For more than 80 years, scientists have known that vitamin D is important for building bones. And for most of those 80 years, people thought this was the only thing it was good for. In the past decade, however, we've learned two important things about vitamin D: it appears to have many other important health effects, and many Americans don't get enough of it. In 2008, new research pointed to a vitamin D deficiency as a possible contributing factor in heart disease. And the suspected link between vitamin D deficiency and cancer grew even stronger. This surely will spur much new research in 2009.
Why is vitamin D deficiency so common? The vitamin is made in our skin when sunlight strikes it. Many Americans—especially those who live in the northern part of the country, are elderly or have dark skin—don't soak up enough sun. And the vitamin isn't found in many foods. The main sources are fatty fish (such as salmon, mackerel, herring and sardines) and milk, cereal and juices that have been fortified with it. Vitamin D deficiency often is unsuspected because it causes no direct symptoms; like high blood pressure, it does its damage silently.
So far, the new findings about vitamin D come mostly from observational studies, which look for correlations between blood levels of the vitamin and the development of certain diseases. Such studies don't prove that the deficiency actually causes a disease nor can they prove that correcting the deficiency will prevent or treat it. But they can provide useful clues; after all, it was in observational studies that researchers first made the link between smoking and lung cancer.
Here's the latest on the vitamin's role in some common health problems:
- Fractures. Broken bones—the bane of the elderly—are less common among people with sufficient levels of vitamin D. And randomized trials suggest that taking vitamin D with calcium can boost bone mineral density and make fractures less likely. Vitamin D deficiency can also weaken muscles, making falls (particularly in frail older folks) more likely.
- Cancer. Vitamin D suppresses uncontrolled cell growth, and higher blood levels appear to be associated with a lower risk of colon cancer, and perhaps other cancers as well. In a small trial, postmenopausal women who received 1,100 IU of vitamin D plus 1,400 to 1,500 milligrams of calcium per day reduced their risk of developing nonskin cancers by 60 percent over four years, compared with a placebo.
- Depression. Several studies reveal links between low vitamin D levels and depression in older adults. A study of overweight people (who tend to have low vitamin D levels) found that taking high doses of the vitamin over one year improved their mood.
- Autoimmune and infectious diseases. Multiple sclerosis, rheumatoid arthritis, type 1 diabetes, asthma, and even influenza all appear to be more prevalent among the vitamin D deprived.
- Heart disease. Earlier this year, at least four studies found a connection between low vitamin D and heart disease. People with low blood levels were twice as likely to have a heart attack or to die of cardiovascular disease as those with normal levels of the vitamin. The risks were greatest in those with deficient levels, and less extreme in those with insufficient levels. It makes biological sense that vitamin D could protect against heart disease: it slows the development of atherosclerosis, lowers blood pressure and blood sugar and reduces inflammation within the artery wall (which, in turn, reduces the risk of heart attacks and strokes).
These studies aren't conclusive, of course. Other factors could be at work—for example, people who rarely exercise may spend less time outdoors and produce less vitamin D, but the lack of activity may be the true culprit rather than the vitamin levels. Randomized trials, in which people are given pills or placebos and followed for years, offer more convincing proof. There have been few randomized trials of vitamin D supplements, and most were small or tested relatively low doses of the vitamin.
Even so, the trials have found evidence that vitamin D supplements may protect against cancer, heart disease and premature death. But larger trials using larger doses are needed to definitively answer these questions.
So should the average person take a vitamin D supplement? A daily dose of 400 IU of vitamin D (the amount in most multivitamin pills) is valuable for its bone-building effects. Many physicians, however, now recommend a higher daily dose of vitamin D (1,000 IU). The evidence that it might help is strong, if not definitive, and the risks are minimal. If in the future large randomized trials confirm the value of a higher dose, starting now will give you a head start on the benefits. If the trials show no benefit, there will be one less pill to take each day.
Stampfer And Manson Are Professors Of Medicine At Harvard Medical School And Professors Of Epidemiology At The Harvard School Of Public Health. Corliss Is Senior Medical Editor For Harvard Heath Publications. For More Information, Go To Health.Harvard.Edu/Newsweek.