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Bethesda, MD, USA. Scientists have demonstrated that quercetin, a naturally occurring substance found in fruits and vegetables, can render subjects less likely to contract the flu. According to a new study using the mouse model orgnism, the findings also indicate that stressful exercise corresponded to increased susceptibility to the flu, but quercetin canceled out negative effects.
Quercetin 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 substance is a close chemical relative of resveratrol, a phytochemical produced by plants that exhibits a number of beneficial health effects.
Quercetin reduces susceptibility to influenza infection following stressful exercise. J. M. Davis, E. A. Murphy, J. L. McClellan, M. D. Carmichael, and J. D. Gangemi. AJP Regulatory Integrative and Comparative Physiology 295(2) R505. doi: 10.1152 / ajpregu.90319.2008
J. Mark Davis and his colleagues at the University of South Carolina and Clemson University carried out the study. [N1-2] The findings appear in the American Journal of Physiology (AJP) Regulatory, Integrative and Comparative Physiology. [C1, N3]
The new study was conducted using mice, but 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, according to Davis.
"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," Davis said.
Earlier mouse studies have found that stressful exercise can increase susceptibility to upper respiratory infections, although it is not yet clear if the same is true for humans. There was also preliminary information that mice may be more susceptible to the flu when they exercise to fatigue. The researchers in the current study hypothesized that exercise would increase the chance of the mice getting the flu but that quercetin would counteract the increased risk.
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 (91% versus 63%).
- The mice that exercised developed the flu much sooner than those that did not (6.9 days versus 12.4 days).
- Mice that exercised and took quercetin had nearly the same rate of illness as those that did not exercise. In other words, quercetin canceled 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.
Although this study was done with mice, a recent human study found that people who took quercetin suffered fewer illnesses following three days of exhaustive exercise compared to those who did not. Unlike the mouse study, the humans were not inoculated with a virus.
"This is the first controlled experimental study to show a benefit of short-term quercetin feedings on susceptibility to respiratory infection following exercise stress," said Davis. "Quercetin feeding was an effective preventive strategy to offset the increase in susceptibility to infection that was associated with stressful exercise."
[C1] Quercetin reduces susceptibility to influenza infection following stressful exercise. J. M. Davis, E. A. Murphy, J. L. McClellan, M. D. Carmichael, and J. D. Gangemi. AJP Regulatory Integrative and Comparative Physiology 295(2) R505. doi: 10.1152 / ajpregu.90319.2008
Abstract
Exercise stress is associated with increased risk for upper respiratory tract infection. We have shown that exercise stress can increase susceptibility to infection. Quercetin, a flavonoid present in a wide variety of fruits and vegetables, has been reported to inhibit infectivity and replication of a broad spectrum of viruses and may offset the increase in susceptibility to infection associated with stressful exercise. This study examined the effects of quercetin feedings on susceptibility to the influenza virus A/Puerto Rico/8/34 (H1N1) following stressful exercise. Mice were randomly assigned to one of four treatment groups: exercise-placebo, exercise-quercetin, control-placebo, or control-quercetin. Exercise consisted of a run to fatigue (140 min) on a treadmill for 3 consecutive days. Quercetin (12.5 mg/kg) was administered via gavage for 7 days before viral challenge. At 30 min after the last bout of exercise or rest, mice (n = 23–30) were intranasally inoculated with a standardized dose of influenza virus (0.04 hemagglutinating units). Mice were monitored daily for morbidity (time to sickness), symptom severity, and mortality (time to death) for 21 days. Exercise stress was associated with an increased susceptibility to infection [morbidity, mortality, and symptom severity on days 5–7 (P < 0.05)]; quercetin offset the increase in susceptibility to infection [morbidity, mortality, and symptom severity on days 5–7 (P < 0.05)] that was associated with stressful exercise. These data suggest that short-term quercetin feedings may prove to be an effective strategy to lessen the impact of stressful exercise on susceptibility to respiratory infection.
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