Jarrow Formulas® Vitamin D3 provides cholecalciferol, which is more biologically active than ergocalciferol (D2). Vitamin D3 is ultimately converted to its hormonally-active form, calcitriol, in the kidney. Calcitriol enhances dietary calcium and phosphorous absorption, stimulates the synthesis of osteocalcin (an important structural protein in bone), and supports a healthy immune response.