Jarrow Formulas Vitamin D (cholecalciferol) may help improve vitamin D status better than equivalent amounts of ergocalciferol (D ). D is also the form produced by skin with Uvb exposure (sunlight). Vitamin D is 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.