Protein supplies the amino acids the body uses to build and repair muscle, tendon, enzymes and hormones. It is found in meat, fish, eggs, dairy, legumes, soy and grains.
The International Society of Sports Nutrition position stand on protein and exercise (2017) reports that a daily intake of 1.4–2.0 g of protein per kg of body weight is sufficient for building and maintaining muscle in most exercising people — roughly double the general adult recommendation — taken in doses of about 20–40 g spread across the day. It states that these intakes have not been shown to harm kidney function or bone health in healthy people, and that both animal and plant sources can meet the target when total intake and variety are adequate.