Quercetin, a flavonoid found in apples and onions, may protect athletes from the flu

October 21, 2008

Quercetin, one of the flavonol-antioxidants found in green tea, red wine, apple skins and onions, may boost the immune system and protect against flu, according to results of a study with mice. The researchers from the University of South Carolina and Clemson University also report that stressful exercise increased the mice’s susceptibility to flu, but Quercetin was found to negate these effects. A study from last year reported that the flavonoid found could help reduce illnesses in people who have carried out extensive exercise. Lead researcher Mark Davis states "Quercetin was used because of its documented widespread health benefits, which include antiviral activity, abundance in the diet and reported lack of side effects when used as a dietary supplement or food additive,"
The researchers started with the premise that strenuous exercise increases the risk for upper respiratory tract infections. Four groups of mice were used in the study - two groups performed consecutive days of running to fatigue on a treadmill: One group of runners received Quercetin and the other did not. The remaining two groups did not exercise, but one received Quercetin, while the other did not. All four groups were then exposed to the common bird flu virus H1N1.
The researchers found that 91% of the running mice not fed the Quercetin were at an increased susceptibility to infection, while Quercetin was found to offset infection risk in 63 per cent of the mice; only 37% developed the flu. Moreover, mice that exercised developed the flu much sooner than those that did not (6.9 days versus 12.4 days).
The same rate of illness in the mice that exercised and took Quercetin was approximately same as those that did not exercise, added the researchers. This suggested that Quercetin supplementation cancelled out the negative effects of the stressful exercise.
The potential health benefits of Quercetin have included reduction of blood pressure in people with hypertension. Researchers from the University of Utah reported results of a randomised, double-blind, placebo-controlled, crossover study last year that found a daily 730 milligram supplement of Quercetin led to significant reductions in blood pressure.

The study, said to be the first to report the blood pressure-lowering activity of this flavonol, was published in the November 2007 issue of the Journal of Nutrition. The newest study is published in a recent issue of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology
Volume 295.