Quercetin is a naturally occurring flavonol that is found in significant quantities in apples, cranberries, blueberries, and onions. There are smaller amounts of quercetin in red wine and fruit juices. A large body of work has documented several positive biologic effects of quercetin such as anti-oxidant, anti-carcinogenic, anti-inflammatory, and cardioprotective properties. Because of the myriad of possible health benefits, researchers are starting to examine the effects in athletes. In one recent study, trained cyclists supplemented with quercetin (1000 mg per day) for 3 weeks while training 3 hours per day. It was shown that almost half the cyclists in the placebo group (9/20) developed upper respiratory tract infections, whereas the incidence was only 1 out of 20 in the group supplemented with quercetin. Researchers are actively studying the mechanisms of how quercetin affects risk for infection and other potential positive effects for physically active individuals.

Nieman DC, Henson DA, Gross SJ, Jenkins DP, Davis JM, Murphy EA, Carmichael MD, Dumke CL, Utter AC, McAnulty SR, McAnulty LS, Mayer EP. Quercetin reduces illness but not immune perturbations after intensive exercise. Med Sci Sports Exerc. 2007 Sep;39(9):1561-9.