Background/Objectives: Muscle strength and mass are key determinants of exercise performance and a hallmark of health span. Recently, several meta-analyses have concluded that protein supplementation timing does not alter muscle strength and mass gains. However, these meta-analyses did not directly compare several supplementation timings within the same study, thus limiting their conclusions. The objective of this study was to conduct a meta-analysis including only studies directly comparing protein intake before and after exercise. Methods: Three databases (PubMed (n = 748), Web of Science (n = 1458), and Scopus (n = 1105)) and reference lists were searched from inception to January 15, 2024 to identify studies where subjects were randomized to consume protein before or after each training session for at least 4 weeks. Risk of bias was evaluated using the critical appraisal checklist for RCT. A meta-analysis was performed using random-effect models. The outcomes were strength and lean body mass. Results: Of 3311 records identified, 6 reports (5 studies) were eligible and all were considered of sufficient quality to be included in the meta-analysis. For the chest press exercise, there was no effect of protein timing on repeated maximum (RM) (SMD: 0.07; 95% CI: -0.248 to 0.395; I2 = 0%, p = 0.653). For the leg press exercise, consuming protein before training increased the RM more than after training (SMD: 0.70; 95% CI: 0.005 to 1.388; I2 = 31%, p = 0.048). However, subgroup analysis did not reveal a significant effect difference (p = 0.07) for leg press and chest press. Lean body mass was not differently modulated by protein supplementation timing (SMD: -0.08; 95% CI: -0.398 to 0.244; I2 = 0%, p = 0.641). Conclusions: Protein timing does not importantly modify exercise-induced changes in lean body mass. While upper and lower limbs strength may respond differently, more investigation is needed to reach a more robust conclusion. The present review was registered in PROSPERO (CRD42023464503).
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