The ABCs of D
Almost everyone needs more of the sunshine vitamin
An immune system link might explain why the flu seems to strike only during the winter. A review of more than 100 studies on vitamin D and respiratory diseases, published in the current Epidemiology and Infection, found that low levels probably allow the viruses to penetrate the immune system. "It's the first comprehensive theory set forth to explain the seasonality of influenza," says vitamin D expert and lead author John Cannell, president of the Vitamin D Council and staff psychiatrist at Atascadero State Hospital in California. What's now needed, he says, is a trial to see if those exposed to flu viruses are less likely to come down with an infection if they take supplements.
The possibility intrigues researchers bracing for an outbreak of avian flu, which quickly kills by triggering an excessive immune response. Victims often suffocate when an onslaught of disease-fighting cells, known as a cytokine storm, results in a rapid buildup of fluid in the lungs. Experts think vitamin D might rev up the part of the immune system that prevents the germs from gaining entry to cells in the first place. "This puts a damper on the part of the immune system that releases the cytokine storm," says Michael Zasloff, an immunologist and vitamin D researcher at Georgetown University in Washington, D.C. Research shows that the mechanism also seems to protect against multiple sclerosis and rheumatoid arthritis, in which the immune system attacks the body's own healthy tissue.
With cancer, it's thought that vitamin D might prevent tumors from rapidly growing by controlling the expression of certain genes that regulate cell division. In a study of more than 46,000 men and 75,000 women reported in September, Harvard University researchers led by Walter Willett found that people who took in the highest amounts of vitamin D cut their risk of pancreatic cancer almost in half, compared with those with the lowest intakes. Earlier, they'd found a similar degree of protection against colon cancer in women. Other researchers are examining vitamin D in breast and prostate cancers. "The epidemiological evidence is very strong; we know there has to be something going on," says Anthony Norman, a professor of biochemistry at the University of California-Riverside who has extensively researched the vitamin D receptor. Is the evidence strong enough to recommend supplements for cancer prevention? "Unequivocally yes," he says.
How much to take? The government last year suggested that African-Americans and the elderly might want more than the guidelines suggest, but it has set 2,000 IUs as its ceiling for safety. Most experts think the limit is too conservative, noting that there's no evidence of toxicity at much higher doses and that 2,000 IUs is a worthy goal for everybody. Consuming 3 ounces of tuna, two glasses of milk, and a glass of fortified orange juice will get you to 500 IUs, and a supplement or two will get you the rest.
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