It's only a simple vitamin, but it sounds like a miracle drug -- one that top researchers say may help you stave off everything from osteoporosis to multiple sclerosis to diabetes, schizophrenia and breast cancer.

A series of recent studies has found that vitamin D, the so-called sunshine vitamin -- once thought to be critical only to bone health -- is useful throughout the body to strengthen the immune system and control cell growth. Yet researchers estimate that as many as half of all Americans are likely deficient in the nutrient.

The problem is particularly serious in people with dark skin, the elderly and those who are diligent about avoiding sun exposure, especially if they adhere to the federal government's current recommended dietary allowance of the vitamin, which many say is far too low.

"What we're finding is really challenging physicians' long-standing knowledge about vitamin D," says Dr. Lisa Bodnar, assistant professor of epidemiology at the University of Pittsburgh. She is co-author of a recent study that found more than 80 percent of pregnant black women and nearly half of pregnant white women -- and their subsequent newborn babies -- to be vitamin D-deficient. The women lived in the Pittsburgh area, but Bodnar says the findings likely apply throughout the northern United States, where there is little wintertime sun, and perhaps to the South, as well.

"If people go to their doctor and say, 'I think I should be taking more vitamin D,' it's very likely their doctor will say they don't need it. But what people have to understand is that the [current] dietary recommendation is not based on the best science. And more science is coming out now confirming that vitamin D intake has got to be higher if we're going to prevent some of these problems."

In fact, current research indicates vitamin D deficiency plays a role in 17 varieties of cancer, plus heart disease, stroke, hypertension, diabetes, depression, chronic pain, osteoarthritis, osteoporosis, muscle weakness, muscle wasting, birth defects, periodontal disease and auto-immune diseases such as lupus. It may not be the lone cause of these problems, the experts caution, but it appears to be a contributor.

One study in Finland, for instance, followed children given a whopping 2,000 international units (IU) of vitamin D per day -- 10 times what the U.S. government currently suggests -- from birth through their first birthday. After three decades, those who had received the supplements were found to have reduced their chance of getting Type I diabetes by 80 percent compared with those who were given no supplement.

"The data are also pretty strong in relating adequate vitamin D levels to decreased risk of developing and dying of colon, prostate, breast and pancreatic cancer," says professor Dr. Michael Holick, director of the Bone Health Care Clinic at Boston University Medical Center and considered one of the world's foremost authorities on vitamin D. "We also [think] now that the reason African-Americans are more prone to getting tuberculosis and have more aggressive disease is that they're more prone to vitamin D deficiency."

Beware of fortified foods

Historically, mankind's main source of vitamin D has been sunlight, which penetrates the skin's uppermost layer and, after processing in the liver and kidneys, becomes cholecalciferol -- the hormone responsible for vitamin D's healthful benefits.

Such a system worked well as long as man spent most of his days outdoors with little clothing. But our relatively recent indoor lifestyle -- as well as dark pigmentation, aging and using sunscreen -- can all interfere with that process. A mere SPF-8 sunscreen, for instance, cuts vitamin D synthesis by roughly 95 percent.

Complicating matters is the fact that few foods offer even modest amounts of natural vitamin D, and those that do are often high in fat so should be consumed in moderation. They include cream, egg yolks, cod liver oil and fatty fish.

And though milk and some cereals are fortified with D in this country, the vitamin is often delivered in the form of synthetic D2, which is more stable but not easily used by the body. Many multivitamins also use D2.

"Make sure you're taking D3, . . . the same compound that is made in sun-exposed skin," advises best-selling author Dr. Andrew Weil, a physician and strong proponent of holistic medicine. "So often, you see orange juice or foods that say they're vitamin D-fortified. But many times, those are fortified with D2. [And] vitamin D has to be taken with fat. Taking a vitamin D pill with orange juice isn't going to work; it won't stick to you."

Too much of a good thing?

Vitamin D has long been known to prevent rickets -- a disease that can leave children with soft and misshapen bones. For years, the United States has based its advice on how much vitamin D people need on the minimum necessary to prevent rickets. Too much D, they warned, could be toxic.

But scientists now say those warnings have been overblown.

"Even at 10,000 units of vitamin D, I know of no adverse health consequences seen in studies," Bodnar says. "There's a study right now going on in pregnancy, for instance, that is looking at the safety and effectiveness of supplementing pregnant women with up to 4,000 units a day. And they're doing it in both black and white women to see, for instance, if black women need more vitamin D simply because of the dark pigment in their skin."

The consensus at this point, though, is that 1,000 to 2,000 units a day is probably a good amount to maintain health. Holick says he -- and everyone in his family -- now takes 1,000 units daily.

Still, the government's official recommendation, last revised a decade ago, is a mere 200 IU for people age 50 and younger, 400 IU for those 51 to 70 and 600 IU for those 70 and older.

Why the discrepancy?

"It has taken science and medicine a long time to figure out how vitamin D really works," explains Dr. Luke Bucci, a biochemist and vice president of research for Schiff Nutrition International. "People were afraid because they thought it accumulates in the body. Well, no, it doesn't.''

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