Protein is the primary nutrient responsible for muscle repair and growth. It provides the essential amino acids necessary for synthesizing new muscle tissue after exercise, especially following resistance training. This process, known as muscle protein synthesis, helps repair damaged muscles and promotes hypertrophy, or muscle increase in size. Adequate protein intake is crucial for athletes and active individuals as it aids in recovery and performance. Additionally, other nutrients like carbohydrates and fats also play supportive roles, but protein is critical for the direct rebuilding of muscle fibers.