"D3 (cholecalciferol), a fat-soluble Vitamin D3 is converted by the kidneys into the hormone calcitriol, the active form of Vitamin D3, which affects a variety of target tissues, including bone, intestine, muscle, brain, skin and immune cells. Calcitriol enhances calcium and phosphorous absorption and stimulates the synthesis of osteocalcin, an important structural protein in bone. "