*4.4. Limitations*

Only one study identified in this review included a measure of protein balance or turnover [57]. Therefore, it is unknown if participants were in a positive protein balance before, during or after the interventions in the remaining studies. Including a measure of protein requirements in future studies can allow for a better understanding of how much additional protein is potentially needed during arduous concurrent training by estimating changes in MPS and whole-body protein balance. The majority of studies failed to include markers of skeletal muscle damage or inflammation when focusing on muscle recovery and therefore, the mechanisms of the effects observed are speculative. The control of dietary intake is critical for comparison between studies involving nutritional interventions. Four studies failed to report the total daily protein intake during the intervention [56,58,59,61], making the comparisons between studies even more challenging, given that this is considered more important than the timing of protein intake [71,83]. It is recommended that these methodological considerations be factored into future studies aimed at investigating the influence of protein supplementation on arduous concurrent training adaptations. The heterogeneity of the meta-analysis results should be acknowledged as this may make it difficult to apply these findings to a specific population. Nonetheless, the findings of this review infer that protein supplementation can support muscle strength, aerobic endurance, and body composition adaptations during concurrent training. However, more population specific randomised controlled trials (RCTs) are needed to build upon these findings.
