A new study published in the American Journal of Physiology −− Regulatory, Integrative and Comparative Physiology may give more scientific backing to the adage, “an apple a day keeps the doctor away.”

Researchers have found that mice who were administered quercetin, a compound found in many fruits and vegetables – particularly apples – were less likely to contract the flu virus, especially after periods of stressful exercise. Mice were divided into four groups, two of which were exercised for an intense period during three consecutive days and two of which were not. One group from the exercised mice and one group from the non-exercised mice received doses of quercetin, after which all groups were exposed to a common flu virus.

The researchers found that stressful exercise significantly increased the mice’s susceptibility to the flu: 93% of mice who exercised contracted the flu, while only 63% of the non-exercised group caught it. However, mice that exercised and received quercetin contracted the flu at the same rate as those that did not exercise, suggesting that quercetin negated the effects of stressful exercise. Of the mice that did not exercise, those that received quercetin also contracted the flu less often than those who did not. Head researcher J. Mark Davis says that this study “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."

A previous study on humans had suggested that receiving quercetin lessened the likelihood of contracting upper respiratory infections in cyclists who exercised strenuously for three days. More research will be needed to determine exactly how quercetin works on the immune system and whether its effect still holds for humans under normal conditions, i.e., without strenuous exercise.

Quercetin is found in apples, red onions, red grapes, leafy greens, and many types of berries.

The study was carried out by researchers at the University of South Carolina and Clemson University.