Health Matters—Hello Spring, Good-bye Flu

By Lisa Marshall

It’s a perennial reality that has puzzled researchers for decades: Despite the fact that people congregate at work, daycare, and shopping malls year-round, influenza A outbreaks are exceedingly rare in warmer months. In 1981, British epidemiologist R. Edgar Hope-Simpson proposed that a mysterious “seasonal stimulus” was to credit for keeping the flu and other respiratory bugs at bay at winter’s end. Now, a team of researchers believes it has identified that stimulus: vitamin D.

According to their review article in the December issue of Epidemiology and Infection, vitamin D boosts expression of antimicrobial compounds in the respiratory tract, helping protect the lungs from infection. In one study, volunteers inoculated with influenza virus during winter were eight times more likely to develop signs of infection than those inoculated during summer. In another, vitamin D–deficient children were 11 times more likely to suffer flu. And numerous studies have shown that ultraviolet radiation and D-rich cod liver oil both reduce the incidence of respiratory infections.

It’s way too soon to throw out the flu vaccine for good and head for the beach. But try and get as much vitamin D exposure in the wintertime as you do in the summertime: Enjoy a few minutes of sunscreen-free time outside everyday. Eat plenty of D-rich foods. And if you live in cloudy climes year-round, are African-American or elderly, or work the night shift and don’t get much sun, take a supplement.