Vitamin D
Of the four fat -soluble vitamins, vitamin D is the one concerned with efficient calcium and phosphorus absorption. Working with various hormones-- in particular, calcitonin and the parathyroid hormone--vitamin D's primary role is to ensure that calcium and phosphorous are absorbed from the intestinal tract into the bloodstream. There, the vitamin helps maintain concentrations of the two minerals in the right proportions (the amounts of calcium and phosphate in the blood--as well as the ratio between them--help determine normal bone growth). In addition, vitamin D also appears to act in the bones themselves, stimulating them to accept calcium into their bony matrices.
Strictly speaking, vitamin D is not one vitamin but is, instead, a group of vitamins--all derived from a parent compound, structurally similar to cholesterol--and it comes in a variety of forms. From a nutritional standpoint, two of these forms are especially important.
The first, vitamin D2 (calciferol), is produced when ultraviolet radiation activates a sterol that is present mainly in yeasts and fungi (sterols are fat-like substances in the steroid family). Vitamin D2, rarely seen in nature, is mostly manufactured in the laboratory specifically to be added to infant formulas and other fortified foods.
The second form—vitamin D3 (cholecalciferol)—is derived from a sterol present in animal tissues, including those in the human body. In humans, in fact, the sterol is converted into vitamin D on the skin's surface, a process activated by ultraviolet rays from the sun. For this reason, healthy adults, exposed to normal amounts of sunlight, may produce enough vitamin D in their own skin to avoid the need for added dietary sources.
As might be expected, the need for vitamin D is substantially greater in childhood. In infants and young children, lack of the vitamin can cause rickets (or rachitis), a bone disorder that results in bowed legs, knock-knees, curved spines and other abnormalities. Children with rickets have been discussed (and painted) since Galen's time and a detailed description of this once very common bone disorder was provided as early as the seventeenth century. The cause of rickets, however, was not discovered until fairly recently.
In 1914, Edward Mellanby (1884-1955), a brilliant young English biochemist and a student of Frederick Gowland Hopkins, was asked by Great Britain's Medical Research Council to concentrate on finding a cure for rickets. Convinced that the disorder had a dietary basis, Mellanby spent the next seven years at Cambridge University conducting feeding experiments on dogs. He finally devised a diet that helped him prove he could cure rickets by adding certain fats to his animals' rations. In 1921, Mellanby wrote that the fats' effectiveness in rickets was due "to a vitamin or accessory food factor they contain," which was probably identical to the fat-soluble vitamin that the American scientist Elmer McCollum was working with.
Shortly afterward, in 1922, McCollum and his associates, by bubbling oxygen through their "fat-soluble vitamin," discovered that it consisted of two separate vitamins-- vitamin A, which was inactivated by the oxygen--and a second vitamin, which remained active and which they named vitamin D. In that same year, McCollum and his group also determined that cod liver oil (which had for years been a folk remedy used to treat rickets) not only was an effective treatment but contained a specific antirachitic substance.
In the next few years, a number of researchers went on to show that, when the skin was exposed to sunlight or ultraviolet light, a substance virtually identical to vitamin D was produced. In 1924, two researchers, Alfred Hess (1875-1933) and Harry Steenbock (1886-1967), working independently, found that foods exposed to ultraviolet light developed substantially greater anti-rickets potency. This discovery led to the practice of irradiating certain foods--especially milk--to help prevent rickets.
In the 1930s, the chemical structures of vitamin D and its several different forms were determined by Adolf Windaus in his laboratories at the University of Göttingen.
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