Can too much exercise be dangerous: what can we learn from the athlete's heart?

Exercise prevents and aids treatment of coronary heart disease, hypertension, heart failure, diabetes mellitus, obesity and depression, reduces cardiac events and improves survival. However, evidence suggests that the relationship between exercise and mortality may be curvilinear, with modest additional benefit at higher levels. Intensive exercise has also been associated with increased atrial fibrillation risk, although its clinical implications are not well understood. Other proposed adverse effects of exercise on the heart, including reduced right ventricular function, elevated cardiac biomarkers, myocardial fibrosis and coronary artery calcification, are less substantiated. Current evidence cannot affirm that extreme exercise is dangerous and future studies should combine large cohorts to obtain a statistically reliable limit. Associations between features of the athlete's heart and cardiovascular morbidity and mortality should also be explored.