Protein supplementation increases adaptations to endurance training: A systematic review and meta-analysis

Background: Trials that assessed the impact of protein supplementation on endurance training adaptations have reported conflicting findings. Objective: To determine the impact of protein supplementation during chronic endurance training on aerobic capacity, body composition and exercise performance in healthy and clinical populations. Design: A systematic database search was conducted for randomised controlled trials addressing the effects of protein supplementation during endurance training on aerobic capacity, body composition and exercise performance in PubMed, Embase, Web of Science, and CINAHL. Meta-analyses were performed to outline the overall effects of protein supplementation with all studies containing endurance training components. The effects of endurance training and add-on effects of protein supplementation were evaluated by the meta-analyses with endurance training-focused studies. Results: Nineteen studies and 1162 participants contributed to the analyses. Compared with the control group, the protein supplementation group demonstrated greater improvements in aerobic capacity measured by mixed peak oxygen uptake (VO2peak) and peak workload power (W-peak) (standardised mean difference [SMD] = 0.36, 95% confidence interval [CI]: 0.05 to 0.67), and VO2peak (mean difference [MD] = 0.89 mL.kg(-1). min(-1), 95% CI: 0.07 to 1.70); had a greater lean mass gain (MD = 0.32 kg, 95% CI: 0.07 to 0.58); and had a greater improvement in time trial performance (MD = -29.1s, 95% CI:-55.3 to -3.0). Secondary analyses showed that, in addition to the substantial improvement in VO2peak (MD = 3.67 mL.kg(-1).min(-1), 95% CI: 2.32 to 5.03) attributed to endurance training, protein supplementation provided an additional 26.4% gain in VO2peak (MD = 0.97 mL.kg(-1).min(-1), 95% CI: -0.03 to 1.97). Conclusion: Protein supplementation further increased aerobic capacity, stimulated lean mass gain, and improved time trial performance during chronic endurance training in healthy and clinical populations. (C) 2020 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.