As spring weather begins to warm the outdoors, many of us around the country are venturing outside to enjoy the bright sunshine. At the same time, exposure to ultraviolet radiation is replenishing our bodies' supplies of Vitamin D.
Today we take sunshine for granted. But after the industrialization of Northern Europe, crowded housing, narrow streets and smoke-filled skies shut out sunlight and produced an epidemic of the D-deficiency disease, rickets, which lasted for many years.
A scientist ahead of his time, the 19th-Century French physician Armand Trousseau, linked rickets to inadequate diet and sunless climate. He prescribed cod-liver oil and exposure to the sun, bolstered by "good general alimentation (nourishment)."
Unfortunately, by 1900 his precepts were forgotten. Most medical authorities believed that rickets was a chronic infectious disease like tuberculosis and advocated development of a vaccine. Cod-liver oil was dismissed as useless.
Two Sources of Prevention
Finally, in 1919, it was recognized that rickets could be cured by exposure to ultraviolet rays. When cod-liver oil also was hailed as an effective treatment, confusion followed. Then proof emerged that there were two distinct sources of prevention: ultraviolet irradiation of the skin, and foods.
These findings came about the time that vitamins were being discovered. The substance in cod-liver oil known to both prevent and cure rickets was considered a food factor and named Vitamin D.
As it turns out, cod-liver oil is one of few naturally occurring sources of the vitamin. The others are egg yolk, fatty fish, liver and butter. But these foods are either not eaten enough or in large enough amounts, or do not contain sufficient quantities of the vitamin to make meaningful contributions. Moreover, the amounts in foods vary considerably, depending on the animal's diet and its exposure to sunlight. Ultraviolet light, then, is the primary way nature intended us to get our Vitamin D.
Research has shown that Vitamin D itself has no biological activity in the body. Before it can be used, it must undergo two reactions, the first in the liver and the second in the kidney. When activated, Vitamin D's most important jobs take place at the intestine, where it facilitates calcium absorption, and in the bone, where it stimulates calcium release.
Vitamin Plays Key Role
Vitamin D is usually associated with bone formation. But it must first maintain blood-calcium levels, critical for the control of nerve conduction and muscle contraction, within strict limits. If insufficient calcium is absorbed from the diet to satisfy these needs, it must be removed from its storage site. Vitamin D plays a key role in this process.
Since it can be produced in the skin, does anyone have to worry about dietary intake? Several factors can hamper Vitamin D formation. In today's society, many people work inside. When they do go outdoors, the sun's rays are often blocked by tall buildings or air pollution, not unlike the situation in post-industrial Europe.
In temperate climates, time spent outside is especially limited during the winter months. Ultraviolet exposure is less intense. Serum levels of Vitamin D are known to drop between fall and spring in the colder areas. Also, dark pigmentation prevents a significant amount of ultraviolet rays from penetrating the skin, thereby limiting Vitamin D synthesis.
This does not mean that Vitamin D supplements are in order. The Recommended Dietary Allowance for children through adolescence, and for pregnant and lactating women, is 400 International Units, an amount provided by a quart of Vitamin D fortified milk. Half that quantity of milk will meet the RDA for other adults. And there are many good reasons to drink that much.
Supplements May Be Needed
For people who are housebound and who do not consume enough milk, supplements may be necessary. Individuals taking anti-convulsant medication, which speeds the metabolic breakdown of Vitamin D, may also require an additional source of the vitamin. Any supplements should be prescribed by a physician.
Whether breast-fed babies should receive extra Vitamin D is the subject of controversy. Mother's milk contains little of the vitamin, but some experts believe that supplements, while not harmful, are superfluous if the mother's intake was adequate during pregnancy. Again, it is the pediatrician who must decide.
Oversupplementation, at levels not all that high above the RDA, can bring toxic effects. Overdosing is therefore extremely dangerous. By contrast, our skin has a built-in safety system. After a certain amount of ultraviolet exposure, Vitamin D synthesis shuts down. The raw materials from which it is manufactured instead produce inactive metabolites, ruling out danger of toxicity from exposure to the sun.
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