But what you eat is just as important as how much more you are eating. “The bottom line for weight gain is increasing total calories from a mixture of nutrient-rich protein, fat, and carbohydrates,” says Cynthia Sass, a registered dietician and virtual private-practice sports and performance nutritionist who has consulted for the New York Yankees. Muscles are built out of proteins, not fats or carbohydrates, so eating enough protein to encourage muscle synthesis is crucial.