Impact of Vitamin C and Other Dietary Antioxidants on Sport Performance

Abstract

PURPOSE: Antioxidant supplementation has been proposed as a treatment to reduce the adverse effects of exercise-induced reactive species, such as muscle damage, immune dysfunction and fatigue. However reactive species can mediate beneficial adaptations to training. Dietary antioxidants may therefore attenuate these adaptations. This review will discuss the effect of dietary antioxidants on athletic performance, including vitamin C, vitamin E, N-acetylecysteine (NAC), polyphenols including quercetin and beetroot; and mixtures of these. Of the antioxidants reviewed, NAC has been studied in an acute manner; the others consumed chronically (greater than one week). It is important to appreciate that chronic antioxidant supplementation may impair training adaptations thus negating any potential acute improvement on a given day. METHODS: The studies included in this review were sourced via Google Scholar and Sport Discus, using the search terms antioxidant and exercise. Additional studies were sourced from reference lists in related articles and books on the topic. RESULTS: Vitamin C in daily doses greater of 400 mg impaired athletic performance in 5 of 7 studies reviewed, possibly by reducing mitochondrial biogenesis, altering fuel utilization via reductions in IL-6 or by acting as a pro-oxidant. Smaller doses of vitamin C (200-400 mg.d-1) may be sufficient to reduce oxidative stress without impairing training adaptations, but further research is required to address this possibility. Vitamin E has little effect on athletic performance, although studies in rodents appear promising. Acute NAC intake improved performance in 2 of 3 studies on endurance cycling and in 2 studies on submaximal muscle force, but can induce toxic side effects. Polyphenol supplementation improved performance in all 9 studies in as little as 7 days, although not all outcomes were clear. Polyphenols may enhance vascular function while quercetin (0.6-1 g.d-1) increases mitochondrial biogenesis. Beetroot juice contains nitrates and anthocyanins, and demonstrated a clear benefit to cycling performance in only 6 days. CONCLUSIONS: Further research is required to investigate antioxidants supplements and their effect on performance, particularly taken acutely. At present the polyphenols are looking promising.