Proteins are needed to build and repair muscles, but carbohydrates also play a  necessary role in maintaining muscle mass, as well as recovery. Carbohydrates provide the energy needed to sustain physical efforts and help prevent muscular fatigue. So, a combination of proteins, carbohydrates, and healthy fats is required to support optimal muscle growth and repair. Protein lays the foundation or building blocks of muscle mass, but carbs sustain and prevent muscle damage while working out. Therefore, both are needed to build muscles.