3.4.1. Performance Adaptations

McAdam et al. [51] observed a greater increase in muscle strength (push-up repetition performance) with protein supplementation (+6.8, 95%CI: 2.9–10.7) compared to a placebo (+2.6, −0.7–6.0 95% CI) during a two-minute maximal push-up test. There was no effect on run time performance (protein: −48.3 s, −63.0–33.6 s 95%; placebo: −74.2 s, −95.5–51.9 95% CI) during a two-minute maximal time trial. Similarly, Ormsbee et al. [55] found a greater increase in 1 RM bench press after six-months of concurrent training with protein supplementation (+27.4 ± 2.4 kg vs. +15.9 ± 2.8 kg, *p* = 0.003) but not 1 RM hip sled performance (protein: 72.3 ± 7.8 kg; placebo: 73.6 ± 9.0 kg). Both groups had a statistically significant increase in . VO2peak at six months compared to baseline but no differences between groups were reported. Taylor et al. [56] reported a statistically significant change in 1 RM bench press performance with additional protein in female basketball players over an eight-week period (protein = +4.9 ± 2.1 kg vs. placebo = +2.3 ± 1.4 kg, *p* = 0.046). Walker et al. [58] reported a statistically significant increase in 1 RM bench press performance (protein: +3.5 ± 5.2 kg; placebo: +1.3 ± 4.4 kg, *p* < 0.05) and the number of push-ups (protein: +5.4 ± 6.8; placebo: +3.2 ± 6.8, *p* < 0.05) performed with protein supplementation over eight weeks of recruit military training. There was, however, no effect of protein on run time performance during a maximal three-mile time-trial (protein: −1.4 ± 0.4 s; placebo: −0.9 ± 3.3 s, *p* > 0.05). The remaining study also observed a greater increase in rowing time to exhaustion with leucine supplementation compared to a placebo (*p* = 0.008) [53]. The remaining three studies reported no statistical effects on exercise performance with protein supplementation compared to a placebo or control condition [54,57]. For instance, Longland et al. [57] reported no impact of protein supplementation on leg and bench press 1 RM or cycling time trial performance. Similarly, no differences in men or women were reported between groups for changes in . VO2peak, 2000 m rowing time trial, leg and bench press 1 RM performance [54]. It was possible to include five and three studies in the meta-analyses for muscle strength [54–58] and aerobic endurance adaptations [54,55,57], respectively. One study was not included in the muscle strength analysis [51] whilst two studies were not included in the aerobic endurance adaptations [51,53] due to no SD being reported. Additionally, another study was removed from the . VO2peak meta-analysis due to assessing time trial performance [58]. The results of the meta-analysis are reported as SMD and showed that protein supplementation improved performance outcomes when muscle strength and aerobic endurance parameters were analysed together (SMD = 0.89, 95% CI = 0.08–1.70). When performance outcomes were analysed independently, protein supplementation was found to enhance muscle strength adaptations during concurrent training compared to placebo (ES = 1.18, SMD = +4.92 kg, 95% CI = −2.70–12.54 kg) (Figure 3). However, the meta-analysis found protein supplementation to not enhance aerobic endurance adaptations ( . VO2peak) with the analysis favoring placebo (ES = 0.79, SMD = −0.37 ml·kg−1·min−1, 95% CI = −1.45–0.71) (Figure 4). The individual study effect sizes for muscle strength and aerobic endurance adaptations can be found in Supplementary Figures S2 and S3. There was substantial heterogeneity between studies for muscle strength (I2 = 94%) and aerobic endurance adaptations (I2 = 95%).

**Figure 3.** Forest plot of the studies which assessed the effects of protein supplementation on muscle strength adaptations. SD = standard deviation, *N* = sample size, ES = effect size, SMD = standard mean difference, CI = confidence interval.

**Figure 4.** Forest plot of the studies which assessed the effects of protein supplementation on aerobic endurance adaptations. SD = standard deviation, *N* = sample size, ES = effect size, SMD = standard mean difference, CI = confidence interval.
