Intensive exercise and its effect on the heart: Is more always better?

Regular physical exercise is undoubtedly beneficial for humans. Since the 1950s, numerous prospective follow-up studies have assessed both all-cause and cardiovascular (CV) mortality in relation to levels of physical activity. The results have always been unambiguous. Both men and women who presented with higher levels of physical activity were found to have a reduced risk of death (by up to 50%). Noteworthy, those who maintain a physically active lifestyle but at the same time present with other risk factors for CV diseases (CVD; smoking, hyperlipidemia, hypertension, diabetes, obesity) appear to be at a lower risk of premature death than sedentary subjects without risk factors [1]. Physical activity reduces the incidence of hypertension, ischemic heart disease and CV events, type 2 diabetes, cancers (in particular colon and breast cancer) and osteoporosis [2]. Furthermore, when applied in the secondary prevention of CVD, regular physical exercise, regular physical exercise reduces all-cause mortality. Whether for health-related reasons or not, the number of people participating in sports activities is increasing and healthy lifestyle has indeed become fashionable in the last decade. In addition, extreme forms of endurance exercise such as marathon, triathlon and ironman triathlon have become increasingly popular among amateurs. Globally, the number of marathon runners increased by 13% in 2009–2014 and 0.5% of the United States population participated in at least one marathon [3, 4]. Amateur participation in these extreme sports is associated with the need for hours of training, often daily, and competitions are exhausting. Thus, a question arises whether in spite of undisputable health benefits connected with physical activity, such intensive forms of endurance sport still continue to be so, or if they can adversely affect the cardiovascular system. Is more always better?