The Food and Nutrition Board’s Adequate Intake for vitamin K is a ridiculously low 90 mcg/day for adults. One cup of cooked kale has more than 1,000 mcg of vitamin K1. A cup of cooked spinach has more than 800 mcg. And a cup of turnip greens has more than 500 mcg. But that is all vitamin K1.
How do we get our vitamin K2? Diet contains little K2 unless one eats fermented soybeans. There is a tiny amount of K2 in organ meat and hard cheeses, but hardly enough to supply the body with all the K2 it needs. Where is all this extra dietary vitamin K2 supposed to come from?
It is commonly believed that the body cannot make vitamin K2 from vitamin K1 so you have to take K2 supplements. In fact, that is one of the main theses of a recent popular book about vitamin K, called Vitamin K2 and the calcium paradox.
Is that true, do we have to take vitamin K2 or is vitamin K1 turned into vitamin K2 by our bodies?
As early as 1994, researchers had discovered that K1 converted into K2 in the body.
In 1998, researchers confirmed that K1 is metabolized into K2 and that metabolism had nothing to do with intestinal bacteria. Sterile mice metabolized ingested K1 into K2.
Ronden JE, Drittij-Reijnders MJ, Vermeer C, Thijssen HH. Intestinal flora is not an intermediate in the phylloquinone-menaquinone-4 conversion in the rat. Biochim Biophys Acta. 1998 Jan 8;1379(1):69-75.
In 2006, researchers confirmed this conversion takes place in humans.
There appears to be two sites of conversion of K1 to K2, one in the intestine and another in peripheral tissues.
Okano T, Shimomura Y, Yamane M, Suhara Y, Kamao M, Sugiura M, Nakagawa K Conversion of phylloquinone (Vitamin K1) into menaquinone-4 (Vitamin K2) in mice: two possible routes for menaquinone-4 accumulation in cerebra of mice. J Biol Chem. 2008 Apr 25;283(17):11270-9. Epub 2007 Dec 14.
Recently, the mechanism by which the body turns vitamin K1 into vitamin K2 was clarified. It occurs through an intermediary molecule, vitamin K3, which is made in the intestine from vitamin K1.
Hirota Y, et al. Menadione (vitamin K3) is a catabolic product of oral phylloquinone (vitamin K1) in the intestine and a circulating precursor of tissue menaquinone-4 (vitamin K2) in rats. J Biol Chem. 2013 Sep 30.
I cannot put my hands on it now, but I read a paper that actually showed K2 content in peripheral organs is higher after vitamin K1 ingestion than K2 ingestion.
It seems that it could be the case that modern humans are deficient in K2 because they do not eat large quantities of vitamin K1 containing foods. If we look at Paleolithic humans, they probably got high amount of vitamin K2 from eating large quantities of kale and spinach-like foods, very high in K1, which then supplied their tissues with all the vitamin K2 they needed.
Does vitamin K2 and supplementation still have a role in clinical nutrition? I think so. There are trials underway looking to see if vitamin K2 can help with a variety of conditions. It may be very important for bone health. There has also been a trial that shows it’s more efficacious than vitamin K1 in raising IGF-1 levels.
Cannell J. RCT: Vitamin D, calcium, and vitamin K’s effect on activity in postmenopausal women. www.vitamindcouncil.org, April 30, 2012.
However, as far as getting enough vitamin K2, the best thing to do is eat your greens.