Effects of Supplementation with Milk Proteins on Body Composition and Anthropometric Parameters: A Systematic Review and Dose–Response Meta-Analysis

Background/Objectives: There is no consensus regarding the impacts of supplementation with milk proteins (MPs) on body composition (BC). This systematic review and dose–response meta-analysis of randomized controlled trials (RCTs) assessed the effects of MP, casein protein (CP), and whey protein (WP) supplementation on BC and anthropometric parameters. Methods: A comprehensive search was performed in several databases to identify eligible RCTs published until October 2025. Random-effects models were applied to estimate the pooled effects of MP supplementation on anthropometric parameters. Results: A total of 150 RCTs were included. MP supplementation substantially increased lean body mass (LBM) (weighted mean difference (WMD): 0.41 kg; 95% CI: 0.19, 0.62; p < 0.001) and fat-free mass (FFM) (WMD: 0.67 kg; 95% CI: 0.40, 0.94; p < 0.001). It also significantly reduced body fat percentage (BFP) (WMD: −0.66%; 95% CI: −1.03, −0.28; p = 0.001), fat mass (FM) (WMD: −0.66 kg; 95% CI: −0.91, −0.41; p < 0.001), and waist circumference (WC) (WMD: −0.69 cm; 95% CI: −1.16, −0.22; p = 0.004). No considerable effects were observed for muscle mass (MM), body mass index (BMI), and body weight (BW). Dose–response analysis revealed that MP dosage was associated with significant changes in BFP, LBM, and MM. Conclusions: MP supplementation was associated with favorable modifications in body composition, including increases in LBM and FFM, as well as reductions in FM, BFP, and WC. These findings provide coherent and consistent evidence supporting the potential role of MP supplementation in targeted body composition management.