High-Protein Foods to Include in Your Fitness Diet Protein is the building block of muscle, and it’s essential for muscle growth, repair, and overall fitness. If you’re aiming to build lean muscle and improve your performance, including high-protein foods in your diet is a must. Here are some top protein-packed foods to incorporate into your fitness diet. 1. Chicken Breast: The Lean Protein Chicken breast is a go-to protein source for fitness enthusiasts.