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Study: Fruits and Vegetables Reduce Flu Risk for Exercisers

By L.K. Regan

The Centers for Disease Control and Prevention warn that September is the month to start working on getting your flu vaccine. But in addition to the poke in the arm, a recent study suggests that you might be able to reduce your flu risk through some healthy eating. And, this may be particularly true for people who engage in strenuous exercise. So, if you're a workout guy who's worried about getting the chills and the pukes, get to work on eating your fruits and veggies.

Researchers at the Arnold School of Public Health at the University of South Carolina, Columbia, and the Department of Microbiology and Molecular Medicine at Clemson University published a study in the American Journal of Physiology: Regulatory Integrative and Comparative Physiology demonstrating that mice given quercetin, a naturally occurring substance found in fruits and vegetables, were less likely to contract the flu than mice that had not received the supplement.

Now, at first glance this would appear to confirm what your mother and every medical study since have already told you: fruits and veggies are good for your health. But here's the twist—the researchers were testing quercetin's ability to counter the immune-suppressing effects of an intensive exercise program. The researchers' theory was that intensive exercise would increase flu risk in mice (this effect has yet to be tested in humans), and that consumption of quercetin would help to counter this effect. Why quercetin? Quercetin is a flavonoid relative of resveratrol, and is found in many of the same places—in fruits and vegetables, especially capers, apples, onions (particularly red onions), broccoli and other leafy green vegetables, grapes, red wine, and tea. Its antiviral properties have already been demonstrated in experiments.

Still, none of this research has tested quercetin's influence on flu virus. "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," said J.M. Davis, lead author on the study. And why exercise? Previous studies have demonstrated that intensive exercise increases risk of upper respiratory infection in mice. So, the mice in the study were separated into two types—one that experienced strain in the form of exercising to fatigue for three days, and one that did not. Some got quercetin, and some did not.

The exercising mice were much more likely to develop the flu than the sedentary mice, and they got the flu sooner, as well. But while 91 percent of the exercising mice got the flu versus 63 percent of non-exercising mice, quercetin cancelled out this effect. So, the exercising mice that got quercetin had the same flu rates as the sedentary mice that got no quercetin. Did quercetin cure the flu? No way. But it did cancel out the immuno-stressing effects of intense exercise.

One problem is that the effects of exercise on upper-respiratory infections in humans are unclear. Mice get the flu when they workout—but that doesn't mean you will. Still, the effects of quercetin on humans have been studied. A recent study found that people on quercetin had fewer illnesses following three days of intense exercise than people who had not taken quercetin. If you're worried that your workouts are depleting your immune system along with your fat cells, you might want to replenish with fruits and veggies...or at least a glass of wine! As Davis puts it, "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."

And, you might want to take this as a reminder to go get your flu shot—September is the season!