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Cardiac health

In 2016, a systematic medical review found that the risk of sudden cardiac death during or immediately after a marathon was between 0.6 and 1.9 deaths per 100,000 participants, varying across the specific studies and the methods used, and not controlling for age or gender. Since the risk is small, cardiac screening programs for marathons are uncommon. However, this review was not an attempt to assess the overall cardiac health impact of marathon running.

A 2006 study of non-elite Boston Marathon participants tested runners for certain proteins that indicate heart damage or dysfunction (see Troponin) and gave them echocardiogram scans, before and after the marathon. The study revealed that, in that sample of 60 people, runners who had averaged fewer than 56 km (35 mi) of weekly training in the 4 months before the race were most likely to show some heart damage or dysfunction, while runners who had done more than 72 km (45 mi) of weekly training showed few or no heart problems.

According to a Canadian study presented in 2010, running a marathon can temporarily result in decreased function of more than half the muscle segments in the heart's main pumping chamber, but neighboring segments are generally able to compensate. Full recovery is reached within three months. The fitter the runner, the less the effect. The runners with decreased left ventricle function had an average peak weekly training distance of 55.1 km (34.2 mi), while those who did not averaged 69.1 km (42.9 mi). The marathon was held in 35 °C (95 °F) weather. According to one of the researchers: "Regular exercise reduces cardiovascular risk by a factor of two or three in the long run, but while we're doing vigorous exercise such as marathon running, our cardiac risk increases by seven."