The study was conducted by J. Mark Davis, E.A. Murphy, J.L. McClellan, and M.D. Carmichael, of the University of South Carolina and J.D. Gangemi of Clemson University.
Quercetin, a close chemical relative of resveratrol, is present in a variety of fruits and vegetables, including red onions, grapes, blueberries, tea, broccoli and red wine.
It has been shown to have anti-viral properties in cell culture experiments and some animal studies, but none of these studies has looked specifically at the flu.
The study was conducted using mice, but Davis believes that if quercetin provides a similar benefit for humans, it could help endurance athletes, soldiers and others undergoing difficult training regimens, as well as people under psychological stress.
For the study, Davis and his colleagues examined four groups of mice. Two groups performed three consecutive days of running to fatigue on a treadmill to mimic a short period of stressful exercise. One group of runners received quercetin, the other did not.
The remaining two groups did not exercise. One non-exercise group received quercetin while the other did not. All four groups were then exposed to a common flu virus, H1N1.
The researchers found that stressful exercise increased susceptibility to the flu. The mice that exercised to fatigue for three days were more likely to develop the flu than the mice that did not exercise. The mice that exercised developed the flu much sooner than those that did not.
Mice that exercised and took quercetin had nearly the same rate of illness as those that did not exercise. In other words, quercetin cancelled out the negative effect of stressful exercise.
The severity of the symptoms among those mice that either did not exercise or those that exercised but took the quercetin was about the same. Quercetin had protective effects for the mice that did not exercise.
"This is the first controlled experimental study to show a benefit of short-term quercetin feedings on susceptibility to respiratory infection following exercise stress. Quercetin feeding was an effective preventive strategy to offset the increase in susceptibility to infection that was associated with stressful exercise," said Davis.
The study appears in the current issue of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology.