NAC boosted exercise performance in men but left inflammation untouched

N-acetylcysteine, better known as NAC, has crossed over from clinical use into the sports supplement aisle on the strength of one idea: hard exercise generates oxidative damage, and NAC supports glutathione, the body's own antioxidant system. If that mechanism translates into real outcomes, NAC would be a cheap way to train harder or recover better. But mechanism and outcome are different things, and the trial literature on NAC in exercising men has never been gathered and judged in one place. This review set out to do exactly that.

Following standard systematic review guidelines, the authors searched Web of Science, Scopus and PubMed for controlled trials comparing NAC supplementation with a control group in adult men, covering publications up to 30 April 2023. From 777 records identified in the search, 16 studies met the inclusion criteria. The trials were quality-checked using an established critical appraisal form alongside a standard risk of bias assessment. Outcomes examined included physical performance, a range of laboratory biomarkers and any adverse effects.

Overall, most of the trials reported beneficial effects of NAC, and no serious adverse events were reported anywhere in the included studies. Men supplementing with NAC showed significant improvements in exercise performance, antioxidant capacity and glutathione homeostasis. The picture was not uniformly positive, though: there was no clear evidence of benefit for haematological markers, the inflammatory response or muscle behaviour. The authors conclude that NAC appears safe and may improve performance through its antioxidant actions, while stressing that further studies are needed to clarify how relevant its use really is.

This is the pattern many antioxidant supplements show: the mechanism holds up, the biochemistry moves, and some performance measures improve, but downstream markers stay stubbornly flat. The review supports the core rationale for NAC, its effect on glutathione, more convincingly than it supports broad claims about recovery or inflammation. Because the findings were synthesised without pooling into a single statistical estimate, the size of any performance benefit remains unquantified. The exclusive focus on adult men also leaves the entire question open for women.

If you have seen NAC promoted as a performance aid, the honest summary is that the evidence in men leans positive on performance and antioxidant status, with a reassuring safety record across these trials. It leans nowhere on inflammation, blood markers or muscle measures, so claims in that territory are running ahead of the data. The absence of any pooled effect size means nobody can yet say how big the performance benefit is, only that most trials found one. For women, there is simply no evidence here either way.