Vitamin K

What is vitamin K?

Vitamin K is an essential vitamin that plays an important role in blood coagulation, bone metabolism, and vascular health, and it is one of the four fat-soluble vitamins (along with vitamin A, vitamin D, and vitamin E). Vitamin K is actually the collective term for several fat-soluble molecules called 2-methyl-1,4-naphthoquinones. There are two naturally occurring forms of vitamin K: vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). K1 is the major dietary form and is found in several plant-based foods including spinach, broccoli, kale, Brussels sprouts, collards, and soybeans. Vitamin K2 is synthesized by bacteria — including gut bacteria in our microbiome — and is found in meat and fermented foods (e.g., nattō; fermented soybeans). Phylloquinone (vitamin K1) is the predominant form used in vitamin K supplements, but menaquinones (vitamin K2) are also used. Another form of vitamin K called menadione (or vitamin K3) is an intermediate molecule in vitamin K metabolism. It is not typically used in human supplements, but it is used in animal feed.

What are the signs and symptoms of vitamin K deficiency?

The symptoms of vitamin K deficiency include bleeding disorders, impaired bone development, and spontaneous rash. The signs of vitamin K deficiency include biomarkers of vitamin K status, such as low serum concentrations of phylloquinone (vitamin K1) and PIVKA-II (protein induced by vitamin K absence or antagonist-II). PIVKA-II is an incompletely carboxylated form of prothrombin, which is a key protein involved in blood coagulation that is carboxylated by vitamin K.

What causes vitamin K deficiency?

In newborn babies, a vitamin K deficiency can arise due to inadequate vitamin K storage or a vitamin K deficiency in the mother’s breast milk. This can lead to a type of hemorrhage called vitamin K deficiency bleeding. Vitamin K deficiency is not common in adults. Still, it can occur due to inadequate dietary intake or the use of drugs known to interfere with the absorption, metabolism, and synthesis of vitamin K (e.g., anticoagulants and drugs that affect the intestinal absorption of dietary fat). It is also possible that antibiotics can inhibit the growth of vitamin-K-producing bacteria in the intestine and increase the risk of vitamin K deficiency.

What are vitamin K’s main benefits?

Due to vitamin K’s role in blood coagulation, bone metabolism, and vascular health, supplementation with vitamin K is claimed to have a range of benefits on blood clotting (coagulation), bone health, cardiovascular health, diabetes and blood sugar, and cancer. The main benefit of vitamin K supplementation is in newborn babies, because vitamin K deficiency bleeding (VKDB) can arise due to inadequate vitamin K storage or a vitamin K deficiency in the mother’s breast milk. To reduce the risk of VKDB, a single 1-milligram (mg) intramuscular injection of vitamin K is routinely administered to newborns. In adults, there is also a relationship between the dietary intake of vitamin K and normal blood coagulation. Observational studies have found that insufficient dietary intake of vitamin K (i.e., lower than the adequate intake) and low serum concentrations of vitamin K are associated with low bone mineral density. It is also generally agreed that there is a relationship between the dietary intake of vitamin K and the maintenance of normal bone health. However, while meta-analyses of randomized controlled trials have found that supplementation with vitamin K can affect markers of bone health, including bone mineral density, the effects on bone fracture risk are inconsistent. Further research is needed to determine whether vitamin K can prevent or treat osteoporosis. Low serum concentrations of vitamin K have been associated with coronary artery calcium progression, a marker of calcification and stiffening of arteries which can cause cardiovascular disease. Consequently, vitamin K has been suggested to support cardiovascular health. However, while low serum concentrations of vitamin K appear to be associated with a greater cardiovascular disease risk and higher mortality, the current evidence does not show a relationship between the dietary intake of vitamin K and the normal function of the cardiovascular system or cardiovascular disease mortality. Furthermore, meta-analyses of randomized controlled trials do not support a benefit of vitamin K supplementation on cardiovascular health. Low serum concentrations of vitamin K have also been associated with a higher risk of type 2 diabetes, but randomized controlled trials show that the effect of supplementation with vitamin K on fasting glucose and insulin resistance is trivial and highly variable. Supplementation with vitamin K might play a role in cancer therapy, but further randomized controlled trials are needed to make firm conclusions.

What are vitamin K’s main drawbacks?

While case studies have shown that injectable forms of vitamin K1 can cause allergic reactions and that high doses of vitamin K3 can cause hemolytic anemia in some people, the consumption of vitamin K is not associated with adverse effects or toxicity in the general population. That said, vitamin K does interact with some drugs, including blood-thinning drugs (anticoagulants) like warfarin and drugs that affect the intestinal absorption of dietary fat, such as colesevelam and orlistat. People who use such drugs should consult their doctor before considering using a vitamin K supplement or altering their dietary intake of foods rich in vitamin K.