N-Acetylcysteine and its effect on Sport Performance: A Meta-Analysis and Case Study

Abstract

Background: N-acetylecysteine (NAC) is a promising antioxidant supplement with potential as a performance enhancing strategy in elite sport. Previous laboratory research suggests the performance benefits are between 2 and 32%, for athletes taking NAC. The true effect of NAC requires further consideration. Also limiting NAC’s use in sport is the lack of information regarding NAC’s tolerability and optimal dose. Purpose: To systematically review the current literature and evaluate the effects of NAC supplementation on sport performance, using a meta-analytic approach. A secondary aim was to evaluate the potential side effects from NAC supplementation. Methods: The literature up to June 2015 was searched on MEDLINE (PubMed), EMBASE, SPORTDiscus, Google Scholar and Scopus databases to identify for all studies investigating the effects of NAC supplementation on exercise performance and/or side effects experienced. Performance outcomes from each study were converted to the percent effect equivalent to mean power output in a time trial. All pooled analyses were based on random effects models generated by Review Manager (version 5.3). Results: A total of 5 studies met criteria for inclusion in the sport performance meta-analysis, and 17 for inclusion in the side effects meta-analysis. The overall mean percent effect of NAC on performance was unclear, with a pooled mean effect of 0.45 (95% CI -0.79-1.69), and evidence of substantial heterogeneity among studies (I2= 68%). The difference in side effects with NAC compared to placebo was also unclear, with a pooled mean effect of 1.11 (95% CI 0.88-1.39) and no evidence the mean effect was influenced by the heterogeneity among studies (I2= 0%,). The sub-analysis of NAC dose suggests an increase in side effects as the dosage of NAC increased however this observation was not significant. Conclusions: NAC supplementation has a range of performance effects from beneficial, to trivial to harmful, and the true performance effect of NAC remains unknown. The risk of side effects from NAC supplementation also remains unknown. Suboptimal reporting and documentation from studies make it difficult to meta-analyse current literature and make clear conclusions regarding the effect of NAC. Further research that is well-designed and practical is needed before NAC can be advocated as a performance enhancing strategy in elite sport.