The primary function of vitamin K is to assist in the process of normal blood clotting. It is also required for the formation of other kinds of body protein found in the blood, bone, and kidneys. However, research has revealed another side to vitamin K that most people were not aware of: It is vital for building healthy bones, which is why a number of calcium supplements are now being formulated with vitamin K. With a shortfall of vitamin K, bone can become weakened because of insufficient levels of osteocalcin, a protein involved in bone hardening. In one study with female athletes, 10 milligrams daily of vitamin K decreased the process of bone breakdown and increased bone formation. These improvements were measured by looking at the amount of osteocalcin (an indicator of bone formation), as well as at by-products of bone breakdown in the bloodstream and urine. A vitamin K deficiency is extremely rare, and there’s usually no need for supplementation unless recommended by your physician. The best food sources are dairy products, meats, eggs, cereals, fruits, and vegetables.