Vitamin D: Actions for the new millennium.

The past two decades have witnessed new facets in vitamin D metabolism. Free vitamin D(3) till now considered to be limited to animal sources, has been shown to occur in plants. Further, the extrarenal synthesis of active vitamin D(3) or calcitriol has been documented in physiological as well as in certain pathological conditions. At the cellular level, calcitriol acts through a dual mechanism. The genomic pathway involves the vitamin D receptor (VDR) which is of ubiquitous distribution and linked with various diseases. Besides, a nongenomic VDR-independent pathway also exists. The biological effects of calcitriol can no longer be dismissed by merely referring to calcium/bone homeostasis. It enhances the perinatal growth and suppresses the abnormal growth, as well. The effects on the endocrine, immune, cardiovascular, reproductive and nervous systems have also been characterized. It is compelling to believe that calcitriol and/or its analogues will find extensive applications at least in the treatment of osteoporosis, perinatal growth retardation, neoplasia, psoriasis, insulin resistance and transplantation.