3.4.2. Body Composition Adaptations

Fat-free mass (FFM) was shown to increase to a greater extent with protein supplementation over an eight-week training period in military recruits (protein: +0.7 ± 1.2 kg; placebo 0.0 ± 0.9 kg, *p* < 0.05) [58]. Similarly, FFM was also shown to increase to a greater extent in female basketball players over an eight-week period (protein: +1.4 kg; placebo: +0.4 kg, *p* = 0.025) [56]. McAdam et al. [51] also observed greater reductions in fat-mass over an eight-week period in military recruits with protein supplementation compared to a placebo (protein: −4.5 kg; placebo: −2.7 kg, *p* = 0.04) after controlling for initial fat-mass [51]. A trend for greater reductions in fat-mass (protein: −1.0 ± 0.3 kg; placebo: −0.3 ± 0.4 kg, *p* > 0.05) and gains in FFM (protein: +2.4 ± 0.3 kg; placebo: +1.9 ± 0.3 kg, *p* > 0.05) were reported by Ormsbee et al. [55]. However, significant differences were observed only at three months into the six-month concurrent training intervention for gains in FFM (protein:+2.6 ± 0.2 kg; placebo: 1.7 ± 0.3 kg, *p* = 0.02) in sedentary men and women. Longland et al. [57] reported greater reductions in fat-mass (protein: −4.8 ± 1.6 kg; placebo: −3.5 ± 1.4 kg, *p* < 0.05) and gains in FFM (+1.2 ± 1.0 vs. +0.1 ± 1.0 kg, *p* < 0.05) with protein supplementation compared to a placebo over a four-week period. Conversely, no effect of leucine supplementation was reported after six-weeks on fat mass (body fat percentage) changes [53]. In total, five out of six studies reported a beneficial impact of protein supplementation on body composition adaptations. Four studies were included in the meta-analysis for changes in FFM [51,55,57–59] whereas three studies were included in the meta-analysis for fat-mass [51,57,58]. One study was excluded from the FFM adaptations analysis due to no SD being reported [56] and one study was excluded from the fat-mass adaptations analysis due to body fat percentage being reported [53]. The meta-analysis found that protein supplementation enhanced gains in FFM (ES = 6.29, SMD = +0.75 kg, 95% CI = 0.44–1.06 kg) (Figure 5). The meta-analysis also found protein supplementation to enhance reductions in fat-mass compared to placebo (ES = −0.99, SMD = 0.60 kg, 95% CI = −1.20–0.45 kg) (Figure 6). The individual study effect sizes for FFM and fat-mass

adaptations can be found in Supplementary Figures S4 and S5, respectively. There was considerable heterogeneity between studies for FFM (I2 = 98%) and fat-mass adaptations (I2 = 91%).

**Figure 5.** Forest plot of the studies which assessed the effects of protein supplementation on fat-free mass (FFM) changes in response to concurrent training. SD = standard deviation, *N* = sample size, ES = effect size, SMD = standard mean difference, CI = confidence interval.

**Figure 6.** Forest plot of the studies which assessed the effects of protein supplementation on fat-mass changes in response to concurrent training. SD = standard deviation, *N* = sample size, ES = effect size, SMD = standard mean difference, CI = confidence interval.
