In young overweight men undergoing a 40% energy deficit and six days per week of combined resistance and high-intensity interval training, changes in serum cortisol are significantly correlated with changes in fat mass (r = 0.39, P = 0.01) and lean body mass (r = -0.34, P = 0.03), but account for only 11–16% of the variance in body composition changes.

When a person eats a lot of protein while losing weight and doing intense exercise, the amino acids from the protein turn on a molecular switch in muscle cells that tells them to build more muscle. This keeps muscle from breaking down, even when the body is low on energy. As a result, the body doesn't need to rely as much on stress hormones like cortisol to break down tissue for fuel, so cortisol levels don't rise as much and don't drive fat loss or muscle loss as strongly.