In recent years it has become increasingly clear that vitamin D is an extremely novel and important prohormone. Not only is vitamin D involved in calcium and bone metabolism including the prevention of osteoporosis, but it has several other functions of equal import. It was discovered by Dr. Michael Holick that vitamin D is converted to 1,25-dihydroxy vitamin D3 (calcitriol) in the liver and kidneys, and this latter hormone is an important factor in inhibiting cancer cell growth. Epidemiological evidence strongly suggests that maintaining adequate vitamin D levels in the blood markedly decreases the incidence of colon, breast, prostate and other cancers. Receptors for vitamin D are found throughout the body. Vitamin D may also play an important role in preventing heart disease. There are receptors for vitamin D in pancreatic islet cells involved in insulin secretion. In fact low vitamin D levels are associated with type I and type II diabetes mellitus. Vitamin D is important in cellular immunity and prevention of the autoimmune diseases. It is actively involved in brain metabolism with links to depression. Fibromyalgia is a musculoskeletal syndrome characterized by muscle pain and fatigue of unknown origin. It has been found that a majority of patients presenting with symptoms of fibromyalgia are deficient in vitamin D. Treatment with vitamin D3 is successful in relieving the symptoms of fibromyalgia in these cases.
In northern latitudes in the United States as well as in Canada, it is very common to develop vitamin D deficiency during fall, winter and spring due to decreased ultraviolet radiation (UVB) from the sun. This is especially true in northern California. Nutritionally there are few sources of vitamin D in the diet, so supplementation with vitamin D in capsule form may be necessary to prevent deficiency of this vitamin. Vitamin D is present in certain fish and fish oils and is synthesized in the skin from exposure of a cholesterol derivative to UVB. Milk products are supplemented with low levels of vitamin D. It is also in egg yolk. The level of vitamin D in the body is assayed by a blood test measuring serum 25- hydroxy vitamin D. This can be used to determine an adequate intake of vitamin D. Vitamin D supplementation is also extremely important during pregnancy to ensure proper development of the fetus and to prevent later life diseases. Forty percent of the American population is vitamin D deficient. A high percentage of pregnant women are also deficient in vitamin D. Consult your physician for advice regarding proper supplementation with vitamin D.
Update: recently conducted research suggests that ingesting a vitamin D supplement during pregnancy may reduce the risk of the child developing schizophrenia later in life. See the abstract for the article: Relation of Schizophrenia Prevalence to Latitude, Climate, Fish Consumption, Infant Mortality, and Skin Color: A Role for Prenatal Vitamin D Deficiency and Infections?
Reference: Vitamin D Deficiency, M. F. Holick, NEJM, 257, 266-281, 2007.