Vitamin K is essential for the process of blood clotting. It takes its name from the first letter of the German word “koagulation.” Vitamin K acts as a coenzyme for chemical reactions that produce proteins involved in blood coagulation and bone metabolism.

Vitamin K at a Glance

Vitamin K has two forms: K1 and K2.

• K1 (phylloquinone) is abundant in plant foods, especially leafy greens. No need to supplement.
• K2 (menaquinones) is scarce in plant foods. Supplementation may be appropriate on a plant-based diet.

Vitamin K is important for the following reasons:

• Essential for blood coagulation and bone metabolism.
• Allows the body to utilize the calcium needed for bone and tooth formation.
• Helps prevent the calcification of soft tissue.
• Low levels are associated with a higher risk of hip fracture or low bone mineral density (BMD).
• Higher intake is associated with a lower risk of bone fractures.

Vitamin K also allows the body to utilize the calcium needed for bone and tooth formation and helps prevent calcification of soft tissue.^1,2 Many studies have associated low vitamin K status with a higher risk of hip fracture or low bone mineral density (BMD),^2-5 and, conversely, higher vitamin K intake is associated with a lower risk of fractures.⁶

Are There Differences Between K1 and K2?

Vitamin K exists in two major forms in nature, K1 and K2. Vitamin K1 (also called phylloquinone) is easy to obtain when following a Nutritarian diet since it is abundant in leafy green vegetables, such as kale, collards, spinach, and mustard greens. Most dietary vitamin K is K1, and up to 25 percent of vitamin K in a typical diet may come from K2.² Vitamin K2 (several different substances called menaquinones) is produced primarily by bacteria, and is scarce in plant foods. K2 is more difficult to get from a Nutritarian diet since it is primarily found in animal products, particularly in dark meat chicken, pork, and fermented foods like cheese and yogurt. The human body can synthesize some K2 from K1, and intestinal bacteria can produce some K2, but these are small amounts, and it is unclear whether any significant amount is absorbed after production by the microbiome.⁷

The evidence is inconclusive on whether there are benefits of vitamin K specific to K1 or K2. There is some evidence that K2 is more bioavailable, and has higher functional activity than K1, particularly for functions outside of blood coagulation. The different chemical structures of vitamins K1 and K2 affect their bioavailability and metabolism, with K2 having a longer half-life in the circulation system. A study comparing K1 and K2 in humans found that the two were similarly absorbed, however, K2 (as menaquinone-7, MK-7) had higher bioavailability. Supplementation with K2 resulted in higher and more stable circulating levels of vitamin K compared to K1, and greater carboxylation of osteocalcin (a measure of noncoagulation vitamin K function).^2,8-10

Research on K2

Bone Health

Conclusions are difficult to draw from trials investigating vitamin K2 supplementation for fracture prevention. There are three reasons for this: First, meta-analyses have combined studies of K1 and K2 supplementation. Second, a few studies from one research group were retracted; and third, most trials haven’t documented K2 status or K2 intake at the start (if participants were already getting plenty of vitamin K as K2, we would not expect to see any further benefit from taking more K2).

Of the five individual trials analyzed in a recent meta-analysis that investigated MK-7 (the most bioavailable form of K2, and the most common form in supplements) supplementation for bone health,^11,12  three reported improvements in bone health indicators such as BMD:

• Postmenopausal women consumed dairy products enriched with calcium plus vitamin D3, and either no vitamin K or supplemental K1 or supplemental K2, for 12 months. All three groups improved total BMD, and the two vitamin K groups also increased lumbar spine BMD.¹³
• In postmenopausal women taking vitamin K2 or a placebo daily for three years, decreases in age-related bone loss were documented at the lumbar spine and femoral neck, but not the total hip. Bone strength also increased.¹⁴
• In postmenopausal women, after one year of K2 supplementation, BMD and bone thickness remained constant, whereas in the placebo group, BMD and bone thickness decreased, suggesting that supplementation helped prevent age-related bone loss.¹⁵

Coronary Calcification

A vitamin K-dependent protein (matrix Gla protein, MGP) inhibits the calcification of soft tissues, including the arterial walls.¹⁶ Coronary artery calcification is a predictor of cardiovascular events like heart attack and stroke.¹⁷ Several observational studies have found a reduced risk of coronary heart disease or coronary calcification associated with higher intake of vitamin K2 specifically.^18-23

Notably, in some of these studies, there was an association between higher K2 intake and lower risk of arterial calcification or coronary heart disease, but not for K1.^20-22 It is unclear why these different results have been found for K1 and K2; it could be due to differences in bioavailability or biological activity between the vitamin K forms.^2,24

Similar to bone health, trials on K2 supplementation measuring arterial calcification are difficult to interpret because most did not assess K2 status or intake prior to supplementation. So far, K2 supplementation trials have not reported reductions in calcification.²⁵ The existing trials on K2 supplementation have been conducted on the general public (not vegetarians or vegans), who are most likely already getting some K2 from animal products.

Should Vegans (and Others With Low K2 Intake) Supplement With K2?

I recommend supplementing with a conservative amount of K2 because this vitamin is not present in most plant foods, and K2 may provide some benefits that K1 does not. Whether K2 supplementation of a plant-based diet (which includes little to no K2) helps prevent fractures or arterial calcification has not been directly studied yet.

Although there is no evidence of toxicity associated with vitamin K1 or K2 supplementation,²⁶ and many trials have used higher doses of K2, I recommend caution with the dose of K2 supplements since vitamin K is a fat-soluble vitamin (fat-soluble vitamins generally accumulate more in the body than water-soluble vitamins). My multivitamin and mineral supplements include approximately one-third of the recommended daily intake for vitamin K as K2 (30 to 40 micrograms).

Originally published on DrFuhrman.com