What Is Vitamin D?
Years ago, very few children in tropical countries developed the malformed bones and teeth characteristic of rickets. Yet many children in temperate climates and large industrial cities did. Why the difference? The sun.
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The human body can make vitamin D naturally with enough exposure to sun.
Children in temperate zones, however, got little exposure to the sun during the winter months, and their skin could not make enough vitamin D. Neither could the skin of children in large, industrial cities because the smoke-filled air filtered out much of the sun's ultraviolet light.
At one time, rickets afflicted large numbers of children in this country as well. Researchers found that the cause was that there was something preventing calcium from being deposited in the bones of these children-some substance that promoted calcium deposition was missing. From this research, investigators concluded that rickets was actually a vitamin-deficiency disease.
However, researchers were perplexed when they discovered that ultraviolet light could also prevent the deficiency. In the 1920s, nutritionists were able to prevent or cure rickets by feeding children cod liver oil or foods exposed to ultraviolet light. They also prevented rickets by exposing children to direct sunlight or the light from a sunlamp.
The explanation for these findings didn't crystallize for several more years. Cod liver oil was effective against rickets because it contains vitamin D. Foods exposed to ultraviolet light were effective because the light changed a substance in plant foods into a form of the vitamin -- vitamin D2.
Today, doctors seldom see cases of rickets in the United States. The few cases that do occur can usually be traced to poverty, neglect, or ignorance. The dramatic drop in rickets cases is primarily due to the increased availability of milk fortified with vitamin D. Choosing to fortify milk made sense because children usually drink lots of it. It's also the single best source of calcium in the American diet, and since vitamin D helps the body use calcium to build strong teeth and bones, milk was the best food to select.
Functions of Vitamin D
Vitamin D is necessary to help the body absorb the minerals calcium and phosphorus, which are needed for the proper growth and development of bones and teeth. It also regulates whether these minerals are deposited into bone or withdrawn out of bone to meet other needs. If minerals are drawn out more than they are put in, this can leave bones soft and weak. Vitamin D signals the kidneys whether to release calcium and phosphorus when the body has plenty or hold onto them when the body is running short.
Whether it comes from food or is made in the skin, vitamin D must be activated before it can be used. It first travels to the liver, where it undergoes a chemical change. Then it moves through the bloodstream to the kidneys, where it undergoes another change to become the active form of the vitamin. This active form -- dihydroxy vitamin D -- is the one that functions.
Society has recognized the importance of Vitamin D over time; but what are the central reasons for making sure you are getting enough Vitamin D? In the next section, we'll discuss the reasons this vitamin is so crucial.
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