• Vitamin K1, also known as phylloquinone or phytonadione, is found in green plants and is the form included in dietary supplements.
• Vitamin K2, also called menaquinone, is produced by bacteria, including some found in the human intestinal tract.
• Vitamin K3 or menadione, a synthetic derivative, is used as a source of vitamin K in animal feeds.

Vitamin K is an essential cofactor for enzymatic activity. It assists in the enzymatic carboxylation of glutamic acid, an amino acid found in most proteins. This carboxylation occurs after the amino acid has been incorporated into the protein chain. The added carboxyl (-COOH) groups provide a site at which calcium can bind to the protein. Through the above activity, vitamin K is involved in converting an inactive precursor of prothrombin (blood coagulation factor II) into biologically active prothrombin.

Vitamin K is similarly involved in the synthesis of at least five other proteins involved in the regulation of blood clotting. In all cases, it is thought that vitamin K assists in activating these proteins through establishment of calcium-binding sites. Other vitamin-K-dependent proteins whose function depends on calcium binding have been identified in bone, kidney, and vascular tissues. In bone, these proteins appear to be involved in bone crystal formation and bone remodeling. As a result, the potential role of vitamin K in osteoporosis has received increasing attention.

The best food sources of vitamin K are green vegetables, including spinach, broccoli, Brussels sprouts, kale, and turnip greens.

No known toxicity is associated with the administration of high doses of the natural phylloquinone form of vitamin K.

Additional Resources:

• Vitamin K - U.S. National Library of Medicine
• Vitamin K - Physicians' Desktop Reference
• Vitamin K - Wikipedia
• Vitamin K - Linus Pauling Institute