Jarrow Formulas 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.