Trained male cyclists who were given 1,000 mg of quercetin per day, before, during and after intense exercise—cycling for 3 hours per day for 3 days—experience 9-times fewer upper respiratory tract infections than cyclists who were given a placebo—1 out of 20 given quercetin versus 9 out of 20 given placebo (5% vs 45%)—according to a study from researchers at Appalachian State University in Boone, North Carolina, USA.

(The cyclists averaged about 1.5 hours of cycling per day, so “the 3-[day] intensified exercise period (9 [hours] of exercise) represented nearly a doubling of their normal exercise workload” the paper noted.)

“Quercetin versus placebo ingestion did not alter exercise-induced changes in several measures of immune function, but it significantly reduced [upper respiratory tract infections] incidence in cyclists during the 2-[week] period after intensified exercise,” the authors of the study concluded.