Researchers See Link Between Vitamin D, Flu Immunity

Vitamin's Vital Role
Michael Holick, a vitamin D researcher, said that immune cells have a vitamin D receptor, and that the cells activate vitamin D as a response to infection.

"What vitamin D really does is play a sentinel role," said Holick, a professor of medicine, physiology and biophysics at Boston University School of Medicine.

First, it is used by immune cells to fight the virus. Then, it helps temper the overall immune response and limit inflammation.

The vast majority of the evidence of vitamin D's role in fighting the flu is observational or based on laboratory research rather than large clinical trials. At the same time, studies continue to find that large numbers of people in the U.S. have deficient or less than optimal levels of vitamin D in their blood.

So vitamin D's role, if any, in defending against the flu is theoretical.

And there are other explanations, also unproven, about why flu persists in the fall and winter.

The two leading contenders: The cold and dryness of winter make it easier for the virus to be transmitted; and people spend more time indoors in the winter, leading to more person-to-person contact.

But vitamin D proponents say those theories are weak. For instance, people congregate indoors at workplaces, day care centers and churches just as much in the winter as in the summer.

And then there are researchers such as Scott Dowell, a physician with the U.S. Centers for Disease Control and Prevention, who don't buy any of those explanations, including the vitamin D theory.

For decades, the seasonality of flu has been one of the most obvious questions that medical science has failed to answer, said Dowell, director of CDC's global disease detection program and international team leader for CDC's response to the H1N1 swine flu outbreak.