What we've known for a long time:
Studies on Vitamin D began when people began to notice that children working in factories in the early years of the Industrial Age had a high incidence of rickets, a bone disease. Since it was also an age of great interest in infectious disease, that was investigated as a possible cause. But, soon, the researchers noticed that children who lived in the countryside were much less likely to have rickets, and eventually it became clear that being outdoors and in clear air (instead of smog) was related to their bone health. A substance was also found in cod liver oil and other foods that was protective against rickets. It was isolated and named Vitamin D. As time went along, more details were filled in. Part of the definition of a vitamin is that lacking that substance in one's diet leads to a specific disease. The RDA for Vit. D was based on the amount needed to prevent rickets, and was set at 400 IU (10 mcg) per day. This is about to be increased to 400 IU/day for infants; 600 for those at ages 1-70; 800 for those over 70.
What we're finding out now:
In some respects, Vit. D is more like a hormone. It is made in the body in a complex process that begins with ultraviolet-B light, in certain intensities, acting on a substance in the skin similar to cholesterol; in other words, being in the sunshine. It then can be activated and sent to the cells to do a variety of things, not just regulate calcium (e g. for strong bones). It appears to influence how cells reproduce themselves, and to modulate some aspects of the immune system. The details on how it does this are currently the focus of intense research. Many diseases, such as cancer, are caused by a combination of factors, not just one thing. But, the more factors you can control, the better you can lower or delay your chance of getting those diseases. Right now, it looks like insufficient Vit. D increases the risk of hypertension (and associated cardiovascular disease); diabetes; multiple sclerosis; lupus; cancer of breast, colon, prostate; and, of course, osteoporosis. It may also have an affect on your general resistance to viral diseases.
Blood tests for the amount of Vit. D in the body have become widely available only recently. As more safety studies are done, it is more and more evident that the optimum amount in the blood and the safe daily intake had been underestimated. Although some researchers believe that higher intakes are safe, none recommend over 30,000 IU/week and most recommend less.
Under ideal circumstances, you can make enough on your own by being in the mid-day sun for a total of 2 hours/week. Of course, this much exposure causes concern about skin cancer! (Although, even 10 minutes before you put on sunscreen helps.) The amount of Vit. D your body can make during exposure to the sun is lower as you get farther from the equator or as the days get shorter during the course of a year. Vitamin D is also sold in supplements. D-3 is most effective.
The bottom line: If you don't get the amount of sun exposure that leads to optimum Vit D levels (which is common), you would probably benefit from supplements of 1000-2000 IU/day of Vitamin D-3. If you are considering taking more, discuss it with your doctor. As with any supplements, discuss with a doctor before giving it to a small child or someone with a chronic illness. (And remember, when a doctor asks what medications you are taking, that includes supplements and over-the-counter, not just prescriptions.)
Humorous side note: Cathy Breedon, at U. of North Dakota, is known to comment that many people don't get enough Vitamin D because of covering themselves up. Some people do this because of concern about skin cancer; some do it because of their religion, and some do it as a public service.
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