Q: I've read several of the Health Bytes emails that mention the health benefits of vitamin K. However, when I went to the health food store to buy some, I saw that there were several different versions – K1, K2, or K3. Can you explain the difference between them?

Dr. Wright: The natural, plant-synthesized dietary source of vitamin K is called phylloquinone, though it's still frequently referred to as vitamin K1. Intestinal bacteria (including normal E. coli and Bacteroides species) also synthesize various forms of vitamin K, previously termed (as a group) vitamin K2 but now generally called menaquinones. Menadione, an entirely synthetic molecule with vitamin K activity, was developed in the 1940s and called (at the time) vitamin K3.

There's usually much more of the menaquinones (vitamin K2) than phylloquinone (vitamin K1) found in our livers. But phylloquinone is the major active form of vitamin K used by our bodies – the menaquinones are not used as effectively. Menadione, being a synthetic molecule, is not normally found in our bodies except when deliberately supplemented.

Both phylloquinone and the menaquinones are fat-soluble and absorbed along with other dietary fats. Menadione (vitamin K3) is water-soluble. Phylloquinone has no known toxic effect; high doses of menadione, however, on rare occasions has caused hemolytic anemia (easily breaking blood cells) and liver toxicity.