It is suggested that men's body fat should be at least below 15% and women's body fat should be at least below 20% before they can start to gain muscle. In addition to the corresponding exercise, you should add some foods with high protein content when you gain muscle. But to avoid eating high-fat, high-calorie foods, you can usually choose nuts, avocados, tuna, chicken breast, beef, salmon, broccoli, eggs, oyster cereal, milk and so on. Some people can also eat protein powder to gain muscle.