Are you getting enough sun?

A spate of new studies suggests vitamin D offers health benefits far beyond strengthening bones. Researchers report that “the sunshine vitamin” may cut cancer risks and help the immune system fight infections. Together these studies raise the possibility that a brief daily dose of sun combined with a vitamin D supplement could help stave off everything from breast cancer to the flu.

Although our skin makes vitamin D when we spend time in the sun, an estimated 1 billion people worldwide are deficient in this essential ingredient, according to Michael Holick, an endocrinologist at the Boston University Medical Center. Most don’t know it because symptoms rarely surface unless the deficiency is severe enough to cause rickets in children or osteomalacia (“soft bones”) in adults. Doctors determine the body’s vitamin D level with a blood test.

Holick, who has pioneered research on how vitamin D works in the body, says deficiencies of this nutrient may underlie susceptibility to infection and illnesses including cancer, autoimmune diseases and cardiovascular disease. “Every tissue and cell in your body has a receptor for vitamin D,” he says. “We know that it is one of the most potent regulators of cellular growth, and that immune cells have receptors that activate it. So it makes sense that vitamin D has many health benefits.”

Here’s a glimpse of recent findings:

• Overall cancer risk: Women who received calcium plus 1,100 international units (IUs) of vitamin D had a 60 to 77 percent reduction in cancer risk compared with those who received calcium alone or placebos, according to a study led by Joan Lappe, associate professor of medicine at Creighton University in Omaha, Nebraska. This study of 1,179 subjects appeared in the online edition of the American Journal of Clinical Nutrition (June 8, 2007).

• Breast cancer: A research team at Harvard Medical School reported that calcium and vitamin D might help protect women from the most aggressive forms of breast cancer before menopause. In a study of more than 30,000 women, the researchers found that premenopausal women who took the most calcium (1,366 milligrams) and vitamin D (948 IUs) were one-third less likely to develop breast cancer compared with the women who consumed the lowest amounts of these nutrients. The findings are published in the May 28, 2007, edition of Archives of Internal Medicine.

• Colorectal cancer: Daily doses of 1,000 to 2,000 IUs of vitamin D could cut the risk of colorectal cancer by one-half to two-thirds, according to a study published in the American Journal of Preventive Medicine (Feb. 6, 2007). A research team led by assistant professor Edward Gorham and his colleagues with the Moores Cancer Center at the University of California, San Diego, based these estimates on a meta-analysis of five studies about the relationship between vitamin D and colon cancer.

• Prostate cancer: Men with the lowest blood levels of vitamin D are at much greater risk for developing a particularly aggressive and deadly form of prostate cancer, according to a long-term study of 14,916 doctors led by Haojie Li and her colleagues at Harvard Medical School. Their research, published in the March 19, 2007, online edition of PloS Medicine, suggests men may be able to reduce this risk by taking vitamin D supplements and getting moderate sun exposure in the summer.

• Influenza and infections: Taking vitamin D may be a good way to fight the flu. After sifting through more than 70 years of research, John Cannell, a psychiatrist at Atascadero State Hospital in California, and his colleagues hypothesize that low vitamin D levels may contribute to a higher risk for upper respiratory tract infections. The team published a review article on their findings in the online edition of Epidemiology and Infection (Sept. 7, 2006). Several recent studies suggest a mechanism for vitamin D’s potentially protective effect: It stimulates white blood cells to produce cathelicidin, a biochemical that kills invading bacteria, fungi and viruses.

So what’s the best way to get your daily D? Holick recommends taking 1,000 IUs of vitamin D every day and getting a short, “sensible” daily dose of sunshine before covering up or applying sunscreen. “Sensible” depends on who you are and where you live. Caucasians can safely make enough daily D by sitting in the sun for a quarter of the time it takes their skin to turn light pink, typically five to 15 minutes in the middle of a sunny summer day, says Holick. People with dark skin need to be in the sun three to five times longer to make enough vitamin D, because dark skin pigment acts as a partial sunscreen (about the equivalent of SPF 8, which blocks 92 percent of ultraviolet radiation).

Holick adds that other studies are underway and these may eventually suggest a therapeutic role for vitamin D in illnesses ranging from diabetes to heart disease. In the meantime, it might not hurt—and certainly could help—to spend a few minutes simply enjoying the sun.

By Kim Ridley

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