The Effect of Dietary Nitrate Supplementation on Endurance Exercise Performance in Healthy Adults: A Systematic Review and Meta Analysis
BackgroundRecent research into the use of dietary nitrates and their role in vascular function has led to it becoming progressively more popular amongst athletes attempting to enhance performance.ObjectiveThe objective of this review was to perform a systematic review and meta analysis of the literature to evaluate the effect of dietary nitrate (NO3 supplementation on endurance exercise performance. An additional aim was to determine whether the performance outcomes are affected by potential moderator variables.Data sourcesRelevant databases such as Cochrane Library, Embase, PubMed, Ovid, Scopus and Web of Science were searched for the following search terms 'nitrates OR nitrate OR beetroot OR table beet OR garden beet OR red beet AND exercise AND performance' from inception to October 2015.Study selectionStudies were included if a placebo versus dietary nitrate only supplementation protocol was able to be compared, and if a quantifiable measure of exercise performance was =30 s (for a single bout of exercise or the combined total for multiple bouts).Study appraisal and synthesisThe literature search identified 1038 studies, with 47 (76 trials) meeting the inclusion criteria. Data from the 76 trials were extracted for inclusion in the meta analysis. A fixed effects meta analysis was conducted for time trial (TT) (n 28) time to exhaustion (TTE) (n 22) and graded exercise test (GXT) (n 8) protocols. Univariate meta regression was used to assess potential moderator variables (exercise type, dose duration, NO3 type, study quality, fitness level and percentage nitrite change).ResultsPooled analysis identified a trivial but non significant effect in favour of dietary NO3 supplementation [effect size (ES) 0.10, 95 Cl 0.27 to 0.06, p 0.05] TTE trials had a small to moderate statistically significant effect in favour of dietary NO3 supplementation (ES 0.33, 95 Cl 0.15 0.50, p 0.01) GXT trials had a small but non significant effect in favour of dietary NO3 supplementation in GXT performance measures (ES 0.25, 95 Cl 0.06 to 0.56, p 0.05) No significant heterogeneity was detected in the meta analysis. No statistically significant effects were observed from the meta regression analysis.ConclusionDietary NO3 supplementation is likely to elicit a positive outcome when testing endurance exercise capacity, whereas dietary NO3 supplementation is less likely to be effective for time trial performance. Further work is needed to understand the optimal dosing strategies, which population is most likely to benefit, and under which conditions dietary nitrates are likely to be most effective for performance.
Sports Medicine