Jarrow Formulas Vitamin D3 supports calcium metabolism, bone health and immune function.* Jarrow Vitamin D3 provides cholecalciferol, which is the form produced by the (human) skin in response to UVB exposure (sunlight) and may also improve vitamin D status better than equivalent amounts of ergocalciferol (D2).* Vitamin D3 is ultimately converted to its hormonally-active form, calcitriol, in the body as needed. Calcitriol enhances dietary calcium and phosphorous absorption, supports the production of osteocalcin (an important structural protein in bone), and supports a healthy immune response.*