Texas, USA: Ankit Rohatgi, 2017) or calculated from baseline values and/or percentage change. Where data were not presented in table or text and authors could not be reached, data were extracted using WebPlotDigitizer (Web Plot Digitizer, V.3.11. Relevant variables included those regarding the study design, details of the RET intervention, participant characteristics, protein supplement information, placebo/control information, performance outcomes, body composition outcomes and any other notable information (eg, sources of bias/conflict of interest). Predetermined relevant variables from each included study were gathered independently by three investigators (RWM, KTM and SRM).
14 Since measures of MPS show good agreement with hypertrophy 13 we theorised that the effect of daily protein intake on RET-induced changes in muscle mass would show a dose-responsive relationship but that this would ultimately plateau. We also undertook an additional rational, mechanism-based analysis that had the aim of answering the following question: is there a protein intake beyond which protein supplementation ceases to provide a measurable benefit in increasing muscle mass during RET? To answer this question, we recognised that the process of muscle protein synthesis (MPS), as the primary determinant of muscle hypertrophy, 13 shows a saturable dose-response relationship with increasing protein intake. The present meta-analysis includes more than double the number of studies and participants than the largest published comprehensive meta-analysis on protein supplementation during RET to date. We used meta-regression to evaluate the impact of important potentially mediating covariates that were decided a priori to the meta-analysis. We conducted a meta-analysis that was more inclusive in nature than previous meta-analyses 6–12 to provide a broad, systematic and evidence-based assessment on whether protein supplementation can augment changes in relevant RET outcomes.
Thus, an evidence-based answer to the main question of the efficacy of protein supplementation, while previously reported, 7 now appears to be controversial. This lack of agreement on the efficacy of protein supplementation 6–12 is likely due to the use of divergent study inclusion criteria and inclusion of subjects with differing: ages, training statuses, total protein intakes, protein sources and protein doses. Despite a large volume of work in this area, narrative reviews 1–5 and even meta-analyses 6–12 yield conflicting results as to the actual effectiveness of protein supplementation to enhance RET-mediated gains in muscle mass and strength.
Recognised as a potent antisarcopenic stimulus, protein supplementation has also been advocated for ageing persons participating in RET. Resistance exercise training (RET) in combination with dietary protein supplementation is a common practice, in athletes and recreational exercisers alike, with the aim of enhancing RET-induced gains in muscle mass and strength.
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