Vitamin D: refers broadly to the organic compounds named vitamin D2 (ergocalciferol), 25-hydroxyvitamin D2, 1?, 25-dihydroxyvitamin D2 and vitamin D3 (cholecalciferol), 25-hydroxyvitamin D3, 1?, 25-dihydroxyvitamin D3.  Vitamin D is not a true vitamin because it can be synthesized by humans with adequate sunlight exposure. The effects of vitamin D on bone mass are supported in the literature. It is likely that the major source of vitamin D for human is not dietary, but results from its manufacture by a chemical photolysis reaction in skin. Vitamin D3 itself is a biologically inert molecule. It must be activated by 25-hydroxylation in the liver to produce the major circulating form of vitamin D, 25-hydroxyvitamin D3. However, 25-hydroxyvitamin D3 is also biological inactive at physiologicl concnetrations, and it is finally activated in the proximal convoluted tubule cells of the kindey to produce 1alpah,25-(OH)2D3 (also called calcitriol).

Functions: The Vitamin D hormones have essential roles in human health which are mediated by the intracellular VDR. In particular, the Vitamin D hormones regulate blood calcium levels by controlling intestinal absorption of dieatry calcium and reabsorption of calcium by the kidenyes. The hormones also participate in the regulation of cellular differentation and grwoth and normal bond formation and metabolism. 

Vitamin D is linked to lower risks of colon, breast, prostate and ovarian cancers. People with higher levels of Vitamin D are also less likely to suffer autoimmune diseases, including type 1 diabetes and MS. Vitamin D also helps mobilize and modulate the immune system by encouraging cells to self-destruct before becoming malignant. Tissues throughout the body have receptors for vitamin D which would seem to indicate a purpose for vitamin D.

Vitamin D, especially its active form calcitriol can affect phosphate homeostasis by directly sitmulaitng interstinal absorption of phosphate. In addition, vitamin D enhances bone resorption through mobilization of calcium and phosphate into the plasma.

Vitamin D Deficiency: Vitamin D deficiency can lead to secondary hyperparathyroidism. It is characterized by abnormally elevated blood levels of parathyroid hormone (PTH) and, in the absence of early detection and treatment, it becomes associated with parathyroid gland hyperplasia and a constellation of metabolic bone diseases. It is a common complication of chronic kideny disease (CKD).

Vitamin D deficiency may also underlie a vulnerability to infections by the microbes that a natural antimicrobial, called cathelicidin, targets.

Regulation: Blood levels of 1,25-dihydroxyvitamin D are preciesly regulated by a feedback mechanisms which involves PTH. The renal 1?-hydroxylase (or CYP27B1) is stimulated by PTH and inhibited by 1,25-dihydroxyvitamin D. When blood levels of 1,25-dihydroxyvitamin D fall, the parathyroid glands sense this change via intracellular Vitmain D receptors and secrete PTH. The secreted PTH stimulates expression of renal CYP27B1 and, thereby, increases production of Vitmain D hormones. As blood concentrations of 1,25-dihydroxyvitamin D rise again, the parathyroid glands attenuate further PTH secretion. As blood PTH levels fall, renal production of Vitamin D hormones decreases. (see US 2009/0209501A1)

Recommended daily allowance of vitamin is 1000 units per day. This can be reached simply by 10 minutes out in the sun with no . So if it is currently believed to be beneficial to include at least some of your activity outside each day.