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Protein is primarily responsible for muscle growth and repair due to its essential role in the synthesis of muscle fibers and tissues. When you engage in resistance training or any form of exercise that stresses the muscles, small tears occur in muscle fibers. The body uses protein to repair these tears, ultimately leading to muscle hypertrophy, which is an increase in muscle size and strength.

Proteins are made up of amino acids, which are the building blocks needed for tissue growth and repair. Specifically, certain amino acids such as leucine are particularly important for stimulating muscle protein synthesis, a process that is crucial for recovery and muscle building. Additionally, consuming adequate protein after exercise ensures that the muscle recovery process is efficient, which supports overall strength and performance improvements.

While carbohydrates and fats are vital for energy and overall metabolic function, they do not play a direct role in muscle repair and growth like protein does. Vitamins are also essential for various bodily functions including metabolism and immune support, but they do not specifically contribute to muscle recovery in the same way that protein does. Thus, protein stands out as the primary nutrient that supports these critical functions related to muscle.