Quercetin is a flavanoid with antioxidant properties that is found in fruits and vegetables including apples, nuts, berries, onions, cauliflower and cabbage, with apples being particularly high in quercetin.

The study also found that quercetin counteracted the increased susceptibility to the flu after stressful exercise.

A recent human study has already established that after three days of exhaustive exercise, participants who took quercetin were less likely to suffer illness than those who did not.

Quercetin intake has also been associated with lower cholesterol levels, and may contribute to a lower incidence of coronary heart disease