His advice: "Be consistent and avoid extremes if you are exercising solely for health." In other words, "Don't be a weekend warrior."
When it comes to suggesting weekly mileage guidelines, Eichelberger hesitated. "I'm not sure there is really a correct distance that applies to everybody. The idea that running less than 20 miles a week is good, and running more than 20 is bad -- I'd view that with a lot of skepticism. It probably truly varies depending on a lot of different factors -- patient age, whether they're in shape or not and other comorbidities, among other factors," he said.
He also noted that the heart's not the only part of the body that's affected by extreme endurance training; it takes a toll on the musculoskeletal system as well.
Lavie, an author on the second study as well as the first, said the endurance athlete study shows that prolonged, intense exercise has its risks.
"We know people are going to want to run marathons and triathlons, and doing this occasionally is probably okay," Lavie said. "But from a health standpoint, it would be best not to become a very frequent long-endurance exerciser."
"Mix it up. Go do some yoga, or strength-training," O'Keefe added.
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