Cholecalciferol is known as vitamin D 3 . As early as 1870 people knew there was something in cod-liver oil that prevented rickets, a disease resulting in soft, deformed bones as a result of calcium deficiency. Sir Edward Mellanby contributed significantly to the understanding of the role of vitamin D: "It was in 1919/20," cites a University of California study, "that Sir Edward Mellanby, working with dogs raised exclusively indoors (in the absence of sunlight or ultraviolet light), devised a diet that allowed him to unequivocally establish that the bone disease rickets was caused by a deficiency of a trace component present in the diet." Mellanby is also credited with confirming that cod-liver oil could provide the missing vitamin. Calciferol was the second vitamin discovered that is soluble in fat; vitamin A was the first.
Normally, it would seem unusual to expose food to ultraviolet (UV) radiation before trying to isolate a nutrient, but starting in the 1890s reports indicated that once exposed to high levels of sunshine, humans were unlikely to develop rickets. Scientists now know that UV exposure is essential for the body to produce cholecalciferol from cholesterol. See Figure 1 for the structures of cholesterol and cholecalciferol. Cholesterol is the steroid lipid often associated with heart disease when too much is present; small amounts of cholesterol are needed to make cholecalciferol and a host of other steroid hormones. Since humans can manufacture all the cholecalciferol needed for good health from exposure to sunshine, vitamin D is commonly referred to as the sunshine vitamin.
Vitamin D is involved in phosphorous and calcium metabolism . Cholecalciferol is converted into compounds that are directly involved in the absorption of calcium by the intestines. As with many steroid compounds, when cholecalciferol breaks down in the body, the resulting molecules are carried into the nucleus of certain cells and determine or change which genes are turned on or off. Recent reports suggest that the steroid hormone character of vitamin D may provide some anticancer activity.
To help prevent vitamin D deficiency, most milk is now enriched with it. Other good sources of the vitamin are meat, poultry, fish, peanut butter, eggs, margarine, and liver, especially cod-liver oil. These sources exist in addition to the vitamin D produced naturally from exposure to sunshine. Unlike water-soluble vitamins, such as vitamin C, fat-soluble vitamins are stored in the liver and fat tissue and can become highly toxic if taken in excess. For these reasons, most nutrition experts do not generally recommend additional dietary supplements for vitamin D. Some individuals taking special medication for cholesterol, or who have dark skin (which reduces the ability of sunshine to produce vitamin D), may require a diet rich in vitamin D or a supplement.
Anderson, Jean, and Deskins, Barbara (1995). The Nutrition Bible. New York: William Morrow.
Metzler, David E. (1977). Biochemistry: The Chemical Reactions of Living Cells. New York: Academic Press.
"The Vitamin D Workshop." Available from http://vitamind.ucr.edu .