The effectiveness of protein supplements on athletic performance and post-exercise recovery − a Bayesian multilevel meta-analysis of randomized controlled trials

Background: Protein supplements are a popular category of dietary supplements among fitness enthusiasts and athletes. However, research providing definitive conclusions on the effects of protein on athletic performance and post-exercise recovery remains limited. Key factors, such as protein source, timing, and optimal dosage, require further investigation to clarify their impact. Method: A systematic search across seven databases identified 6,129 studies, which were screened using the Covidence online tool. After independent selection, data extraction, and risk of bias assessment by two reviewers, 75 studies involving 1,206 athletes were included in the meta-analysis. A multilevel meta-analysis synthesized data from the included studies using a Bayesian hierarchical model with the brms package. Publication bias was assessed using a funnel plot generated with the PublicationBias package and by calculating the P value of Egger's test through the metafor package. Additionally, a moderation analysis with the brms package was conducted to examine the relationship between seven moderators and effect sizes. Results: The results demonstrated that the effects of protein-carbohydrate supplements showed statistical significance in comparison to the placebo group [μ(SMD): 0.57, 95% CI: 0.2 to 0.93] in enhancing endurance performance. Pure protein supplements demonstrated statistically significant effects compared to the placebo group in both endurance performance [μ(SMD): 0.37, 95% CI: 0.02 to 0.71] and muscle strength [μ(SMD): 0.72, 95% CI: 0.18 to 1.27]. For post-exercise recovery, pure protein supplements also showed statistically significant effects compared to carbohydrate supplements for maintaining glycogen resynthesis [μ(SMD): 0.83, 95% CI: 0.21 to 1.46]. However, the results indicated that all significant effects were observed in randomized controlled trials where the energy intake between the intervention and control groups was not matched. Conclusion: The effects of protein supplementation on athletic performance and post-exercise recovery appear to be limited. Protein supplements showed beneficial effects compared to no supplementation. However, all statistically significant results were derived from studies in which energy intake was not matched between groups. This suggests that the observed benefits may not be attributable to protein per se. An additional intake of 1 g/kg/day of protein from supplements, resulting in a total daily protein intake of approximately 2 g/kg/day, appears to be most effective for enhancing athletic performance. Registration: Registered at the International Prospective Register of Systematic Reviews (PROSPERO) (identification code CRD42024608194).