Quercetin in culture with target cells and pathogens exerts anti-pathogenic activities against a wide variety of viruses and bacteria. A few small-scale human quercetin supplementation studies have produced conflicting results regarding quercetin's effects on upper respiratory tract infection rates, and little is known regarding the appropriate human dose. The purpose of this randomized, double-blinded, placebo-controlled trial was to measure the influence of two quercetin doses (500 and 1000 mg/day) compared to placebo on upper respiratory tract infection (URTI) rates in a large community group (N=1002) of subjects varying widely in age (18-85 years). Subjects ingested supplements for 12 weeks and logged URTI symptoms on a daily basis using the Wisconsin Upper Respiratory Symptom Survey (WURSS). No significant group differences were measured for URTI outcomes for all subjects combined, or when analyzing separately by gender, body mass index, and age categories. Regression analysis revealed that the strongest interaction effect with group status was self-reported fitness level. A separate analysis of subjects 40 years of age and older rating themselves in the top half of the entire group for fitness level (N=325) showed lower URTI severity (36% reduction, P=0.020) and URTI total sick days (31% reduction, P=0.048) for the Q-1000 group compared to placebo. In summary, for all subjects combined, quercetin supplementation over 12 weeks had no significant influence on URTI rates or symptomatology compared to placebo. A reduction in URTI total sick days and severity was noted in middle aged and older subjects ingesting 1000 mg quercetin/day for 12 weeks who rated themselves as physically fit.
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