Whilst moderate exercise in humans appears to enhance immune function and resistance to viral infections, there is evidence that exhausting exercise suppresses the immune response and increases susceptibility to infections.
Intense exercise stress has also been shown to increase susceptibility to upper respiratory tract infection in mice. A new study shows that quercetin can lessen the impact of stressful exercise on the susceptibility to influenza virus A infection in mice.
Quercetin is a naturally occurring flavonol found in many fruits and vegetables, and is known to have antioxidant and anti-inflammatory activity.
A recent study in trained cyclists has also shown that quercetin can help reduce illness and maintain mental alertness in physically stressed human subjects. Although quercetin did not alter several measures of immune function, it did significantly reduce the incidence of upper respiratory tract infections in the cyclists during the 2 week period following intense exercise.