Lean beef, skinless chicken and cottage cheese are some of the foods that are good muscle weight gainers. All three are relatively high in protein, which is essential in muscle formation.
Lean beef is top on the list, as it contains relatively few calories yet has high amino acid content, which tandems with insulin to promote muscle growth. Skinless chicken, likewise, is a good source of protein, which is essential to muscle and weight maintenance. Cottage cheese, on the other hand, contains a slow digesting protein called casein. This is very useful, especially for those who cannot eat for long periods of time.