The sunshine vitamin
Vitamin D has long been known as the "sunshine vitamin" because the body can produce it when the skin is exposed to ultraviolet rays. In fact, the ability to synthesize vitamin D may have played a role in human evolution. "There's reason to suspect that the origin of human populations with fair skin may have involved an evolutionary selective advantage as they migrated to higher latitudes," explains David E. Fisher, M.D., Ph.D., chief of Massachusetts General Hospital's Department of Dermatology. "Those with lighter skins were able to produce more vitamin D from minimal UV doses, whereas those with darker pigment would not have been able to produce enough vitamin D to stay healthy at northern latitudes."

It is estimated that it takes about three to eight minutes of exposure to midday summer sun (without sunscreen) for the body to synthesize 400 international units (IUs) of vitamin D. And while that may sound like the easiest way to get your vitamin D, the method is not without drawbacks. "Sunlight comes with a carcinogen," says Fisher. "Exposure to UV rays is linked to both melanoma and non-melanoma skin cancers." The other problem with using the sun to help your body produce vitamin D is that it's not an exact science. There are too many variables -- the color of your skin, the time of day, the time of year, where you live, how much skin is exposed, the weather -- to be able to accurately prescribe an exact daily dose. "Those variables could have a huge impact on how much vitamin D you're really getting," says Fisher. "That is not the way we practice modern medicine."

Deficiency -- more likely than overdose
The question of how much vitamin D we really need is still hotly debated. The current government recommendations call for just 200 to 400 IUs daily. And while that may be enough to maintain strong bones, all of the research looking into the myriad other possible health benefits (including lower risks of diseases such as multiple sclerosis, heart disease, cancer, and respiratory and bacterial infections) has subjects taking significantly higher doses. "The optimal blood level of vitamin D for an adult is between 50 and 70 nanograms per milliliter," says Bruce W. Hollis, M.D., professor of biochemistry and molecular biology at Medical University of South Carolina. "In order to maintain that, you need to be getting between 3,000 to 6,000 IUs a day." Hollis says that in all of the research he has seen -- in populations including adults, pregnant women, and older men and women -- there is no evidence of adverse effects, even at high dosages. "You're much more likely to be deficient in D than you are to overdose on it," he says.

That said, it's worth consulting with your doctor before you start taking megadoses of any nutrient, including vitamin D. A quick test can assess your blood levels of vitamin D, and your doctor can then recommend whether, and to what degree, you should supplement. And while vitamin D can be found in certain foods -- things like fortified milk, sardines, salmon and cod liver oil are good sources -- experts say most people probably don't get enough from diet alone.

Stay tuned for new, higher government recommendations for vitamin D in the near future. The Institute of Medicine is currently looking at the guidelines and a new number is pending.

Sally Wadyka is a freelance writer based in Boulder, Colorado, who specializes in health, nutrition and fitness. Her work has been featured in such publications as Shape, Real Simple, The New York Times and Runner's World, as well as on MSN. When not writing, she can be found doing activities that feed her body, mind and soul -- such as hiking, trail running, rock climbing and skiing in the Rockies.