Cardiac risks of excessive exercise: can too much physical activity damage the heart?

Although regular exercise is a crucial part of a healthy lifestyle, new research suggests that very high amounts of endurance training may cause damage to the heart when performed over many years. In this talk from November 2017 Professor David Stensel, Professor of Exercise Metabolism at Loughborough University, examines the most recent research in this area and questions whether it is now necessary to set an upper ‘safe limit’ for exercise.

[@rayF4rio]

I guess these studies do account for people who are more likely to take up endurance exercise or intense exercise habits due to the fact that they may have a family history of some heart disease, or they have a personal history of poor diet, weight gain/loss, metabolic syndrome or a sedentary history since childhood. Meaning that intense exercise may not totally negate unhealthy habits or family history risks, or that endurance exercise self selects for a less healthy population to start with.
Also, all cause mortality also includes accidents and injuries (hip/knee) that then result in knock on health impacts which could severely increase mortality.
But the takeaway does remain, more is not always better and the answer usually includes an "it depends" clause.