Most Americans do not get enough of this potential immune booster

By all rights, vitamin D should be the one essential nutrient we don't need to fret about. During the summer months, most of us need only about 10 to 20 minutes of direct sun on the hands and face for our skin to manufacture enough D for the whole day. To make up for any shortfall, especially in young children, milk has been fortified with D since the 1930s.

So it came as a shock a few years back when surveys found that a large number of Americans aren't getting nearly enough. In one study, for example, 32 percent of healthy young adults in Boston were found to be vitamin D-deficient.

What it does:

Early on, most of the concern focused on bones, since vitamin D, working along with calcium, helps build and maintain them. But now there are new worries. "Receptors for vitamin D are being found on all kinds of cells where we never expected to find them," says Hector DeLuca, M.D., professor and chair in the department of biochemistry at the University of Wisconsin-Madison. "That suggests it plays a much broader role in the body than we imagined." In fact, vitamin D regulates more than 50 different genes, affecting tissues throughout the body. Falling short may be risky in ways scientists never suspected.

The first disease to be linked to vitamin D deficiency was rickets, which occurs when children's bones don't get enough calcium to form normally. In adults, a shortfall in D can lead to soft and painful bones because D plays a crucial role in maintaining steady levels of calcium in the blood.

New investigations are turning up other essential functions: maintaining the immune system, controlling blood pressure, and ensuring the secretion of insulin in order to control blood-sugar levels.

Studies show that too little vitamin D is associated with a greater risk of several autoimmune diseases, including multiple sclerosis and rheumatoid arthritis. Last year, researchers at the Harvard School of Public Health reported that women who got at least 400 international units of vitamin D daily - the amount found in the typical multivitamin supplement - were 40 percent less likely to get MS than those not taking multivitamins.

Inadequate levels have also been linked to increased danger of several cancers, including colon and prostate cancer, and both insulin resistance and metabolic syndrome, the constellation of conditions that dramatically increases heart-disease risk.

How much you need:

Guidelines on vitamin D are currently under debate. The official recommendations now call for 200 international units for children and 600 IU for people over 71, with other groups falling somewhere between. But many researchers reviewing the latest findings think most adults should be getting around 1,000 IU per day and some suggest even dramatically higher levels. Currently the safe upper limit is 2,000 IU, and until new recommendations are issued, people should not exceed this level unless taking the supplement under doctor supervision; too much vitamin D can be toxic.

One way to boost vitamin D levels is simply to catch some rays when the sun is high without putting on sunscreen. But that advice is extremely controversial, since too much unprotected sun exposure can cause skin cancer. And there's another problem. "In the northern part of the country, the sun isn't strong enough in the winter months to trigger vitamin D production," says DeLuca, even if you are crazy enough to sunbathe on a rooftop in Boston in December.

Food sources of vitamin D:

We rely on fortified milk and breakfast cereals to get most of our dietary vitamin D-an 8 ounce glass of milk has 100 IU. And apart from a few kinds of fish that are naturally rich in D, including herring and sardines, there aren't many natural food sources. "That leaves supplements," says DeLuca. Like a growing number of experts, he thinks supplements, especially during winter and spring when people are most likely to be deficient, are a good idea. Standard multivitamins contain 400 IU. If you don't drink much milk, it might be worth taking an additional 400 IU through a vitamin D capsule.

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