*4.2. Aerobic Adaptations*

Protein requirements are elevated in endurance athletes to 1.6–1.8 <sup>g</sup>·kg−1·day−<sup>1</sup> [74] but may be higher (1.7–2.2 <sup>g</sup>·kg−1·day−1) during periods of intense and/or high volume training [72]. Protein feeding has been shown to facilitate recovery and performance adaptations to endurance training [17,64]. Nonetheless, the effects of protein supplementation on aerobic performance adaptations during an arduous concurrent training programme are unknown. Similar improvements in run time performance was observed in military recruits with total daily protein intake of 2.8 ± 0.5 and 1.6 ± 0.4 <sup>g</sup>·kg−1·day−<sup>1</sup> in the protein and placebo conditions, respectively [51]. As such, the dietary protein requirements to facilitate endurance-based adaptations were likely met in both groups, therefore, between groups differences were not observed. This suggests that to promote endurance-based performance adaptations, additional protein intake is not warranted when total habitual intake is ≥1.6 <sup>g</sup>·kg−1·day−1. Walker et al. [58] also observed no between group differences in run time performance in military recruits supplemented with whey protein or CHO for eight-weeks. However, the total daily protein intakes were not reported, and it is unknown if protein requirements were met. In contrast, one study found leucine supplementation (45 mg·kg−1·day−1) for six-weeks improved exercise time to exhaustion in canoeists [53]. The reported total daily protein intake was 0.85 ± 0.06 <sup>g</sup>·kg−1·day−<sup>1</sup> and 0.85 ± 0.05 <sup>g</sup>·kg−1·day−<sup>1</sup> in the protein and placebo groups, therefore, participants likely benefited from the elevated leucine intake given that this amount of protein per day is much lower than the recommended amount [72,74]. Leucine is the key amino acid which stimulates MPS through the mTOR pathway [39,75], but given that the other essential amino acids are required to support this process [36,39], it is unclear how leucine supplementation improved exercise time to exhaustion. The results of the meta-analysis sugges<sup>t</sup> that there is limited evidence to support the use of protein supplementation for aerobic endurance adaptations in response to concurrent training compared to placebo. However, based on the limited number of studies identified, more work is needed to confirm this.
