Vitamin K is a fat-soluble vitamin usually formed in the body by intestinal bacteria but also available from some plant and animal sources.

Function: Essential in the formation of prothrombin, a substance necessary for proper clotting of blood, and at least five other blood-clotting factors.

Sources: All green leafy vegetables (including lettuce, spinach, kale, and cabbage), eggs, meats, cereal grain products, fruits, and milk and dairy products.

Deficiency: vitamin K deficiency may cause bleeding disorders in premature infants with inadequate amounts of stored vitamin K, and in people on blood-thinning medications and those with fat malabsorption syndromes.

Excess: Natural forms have no known toxic effects; large doses of the synthetic version, menadione, and its derivatives cause anemia and kernicterus, a condition characterized by jaundice, in infants.