Vitamin D belongs to the group of fat soluble vitamins. It is a derivative of a steroid. For centuries, the role of vitamin D in promoting the bone growth has been known. Several forms of vitamin D are available. The active form of vitamin D is called 1,25-dihydroxycholecalciferol or simply as calcitriol. This compound acts as a hormone and is produced in the body as a result of the combined actions of the enzymes present in the liver and the kidneys. The last step in the formation of calcitriol takes place in the kidneys and this step can also be brought about in the monocyte macrophages.
Like all hormones, calcitriol also requires receptors for its effects to be felt. These receptors are located in the nucleus of the target cells. Intestinal cells are obviously target cells. In the intestine, calcitriol promotes the formation of transport proteins like calbindin which promote the absorption of calcium. In addition to the role in calcium metabolism, vitamin D also has other roles. It acts as a cytokine in the macrophages where also it is produced. Thus, it is concerned in the body defense against microbial invasion. Thus vitamin D deficiency makes one susceptible to infections like tuberculosis. Vitamin D deficiency can also give rise to autoimmune diseases and cancer. The deficiency also makes one susceptible to cardiovascular diseases and hypertension.
Vitamin D deficiency is seen in areas with minimum sunlight and during periods of winter when the sun is not out. This is due to the fact that exposure to the sunlight will convert the 7-dehydrocholesterol in the skin to vitamin D. The role of vitamin D in producing rickets in the children and osteomalacia in the adults is already a well known fact. Vitamin D is readily available from fatty fish, eggs and mushroom. Vitamin D deficiency leads to drop in the levels of serum calcium. This in turn stimulates the parathyroid glands to secrete the parathyroid hormone which causes the release of calcium from the bones. This causes a decrease in the bone density and one easily becomes susceptible to fractures. This is all the more relevant to the post menopausal women who are suffering from osteoporosis. Hence, it is vital to maintain sufficient levels of vitamin D in the body.