Medical Information On Vitamin D

Vitamin D is a group of fat-soluble vitamins, the two main forms of which are vitamin D2 and vitamin D3. The former is formed by ultraviolet irradiation of ergosterol in plants while the latter is derived similarly by the action of ultraviolet radiation on 7-dehydrocholesterol in the skin of animals. Under most conditions, synthesis of vitamin D in this way is the main source of the vitamin, especially considering that very few dietary sources exist.

Metabolism

Vitamin D taken with food is absorbed in lipid micelles and incorporated into chylomicrons. Whether obtained from food or synthesized in the skin, vitamin D is first hydroxylated to 25-hydroxyvitamin D in the liver and then undergoes a second hydroxylation reaction in the kidneys to yield 1,25-dihydroxyvitamin D, the active metabolite involved in calcium homeostasis. 1,25-(OH)2D is transported to the various organs via a carrier protein known as vitamin D binding protein, and it mediates its effects by binding to the vitamin D receptor located in the nuclei of target cells.

Calcium Homeostasis

The main physiological role of vitamin D is regulation of the plasma concentration of calcium. 1,25-(OH)2D, also known as calcitriol, achieves this by augmenting intestinal calcium absorption, mobilizing osteoclast activity and inhibiting calcium excretion. Calcium is essential for the proper functioning of cells in practically all tissues of the body, including muscles, nerves and glands.

Other Roles

M.F. Holick, in "Advances in Experimental Medicine and Biology," points out that as various non-calcium-regulating tissues, such as the prostate and colon, express 1α-hyrdoxylase, they locally convert 25-OHD to calcitriol, and this has the potential to regulate genes and facilitate cell growth and differentiation. Calcitriol is also implicated in modulating insulin, parathyroid and thyroid hormones, inhibition of interleukin and immunoglobulin production and is necessary for the maturation of monocyte precursor cells.

Vitamin D Deficiency

Vitamin D deficiency in children can lead to rickets, while the equivalent condition in adults is osteomalacia. In rickets, the bones are not mineralized properly as a result of poor absorption of calcium due to inadequate levels of calcitriol. In osteomalacia, because of low levels of calcitriol, the bones are demineralized and progressively weaken.

Supplementation

E.M. Nunnelley Hamilton, F.S. Sizer, E. Whitney, and E.N. Whitney warn that supplementation of vitamin D should be done with care because it is the most potentially toxic of all vitamins. Chronic ingestion of too much vitamin D can lead to the deposition of calcium in soft tissues, such as the heart and blood vessels, potentially causing death. Some infants are particularly sensitive to high levels of vitamin D. Unlike high-dose supplements, sunlight poses no risk of vitamin D toxicity because sunlight itself breaks down excess vitamin D synthesized in the skin.