Testosterone Replacement Therapy as a Foundation for Body Composition Remodeling: Synergistic Roles of Resistance Training and Protein Intake.

Testosterone plays a central role in the regulation of body composition, skeletal muscle metabolism, and metabolic health in men. Testosterone deficiency is frequently associated with increased adiposity, reduced lean body mass, impaired physical performance, and adverse metabolic profiles, contributing to the development of sarcopenia and cardiometabolic disease. Testosterone replacement therapy (TRT) has emerged as an effective intervention to restore physiological androgen levels and improve body composition by promoting increases in lean mass and reductions in fat mass. This review proposes a conceptual framework in which TRT functions as the biological foundation upon which lifestyle interventions exert amplified anabolic effects. Mechanistic and clinical data demonstrate that TRT enhances muscle protein synthesis, satellite cell activation, and mitochondrial function, thereby supporting both the quantity and quality of skeletal muscle. When combined with resistance exercise, TRT amplifies hypertrophic responses and functional performance, while adequate protein intake provides the necessary substrates to sustain muscle remodeling and preserve fat-free mass. This integrated framework highlights the limitations of relying solely on body weight as a clinical metric and underscores the importance of evaluating body composition changes in the context of metabolic health. When appropriately prescribed and combined with targeted lifestyle interventions, TRT may represent a comprehensive strategy for improving musculoskeletal integrity, enhancing metabolic function, and reducing the burden of hypogonadism-related complications. Further research is warranted to refine patient selection, optimize treatment protocols, and clarify long-term clinical outcomes.