Role of ingested amino acids/protein in the promotion of resistance exercise-training adaptations in aging: analysis of meta-analyses

A current lack of consensus exists regarding the effect of protein supplementation during resistance exercise on the phenotypic adaptation in aging adults. Thus, we critically assessed the collection of meta-analytic evidence to provide clarity regarding the differences between meta-analyses examining the combined effectiveness of chronic exposure to PRO/AA supplementation and resistance exercise to enhance the adaptive response. Thirteen meta-analyses, with relatively similar titles, presented different results on the topic. This divergence is mainly due to an inconsistent study selection process resulting in distinct study populations and varied types of protein-focused nutritional interventions and not RCT study quality. The methods applied to extract and estimate effects from RCTs with incorrectly formatted data for meta-analyses likely provide an additional reason for divergent results. PRO/AA supplements (when combined with resistance exercise training) produced a positive, albeit minor effect on the promotion of whole-body lean mass growth, yet a minimal and inconsistent effect on muscle mass, muscle strength, or functional capacity. The lack of an effect was skewed in studies with a higher proportion of obese and overweight participants and somewhat less noticeable in those containing sarcopenic and frail older adults, who would have the greatest need for an intervention to enhance muscle mass. Researchers are encouraged to provide the change scores mean and standard deviations for all their outcomes by group or even making the data sets available to improve future meta-analyses and advance the field.