Vitamin D and Antioxidant Quercetin Effects on Muscular and Cardiorespiratory Performance in Physically Active Adults

Exercise results in an increased production of reactive oxygen species (ROS) which may cause cellular damage and impaired performance. As an antioxidant, quercetin supplementation may reduce ROS, leading to improved physical performance. The purpose of the first investigation was to determine the effects of quercetin supplementation on oxidative status and cardiorespiratory performance. We hypothesized that an improved oxidative status will be associated with enhanced performance. Physically active adults received quercetin or placebo daily for six weeks. Following the supplementation period, there was a significant group interaction for serum malondialdehyde (placebo +0.88 ± 0.53 pmol/mg protein, quercetin -0.69 ± 0.22 pmol/mg protein; p = 0.019), suggesting reduced oxidative stress in the quercetin group. However, this decrease did not significantly result in improved cardiorespiratory fitness. Insignificant results could be due to a small effect size and quercetin’s use in isolation. It is possible the addition of other supplements with similar proposed ergogenic outcomes, will affect multiple biological targets leading to increased performance. Up to 91% of athletes have insufficient vitamin D status, which has been associated with low cardiorespiratory fitness and impaired muscular strength, possibly through its role in Ca2+ signalling. Therefore, we designed a second investigation to determine whether dietary supplementation with quercetin and vitamin D, alone or in combination, improves physical performance. We hypothesized that quercetin and vitamin D supplementation would increase testosterone and 1,25(OH)2D3, improve antioxidant status and decrease oxidative stress leading to improved cardiorespiratory fitness and muscle function. Physically active adults received vitamin D, quercetin, vitamin D plus quercetin, or placebo daily for eight weeks. Although an increased vitamin D status (84.6%, p \u3c 0.001) was accompanied by an apparent increase in testosterone concentration (19.7%, p \u3c 0.557), no improvements in cardiorespiratory fitness or muscle function were found. These findings demonstrate that supplementation with quercetin and vitamin D does not significantly change markers of oxidative status, 1,25(OH)2D3, or testosterone levels, possibly limiting any effect on muscular and cardiorespiratory performance. As a result, these findings do not support supplementation of quercetin and vitamin D in subjects with presumed adequate antioxidant status and vitamin D levels for the purpose of performance enhancement. Our failure to support our hypotheses may be, in large part, due to our subjects’ demographics – young, physically fit, and sufficient baseline antioxidant and vitamin D status