Journal Title (Medline/Pubmed accepted abbreviation): Med. Sci. Sports Exerc.
Page numbers: 2396-2402
Summary of background and research design:
Background: Eleven studies have been performed testing the effects of quercetin on maximal energy intake and/or exercise performance.
Purpose: to calculate the net effect of quercetin on all the test subjects in the 11 studies and to determine 1) if quercetin provides performance benefits, and 2) if differences in findings correlate with identity of the subjects, the exercise protocol, or the supplementation routine.
Procedure: Studies were sought that investigated the effects of quercetin and endurance exercise capacity up until July 2010. Eleven fit the inclusion criteria. The "effect size" (the difference between quercetin vs. the placebo) was mathematically calculated, with the details provided in the study. Regression analyses were performed where appropriate (ex. effect as a function of dose). Effect of supplement vehicle (drink with or without carbohydrates, powder, gel, solid) was assessed, but was difficult to determine due to the small number of studies in each category.
Summary of research findings:
- Six studies assessed VO2max and eight assessed endurance performance (three did both). In total, 129 subjects were evaluated for VO2max, 194 for endurance performance (51 for both). Most studies were on young adults, male or male + female.
- There were 9 of 11 studies that used 1000 mg/day (range 600-2000 mg). The duration of supplementation varied to a greater extent (1-54 days).
- The "effect size" range was -0.10 to 0.59. In regard to effects on VO2max, 3 studies favored the placebo (not statistically significant) and 8 studies favored quercetin. Overall, there is a significant effect that favors quercetin in regard to exercise capacity.
- The overall "effect size" for endurance performance was 0.23. There was a 5% average increase in performance. Six of the 8 studies favored quercetin over the placebo in regard to endurance exercise performance.
- No variables could be identified (ex. subject fitness level, plasma quercetin level) that affected the results.
- The effect size of quercetin was 0.1 (<0.2 = trivial to small). To put this in perspective, the effect size of caffeine is about 0.4-0.5.
Key practice applications:
Overall, the size of the effect of quercetin on endurance capacity equates to about 3%. Practically, this effect is very small. Quercetin improved endurance performance about 5%, which is not a very large amount, but could benefit very competitive athletes or the results of very close races.
- One problem that always exists is a publication bias: Statistically significant results are more "exciting" than no-effect results, therefore being published at a higher rate. There is a chance that some "no effect" quercetin studies were not published and, because of the publication bias, there is a greater chance that studies that show no difference were not published. In fact, in this meta-analysis, 2 abstracts from conference presentations were included (not from a peer-reviewed journal) and both of these generally showed a lower effect than the peer-reviewed, published studies.
- Because quercetin is mostly insoluble in water, all the studies added another ingredient to the supplement to enhance uptake. If any of these ingredients affected athlete performance then it could skew the results.