**3. Results**

#### *3.1. Study Quality and Risk of Bias Assessment*

Table 1 outlines the quality assessment scores for each study. All 11 studies were considered eligible for this review based on their COSMIN quality assessments scores.

**Table 1.** The individual and mean reviewer quality assessment scores for each study.


Yes = Y; 1 = poor, 2 = fair,3 = good,4 = excellent.

Egger's regression analysis found asymmetries in the funnel plot (Figure 2) suggesting that results might be influenced by biasing factors such as publication bias. One study was particularly responsible for this asymmetry [57] as they showed the strongest beneficial effects of the treatment group when compared with the placebo group. When this outlier was removed the funnel plot was symmetric (*p* > 0.05). The funnel plot with this outlier included can be seen in Supplementary Figure S1.

**Figure 2.** Funnel plot of the comparison of the effect of protein supplementation vs. placebo on muscle strength, aerobic endurance, fat-free mass (FFM) and fat-mass (FM) adaptations. CES = combined effect size.

#### *3.2. Participant Characteristics and Study Interventions*

Details of the studies' characteristics are provided in Tables 2 and 3. The sample sizes ranged from 10 to 387 with a total participant sample size of 681 (645 men, 61 women) for all studies. The reported mean age of participants ranged between 18 and 31 years. Intervention durations ranged from one day to six months with seven studies using a standardised concurrent endurance and resistance training programme [53–58], two studies using a military training programme [51,58] and two studies using an acute loaded march protocol [59–61].

**Table 2.** The impact of protein on performance and body composition during concurrent exercise training.


Data reported as mean ± standard deviation where possible. U.S. = United states, <sup>g</sup>·kg−1·day−<sup>1</sup> = grams per kilogram of body mass per day, PLA = placebo, PRO = protein, WP = whey protein, CHO = carbohydrate, CON = control, AM = morning.

> **Table 3.** Concurrent exercise training and the impact of protein on muscle recovery.


Data reported as mean ± standard deviation. U.S. = United States, <sup>g</sup>·kg−1·day−<sup>1</sup> = grams per kilogram of body mass per day, PLA = placebo, PRO = protein, WP = whey protein, CHO = carbohydrate, CON = control. \* maximum across three timepoints.
