Chicken is the go-to muscle-building protein. It’s a lean source of all necessary amino acids and can be prepared in myriad ways. A six-ounce chicken breast yields 54 grams of protein and four grams of leucine, the amount needed to max out protein synthesis in a given meal. Chicken breasts and thighs have similar protein contents but different flavor profiles due to differences in their fat contents. Opt for boneless, skinless chicken breasts or thighs to minimize the time needed to cook and prepare this essential protein source.