Vitamin D Fights Cancer--and That's Not All
But having too little appears to cause the immune system to weaken as well. A landmark study published in the March issue of Science found that cells from African-Americans (whose dark skin doesn't efficiently absorb UV rays) churned out 63 percent less of a protein needed to fight off tuberculosis than expected. When added to the cells, vitamin D appeared to signal the cells to produce normal levels of the protein.
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 GeorgetownUniversity 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 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.