Fourteen days of low energy availability (22 kcal/kg fat-free mass/day) in female endurance athletes increases NADPH oxidase 2 protein content in peripheral blood mononuclear cells by 94% and amplifies exercise-induced hydrogen peroxide production by up to 341%, without altering resting levels of hydrogen peroxide or antioxidant enzyme expression.

When the body doesn't get enough energy for its training demands, it releases more of a stress hormone called cortisol. This hormone tells immune cells in the blood to make more of a protein that generates hydrogen peroxide. During exercise, these immune cells produce much more hydrogen peroxide than normal, but at rest, their output stays the same. The immune cells don't change how they fight back with antioxidants, so the extra hydrogen peroxide builds up and creates oxidative stress only when the body is active.