Regular aerobic exercise can be among the biggest challenges facing heart failure (HF) patients, because a weakened heart can make patients tire quickly. Many individuals with HF also face mental challenges, as an estimated 40 percent of people with heart failure suffer from depression. But a recent study in the Aug. 1 issue of the Journal of the American Medical Association found that exercise training can help relieve moderate depression symptoms, as well as improve the energy level and quality of life of HF patients. Michael Crawford, manager of Cleveland Clinics Cardiac Rehabilitation Program, says there may be multiple explanations for exercises positive mental outcomes in HF patients. One reason is that exercise releases feel good hormones such as serotonin, he explains. The primary function of serotonin is regulation of our emotions. Low levels of serotonin are thought to cause depression, and according to the American Medical Association, exercise may increase serotonin levels resulting in reduced feelings of depression.