"So in the Breuckmann study, they recruited 102 active marathon runners. To be a marathon runner (and perhaps to maximize their power to show how healthy "cardio" is) required at least 5 marathons in the past 3 years. Many had run dozens or more in their lifetime. Anyone with a known history of heart disease or diabetes was excluded. The average age was 57 with age 50-72. The median number of marathons was 20. Weekly mileage was 35 (55km). Mean work was 4700 METs per week.
"There were 102 totally asymptomatic age-matched controls, also with no history of diabetes, who had no significant history of vigorous exercise.
"All subjects had cardiac MRI with LGE imaging. Those with LGE abnormalities were called back to have perfusion imaging as well to help tell if they had evidence for ischemia (which might indicate narrowing of a coronary artery).
"What do you think they found? After all, these were a bunch of completely asymptomatic runners.
"Would you believe 12% of asymptomatic marathon runners had evidence of myocardial damage on LGE?
"Would you believe that among the sedentary age matched controls only 4% had abnormal LGE?"
I definitely suggest reading the whole thing. I don't agree 100% with Dr. Harris on this topic, but he covers most of the objections I would make in his article, along with his replies.
I do think that he's a thoughtful person, and his ideas deserve careful consideration.
What I would love to see is a reconciliation by him of the studies he discusses in this article with the Stanford Running Study, which has contradictory results.