Purpose: To investigate the effects of quercetin supplementation on incidence of upper respiratory tract infections (URTI) and exercise-induced changes in immune function. Methods: Trained male cyclists (N = 40) were randomized to quercetin (N = 20) or placebo (N = 20) groups and, under double-blind procedures, received 3 wk quercetin (1000 mg[middle dot]d-1) or placebo before, during, and for 2 wk after a 3-d period in which subjects cycled for 3 h[middle dot]d-1 at approximately 57% Wmax. Blood and saliva samples were collected before and after each of the three exercise sessions and assayed for natural killer cell activity (NKCA), PHA-stimulated lymphocyte proliferation (PHA-LP), polymorphonuclear oxidative-burst activity (POBA), and salivary IgA output (sIgA). Results: Pre- to postexercise changes in NKCA, PHA-LP, POBA, and sIgA did not differ significantly between quercetin and placebo groups. URTI incidence during the 2-wk postexercise period differed significantly between groups (quercetin = 1/20 vs placebo = 9/20, Kaplan-Meier analysis statistic = 8.31, P = 0.004). Conclusion: Quercetin versus placebo ingestion did not alter exercise-induced changes in several measures of immune function, but it significantly reduced URTI incidence in cyclists during the 2-wk period after intensified exercise.
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