Jarrow FORMULAS® Vitamin D3 provides cholecalciferol, the most bioavailable form of vitamin D. Vitamin D3 is converted by the kidneys into the hormone calcitriol, the active form of vitamin D3. Calcitriol enhances calcium and phosphorous absorption, stimulates the synthesis of osteocalcin (an important structural protein in bone), and is involved in proper cell differentiation (changing of cell function), including in prostate cells and immune function.*