 at birth to prevent vitamin K deficiency bleeding
• Exclusively breastfed infants, given the low vitamin K content of breast milk
• People with malabsorption syndromes and gastrointestinal disorders (cystic fibrosis, celiac disease, ulcerative colitis, short bowel syndrome) who may need monitoring and, in some cases, supplementation
• People who have had bariatric surgery

Food sources

Green leafy vegetables and vegetable oils are the richest sources of phylloquinone. Examples per serving: natto 850 mcg (3 oz, as MK-7), frozen boiled collards 530 mcg (1/2 cup), frozen boiled turnip greens 426 mcg (1/2 cup), raw spinach 145 mcg (1 cup), raw kale 113 mcg (1 cup), boiled broccoli 110 mcg (1/2 cup), roasted soybeans 43 mcg (1/2 cup), carrot juice 28 mcg (3/4 cup), soybean oil 25 mcg (1 tbsp), edamame 21 mcg (1/2 cup), canola oil 10 mcg (1 tbsp). Meat, dairy, and eggs contain low phylloquinone but modest menaquinones (MK-4). The most common U.S. dietary sources are spinach, broccoli, iceberg lettuce, and soybean and canola oils.

Forms and absorption

Supplement forms include vitamin K1 as phylloquinone or phytonadione (synthetic K1) and vitamin K2 as MK-4 or MK-7. Few data are available on the relative bioavailability of the different forms; one study found both phytonadione and MK-7 are well absorbed, but MK-7 has a longer half-life. Free-form phylloquinone is absorbed at roughly 80%, but absorption from foods is significantly lower because plant phylloquinone is tightly bound to chloroplasts (the body absorbs only about 4% to 17% as much phylloquinone from spinach as from a tablet). Consuming vegetables with some fat improves absorption. Menadione (K3) is no longer used in supplements or fortified foods. Some single-nutrient vitamin K supplements provide very high amounts, such as 4,050 mcg (5,063% of the DV).

Safety, excess and interactions

Too much

The Food and Nutrition Board did not establish a Tolerable Upper Intake Level for vitamin K because of its low potential for toxicity. The FNB reported that no adverse effects associated with vitamin K consumption from food or supplements have been reported in humans or animals. Note that this low-toxicity statement applies to phylloquinone and menaquinones, not to menadione (K3), the synthetic form that was shown to damage hepatic cells and is no longer used.

Interactions to know

• Warfarin (Coumadin) and similar vitamin K antagonists (phenprocoumon, acenocoumarol, tioclomarol): these anticoagulants antagonize vitamin K activity and deplete vitamin K-dependent clotting factors. Sudden changes in vitamin K intake can increase or decrease the anticoagulant effect, so intake must be kept consistent.
• Antibiotics: can destroy vitamin K-producing gut bacteria and may decrease vitamin K status; cephalosporins such as cefoperazone (Cefobid) may also inhibit vitamin K action. Supplements are usually needed only with prolonged use (beyond several weeks) plus poor intake.
• Bile acid sequestrants (cholestyramine/Questran, colestipol/Colestid): can reduce absorption of vitamin K and other fat-soluble vitamins; status should be monitored with long-term use.
• Orlistat (Alli, Xenical): reduces dietary fat absorption and can lower absorption of fat-soluble vitamins including vitamin K; combined with warfarin it may significantly increase prothrombin time.

How status is measured

Vitamin K status is not routinely assessed except in people who take anticoagulants or have bleeding disorders. The only clinically significant indicator is prothrombin time (the time it takes for blood to clot). Fasting plasma phylloquinone has been reported to range from 0.29 to 2.64 nmol/L in healthy people, but it is unclear whether this can quantitatively assess status, and no normal ranges for menaquinones are available.

Regulated as a dietary supplement ingredient under DSHEA, not as a drug; supplements are not FDA-approved to treat, cure, or prevent disease. The FDA Daily Value (DV) for vitamin K is 120 mcg for adults and children age 4 and older. The FDA has not authorized a health claim for vitamin K in the United States (the European Food Safety Authority has approved a bone-maintenance claim). Menadione (K3) is permitted only in animal feed, not human supplements or fortified foods.

What the evidence supports

We grade the strength of evidence behind each common reason people take vitamin k, mirroring how the NIH describes it.