Vitamin D, an essential nutrient with versatile functions in nearly all organs.

For decades, vitamin D has been known to be essential in the development, function, and maintenance of healthy bones through the regulation of calcium homeostasis throughout life. Sufficient vitamin D prevents the occurrence of rickets in children and osteomalacia in adults. The adequate nutritional intake of vitamin D and calcium are the basis for the prevention and management of osteoporosis, a disease producing brittle bones that are prone to fractures. Vitamin D has been implicated in the regulation of neuromuscular function and in reducing the risk of falls, a major cause of bone fractures. Thus vitamin D may be a central component of musculoskeletal health through its beneficial effects on muscle function and bone stability. The action of vitamin D by the active metabolite 1,25-dihydroxyvitamin D [1,25(OH)2D], however, is not limited to its endocrine function in bone metabolism. The active metabolite behaves as a hormone and binds to the vitamin D receptor (VDR) present in nearly all tissues of the human body. In addition, the 1-alpha-hydroxylase enzyme is present not only in the kidney but also in many other organs. Both vitamin and enzyme exert their biological effects via paracrine/autocrine actions related to cardiovascular disease, diabetes, cancer, and the immune system. Thus vitamin D may show favorable effects in many organs and play a significant role in the maintenance of general health.