Shining Light on Vitamin D Health Article

Vitamin D is so critical to our health that nature designed a fail-safe way to obtain it: from the sun. Throughout history, exposure to the sun gave humans the vital doses needed to build bones, protecting children against the characteristic bowed legs of rickets and adults from osteomalacia, or softening of the bones.

Certainly, some of our vitamin D is provided by foods—fatty fish and egg yolks, for example. But until 1931, when milk began to be fortified with this fat-soluble vitamin, the sun was our main source, its ultraviolet B rays penetrating the skin’s uppermost layer, causing skin cells to produce a vitamin D precursor. (The precursor, along with vitamin D from food, is processed by the liver and kidneys and converted to D3—the active form of the vitamin.)

Now, we spend more time indoors, in cars or behind computers. We drink less milk and when we do go out, slather on sunscreen. It is no coincidence, experts say, that rickets (which had been virtually wiped out until the 1990s) is making a comeback. Scattered cases of rickets in African-American infants and breast-fed babies have been documented as far south as Georgia; just last February, researchers at the University of Pittsburgh found that over 80 percent of pregnant black women and nearly half of pregnant white women (and the babies they later gave birth to) were classified as "insufficient" or "deficient" in vitamin D. Older people in hospitals and nursing homes are especially likely to lack the vitamin; in one study, 57 percent of elderly patients admitted to a Boston hospital were found to be vitamin D-deficient, according to blood samples and diet records. What’s more, dietary surveys suggest that most Americans, young and old, aren’t getting recommended amounts of the vitamin.

Ironically, these deficiencies have been coming to light at the same time research is uncovering promising new roles for vitamin D—much of it suggesting that higher (often much higher) daily doses of the vitamin are needed for optimal health. Researchers have long been urging policy makers to rethink current dietary recommendations, and in recent months their voices have been getting louder (see "The New D Debate," page 50). Here, some light on the debate about vitamin D—and what you can do now to stay healthy.

The Promise of D

Vitamin D is essential for helping bones absorb calcium, keeping them strong and preventing osteoporosis. Vitamin D also helps maintain muscle strength and balance and lowers the risk of bone fractures in older people. But it seems meant to do more: there are receptors for vitamin D in almost every cell in the body, and evidence is growing that the vitamin helps the immune system function and regulates cell growth.

One clue is the success of vitamin D in treating psoriasis: topical applications of the vitamin help suppress the proliferation of skin cells, reducing the size of the disease’s lesions. Likewise, vitamin D may help quell the uncontrolled growth of cancer cells.

Exciting new research suggests that vitamin D may offer protection against some types of cancer, including breast and prostate cancers and, in particular, colorectal cancer. How vitamin D fights cancer isn’t known for sure, but it "helps reduce cell proliferation and differentiation, and it may reduce inflammation," says Edward Giovannucci, M.D., professor of nutrition and epidemiology at the Harvard School of Public Health in Boston. Giovannucci, whose work has reported increased risks of digestive system cancers among people with low vitamin D levels, threw down the gauntlet in a keynote speech at the American Association of Cancer Research in 2004. "I would challenge anyone to find an area or nutrient or any factor," he said, "that has such consistent anti-cancer benefits as vitamin D."

Research is also revealing vitamin D’s promise in autoimmune disease. For example, a sufficient level of vitamin D may confer some protection against developing multiple sclerosis (MS), a neurologic disease affecting over 2 million people worldwide. Last year, in a study based on blood samples from more than 7 million military personnel, researchers from the Harvard School of Public Health found that those with the highest blood levels of the vitamin were 62 percent less likely to develop MS than those with the lowest vitamin D levels.

Diabetes is also considered an autoimmune disease, since it involves immune-cell attack on the insulin-secreting cells of the pancreas—and vitamin D may have a protective role. In a landmark study from Finland, researchers found that children who were given high-dose vitamin D supplements in their first year of life were nearly 80 percent less likely to have developed diabetes 30 years later when compared with a similar group that did not receive supplements.

Experts are not sure about how vitamin D protects against autoimmune diseases, but believe that it may serve as a brake on the overactive immune cells. "Vitamin D may decrease the development of type 1 T-helper cells," explains Charles Stephensen, Ph.D., a research scientist at the USDA’s Western Human Nutrition Research Center at the University of California, Davis. These cells are involved in protective immune responses, "but they may also initiate autoimmune disease, especially in people who may have a genetic predisposition."

Other, preliminary research hints at a connection between inadequate prenatal vitamin D and asthma in young children. Recent data even suggest that low vitamin D may be linked with epidemic influenza (which tends to strike, after all, during sun-deprived winter months).

How Much is Enough?

For now the U.S. recommendations for adequate intake of vitamin D are 200 international units (IU) per day for adults under age 50, 400 IU for adults aged 50 to 70 and 600 IU for adults over 70.

"How much you need from your diet is inversely proportional to what you make from the sun," explains Giovannucci. "Lifeguards probably don’t need any from their diet." But the rest of us need to be more vigilant, unless we take supplements or love drinking milk. (Take our quiz on page 49 to get an unscientific, but helpful, sense of your vitamin D status.)