In young overweight men undergoing a 40% energy deficit and six days per week of combined resistance and high-intensity interval training, a protein intake of 2.4 g/kg/day results in lean body mass gain, while a protein intake of 1.2 g/kg/day maintains lean body mass, demonstrating that protein intake at the upper end of the Recommended Dietary Allowance prevents muscle loss under extreme caloric restriction.

When a person eats a lot of protein while exercising intensely and burning more calories than they consume, the amino acids from the protein, especially leucine, turn on a cellular switch that tells muscle cells to build more protein. This switch stays active longer because protein is eaten frequently throughout the day, and the workouts make the muscle more sensitive to it. As a result, the muscle builds more protein than it breaks down, so lean mass increases or stays the same even when the body is in a calorie deficit.