Journal Title (Medline/Pubmed accepted abbreviation): Intl. J. Sport Nutr. Exerc. Metab.
Page numbers: 91-96
doi (if applicable):
Summary of Background and Research Design
Background: Xanthine oxidase is an enzyme that converts hypoxanthine to xanthine and then xanthine to uric acid. It is a major source of reactive oxygen species (ROS) during exercise, especially sprinting. ROS cause oxidative stress, which can cause an increase in inflammatory cytokines (signaling molecules). Quercetin has been identified as a potential inhibitor of xanthine oxidase.
Hypothesis: 1) Quercetin will increase performance during repeated sprints. 2) Quercetin will attenuate the increase in xanthine oxidase activity after sprints. 3) Quercetin will attenuate the rise in IL-6, an inflammatory cytokine, after sprints.
Subjects: A total of 15 males trained for team sports, age 23.3 ± 2.6 yrs.
Experimental design: Randomized, double-blind, crossover
- Dry powder packet containing Gatorade (6% carbohydrate) with 500 mg of quercetin-3-glucose. The powder was reconstituted in 591 mL water. Two servings were consumed per day to total 1000 mg quercetin per day.
- Placebo – same as above but without quercetin
Protocol: Subjects provided an initial blood sample and were familiarized with the testing procedures. During week 1, participants either consumed quercetin or the placebo. Week 2 was a washout period, and then week 3 they consumed the other treatment. One sprint trial for each treatment was performed after the week of supplementation.
On the day of the sprint trial, subjects arrived after an overnight fast. They provided a blood sample and ingested the beverage that they have been consuming all week. They warmed up and then executed 12 × 30 m sprints with 35 sec rests in between. After the sprints, participants reported their perceived exertion. Additional blood draws were taken within 2-3 min of completing the sprints and then after 1 hr.
Summary of research findings
- Average sprint times were the same for both treatments and increased at the same rate (over the 12 sprints).
- The fastest sprint time occurred under quercetin supplementation (4.62 ± 0.22 sec vs. 4.68 ± 0.26 sec for placebo, p = 0.132).
- Unexpectedly, percent fatigue decrement (%FD, or the increase in sprint times occurring over the course of the repeated sprints) was higher with quercetin supplementation.
- Serum levels of xanthine oxidase, IL-6, and uric acid (product of nitrogen-containing compound degradation, like protein) increased immediately following the sprints to a similar magnitude for both treatments.
Interpretation of findings/Key practice applications
Quercetin supplementation at 1000 mg per day for 7 days did not increase performance in repeated sprints. Quercetin supplementation at this level also did not have any apparent effects on reduction of xanthine oxidase activity and therefore levels of oxidative stress. One potential explanation for these findings is that quercitin is metabolized to compounds that have less inhibitory activity against xanthine oxidase. It is also possible that quercetin may be more effective at increasing performance in aerobic events via increasing mitochondrial density in muscle cells. It is likely, though, that a longer period of supplementation would be required for this type of an effect to be observed.