Researchers have found no evidence of elevated cardiac risk in runners who completed a 24-hour ultramarathon (24UM), despite the transient elevation of blood biomarkers that measure cardiac health. According to the study in the journal Heliyon, published by Elsevier, trained runners were more likely than their novice counterparts to experience raised levels, reflecting the greater cardiac load and pituitary-adrenocortical response to extremely strenuous exercise.
Source: Extreme exercise can strain the heart without causing permanent damage