Protein Supplements Boost Endurance Stamina But Won't Shrink Your Waistline

Protein supplementation is well-established for resistance training, but its role in endurance sports has been less clear. This 2025 meta-analysis from Wuhan Sports University set out to resolve that ambiguity by pooling data from 23 randomized controlled trials covering 1,146 healthy adults aged 18–65, with studies drawn from Web of Science, PubMed, and SPORTDiscus databases up to April 2025.

The standout finding is a statistically significant improvement in time to exhaustion (TTE) — the ability to sustain exercise until failure — with a standardized mean difference (SMD) of 0.45 (95% CI: 0.15–0.76, p<0.01). This is a moderate effect size and is clinically meaningful for endurance athletes who need to sustain high-intensity efforts. The likely mechanism involves protein blunting muscle protein catabolism and supporting branched-chain amino acid availability during prolonged exercise, which can account for up to 6% of total energy expenditure in sessions exceeding two hours.

Lean body mass showed a small positive trend (SMD=0.13, 95% CI: −0.01 to 0.28, p=0.07) that fell just short of statistical significance. This is biologically plausible — endurance training alone produces minimal muscle hypertrophy, so protein's anabolic signal is modest. Body weight and body fat percentage were not significantly altered, suggesting protein supplementation doesn't drive fat loss or unwanted weight gain in this context.

VO2max, the gold standard of aerobic capacity, was not significantly improved overall. However, a subgroup analysis revealed that untrained individuals may derive greater VO2max benefit (SMD=0.21) compared to trained athletes, who likely have less room for aerobic improvement. Anaerobic capacity measures also showed no significant change with protein supplementation.

The authors note this review updates and extends a 2020 meta-analysis by Lin et al., which had concluded protein improved aerobic capacity and lean mass. The current analysis, incorporating newer trials and subgroup moderator analyses by training status, sex, and protein type, presents a more nuanced picture. Protein type (e.g., whey vs. other sources) and timing were not consistently reported across trials, limiting definitive guidance on optimal protocols. The authors recommend future research clarify protein intake habits, explore populations with chronic disease, expand sample sizes, and consider carbohydrate co-ingestion effects.