‘All things are poison and nothing is without poison, only the dose makes a thing not a poison’.

When Phillipus Aureolus Theofrastus Bombastus von Hohenheim—better known as ‘Paracelsus’—a Swiss German Renaissance physician, botanist, astrologer and philosopher coined this well-known quote about 500 years ago, he might not have thought about dose-dependent ‘toxic’ effects of regular sport. Nevertheless, these words perfectly fit a highly debated hot topic in sports cardiology: where is the upper limit of ‘healthy’ physical exercise and up from which ‘dose’ it might be even harmful?

Although quite much is known about the minimal efforts of physical exercise that have to be provided to gain health benefits, so far no ‘upper limit’ has been defined.1 However, adverse longtime effects of regular strenuous endurance training (eg, in cross-country skiers and bicycle racers) could be defined: as such, a multitude of studies proved an increased rate of atrial fibrillation in endurance athletes at an older age compared with age-matched controls.2 ,3 The underlying morphological substrates are miscellaneous, consisting of inflammation, fibrosis and cavity dilation (figure 1).3

Another important ‘weak spot’, particularly in ambitious endurance athletes’ hearts, is the right ventricle. As such, postrace dilation and decreased function of the right ventricle have been demonstrated impressively in cyclists, recently.4 Moreover, these changes are regressive but it might take a few weeks or months for the right ventricle to regain its …