Cortisol is a steroid hormone that regulates a wide range of vital processes throughout the body, including metabolism and the immune system. It also has a very important role in the body’s stress response. However, prolonged high levels can have a catabolic effect, leading to the breakdown of muscle tissue, and may suppress the immune system. The immunosuppressive effects of intense exercise have been linked to high plasma cortisol levels that continue after prolonged intense exercise.