**4. Discussion**

This review identified 11 studies which investigated the effects of protein supplementation on exercise performance, body composition and muscle recovery adaptations to concurrent exercise training compared to a placebo in healthy adults, confirming the need for more work in this area. The key findings from the literature that met our inclusion criteria demonstrated that protein supplementation had a large effect on muscle strength and FFM adaptations to concurrent exercise training. There was limited evidence to sugges<sup>t</sup> that protein supplementation can support aerobic endurance and muscle recovery adaptations.

#### *4.1. Muscle Strength and Body Composition Adaptations*

Longland et al. [57] reported no impact of protein supplementation on muscle strength adaptations despite a greater increase in FFM compared to a placebo condition. This was the only study to purposely induce a negative energy balance while participants consumed a total protein intake of 2.4 <sup>g</sup>·kg−1·day−1. The study duration (four weeks) may have been too short for differences in strength development to be detected, particularly as protein supplementation is suggested to promote gains in FFM and muscle strength as the duration of training increases [62]. Forbes and Bell [54] also reported no effect of an additional 2.0–2.4 <sup>g</sup>·kg−1·day−<sup>1</sup> of protein on muscle strength and body composition adaptations over a six-week period (Table 2). It may be that the findings were also confounded by the intervention duration, given that it was shorter than each study reporting a positive effect of protein supplementation. The results were analysed by sex and the small sample size (15 women and 16 men) may have also limited the statistical findings as acknowledged by the study authors. Furthermore, the participants in the control condition consumed 1.2–1.4 <sup>g</sup>·kg−1·day−<sup>1</sup> of protein, which may have been adequate to meet the demands of training, however, with no measure of nitrogen balance or protein turnover, this cannot be confirmed. The final study which observed no effect of protein supplementation on body composition adaptations may have had low total daily protein intakes (0.85 <sup>g</sup>·kg−1·day−1) [53].

The studies that reported a positive effect of protein supplementation on muscle strength and body composition adaptations provided protein to participants immediately post-exercise [51,55–58]. This likely maximised myofibrillar protein synthesis in response to concurrent training [33,35,38,39] and modulated muscle strength and FFM adaptations [63,64]. Promoting MPS post-exercise is an important factor at enhancing skeletal muscle remodelling and adaptation [39,42,64,65]. Subsequently, this could have attenuated the interference effect of endurance training on strength adaptations [35,66] by promoting the activation of mTOR and inhibiting the activation of the tuberous sclerosis complex [27,28]. Skeletal muscle is sensitive to protein feeding for 24 hours postexercise and thus, consuming protein in 20–40 g doses evenly throughout the day is recommended [64,67,68]. More recently, it has been shown that consuming protein prior to sleep also augments MPS throughout the night [69]. Consuming protein prior to sleep, and subsequently increasing total daily protein intake may be advantageous at optimising MPS responses and supporting muscle strength and body composition adaptations when undertaking concurrent training [70].Nevertheless, it should be acknowledged that acute changes in MPS does not necessarily predict changes in muscle strength and FFM [71]. Instead it is likely the chronic and repetitive changes in MPS and muscle protein breakdown (MPB) which contribute to these [65]. Two studies that reported a greater increase in muscle strength reported a larger reduction in fat-mass with protein intakes ≥2.2 <sup>g</sup>·kg−1·day−<sup>1</sup> compared to a placebo [51,55]. This suggests that individuals undergoing arduous concurrent training may benefit from protein intakes higher than the current recommendation of 1.8–2.2 <sup>g</sup>·kg−1·day−<sup>1</sup> [72]. Mechanistically, it is speculative as to how an elevated protein intake promoted a greater loss in fat-mass, but previous work suggests that the greater

thermic effect of protein may play a key role [73]. However, despite similar daily energy intakes between groups in both studies, neither included a measure of energy expenditure, and therefore, it is unclear if participants were in energy balance. Consuming a protein intake >2.2 <sup>g</sup>·kg−1·day−<sup>1</sup> and possibly higher than 3.0 <sup>g</sup>·kg−1·day−<sup>1</sup> while restricting energy intake has been suggested to maximise the loss of fat-mass and promote the maintenance of FFM [72]. It is unclear if the greater reduction of fat-mass promoted greater improvements in muscle strength performance in studies included in this review [51,55]. More work is needed to better determine the impact of protein intakes higher than the current recommendations (1.7–2.2 <sup>g</sup>·kg−1·day−1) on body composition adaptations, and how this may influence exercise performance in individuals undergoing arduous concurrent training. The studies identified in this review sugges<sup>t</sup> that protein supplementation may be an effective strategy at augmenting muscle strength and body composition adaptations in healthy adults undertaking concurrent training. It is likely that this effect is facilitated by maximising the MPS post-exercise and attenuating the potential interference effect of endurance training on muscle strength and FFM adaptations. However, more work which includes measures of nitrogen balance or protein turnover are needed to confirm this. Furthermore, future work should also consider factors such as the timing of protein intake around exercise, energy intake/expenditure, and the duration of the training intervention.
