Vitamin K1 (phylloquinone) is dietary vitamin K. Sources include green leafy vegetables (especially collards, spinach, and salad greens), soy beans, and vegetable oils. Dietary fat enhances its absorption. Infant formulas contain supplemental vitamin K. After the neonatal period, bacteria in the gastrointestinal tract synthesize vitamin K, which is absorbed and used by the body.
Vitamin K2 refers to a group of compounds (menaquinones) synthesized by bacteria in the intestinal tract; the amount synthesized does not satisfy the vitamin K requirement.
Vitamin K controls the formation of coagulation factors II (prothrombin), VII, IX, and X in the liver (see table Sources, Functions, and Effects of Vitamins). Other coagulation factors dependent on vitamin K are protein C, protein S, and protein Z; proteins C and S are anticoagulants. Metabolic pathways conserve vitamin K. Once vitamin K has participated in formation of coagulation factors, the reaction product, vitamin K epoxide, is enzymatically converted to the active form, vitamin K hydroquinone.
The actions of vitamin K–dependent proteins require calcium. The vitamin K–dependent proteins, osteocalcin and matrix gamma-carboxy-glutamyl (Gla) protein, may have important roles in bone and other tissues. Forms of vitamin K are common therapy for osteoporosis in Japan and other countries.
(See also Overview of Vitamins.)