Revisiting Protein Intake in Fitness Training: New Evidence on Dose, Timing, and Skeletal Muscle Adaptation

Protein intake is a fundamental nutritional determinant of skeletal muscle adaptation in response to resistance-based fitness training. Over the last two decades, advances in exercise physiology and nutritional science have substantially refined the understanding of how protein dose, distribution, and timing influence muscle protein synthesis, hypertrophy, and strength development. The purpose of this article is to revisit and critically synthesize contemporary evidence on protein intake in fitness training, with a specific focus on optimal dosing strategies, temporal distribution relative to exercise, and their interaction with resistance training–induced skeletal muscle adaptation.This review integrates findings from recent systematic reviews, meta-analyses, and international position stands to examine the dose–response relationship between dietary protein intake and hypertrophic outcomes. Evidence indicates that daily protein intakes above traditional recommendations, typically in the range of ≥1.6–2.2 g·kg⁻¹·day⁻¹, are associated with greater increases in fat-free mass in resistance-trained individuals. Furthermore, emerging research challenges the traditional concept of a narrow post-exercise “anabolic window,” suggesting that total daily protein intake and even distribution across meals may exert a more pronounced influence on long-term muscle adaptation than precise nutrient timing alone. Mechanistic insights related to amino acid availability, leucine threshold signaling, and regulation of muscle protein turnover are discussed to contextualize these findings.In addition, individual moderators such as training status, age, energy balance, and training volume are examined for their role in shaping protein requirements and adaptive responses. By integrating physiological mechanisms with applied evidence, this article aims to clarify ongoing controversies and provide an updated, evidence-based framework for protein intake strategies in fitness training. The findings support a shift from rigid timing-focused models toward a holistic approach emphasizing adequate total protein intake, appropriate per-meal dosing, and alignment with resistance training demands to optimize skeletal muscle adaptation.