Back in 2011, former Boston Marathon champion and longtime Runner’s World journalist Amby Burfoot interviewed a leading cardiologist about emerging claims that too much running might be bad for your heart. The controversy hadn’t really blown up yet, and it wasn’t until the following year that the scary headlines hit mainstream media outlets: “One Running Shoe in the Grave,” “Running Farther, Faster and Longer Can Kill You,” and so on.
But the questions were starting—and Burfoot, for one, was already getting tired of them. Small studies seemed to hint that high volumes of endurance exercise might negatively affect various proxy markers of health like the thickening of heart tissue and calcium levels in hardened arteries. Burfoot wanted more tangible evidence, though: “If you think you’ve got something,” he suggested, tongue in cheek, “show me the bodies in the streets.”
In the eight years since then, as fears of the supposed dangers of too much running waxed and then gradually waned, I’ve often thought about Burfoot’s challenge. Researchers have spent a lot of time searching for those bodies, sifting through long-term data from tens of thousands of people to see if the heaviest exercisers were expiring at a higher rate than expected. Overall (as I found when I took a deep dive into the research back in 2016), the picture looks fairly reassuring for runners. But there’s a notable gap in the data: in most studies, even just an hour of exercise per day puts you in the extreme upper category. For a lot of marathoners, ultrarunners, triathletes, and other endurance junkies, that’s where the fun is just getting started.
That’s what makes some new data presented at an American Heart Association conference in Philadelphia last week so interesting: it includes a bunch of truly hardcore endurance athletes who were averaging more than five hours of exercise per day and had been doing it for decades. The good news: their hearts for the most part looked fine.
The new analysis is a follow-up to a previous study, published earlier this year, from a team led by Laura DeFina at the Cooper Institute in Dallas, working with cardiologist Benjamin Levine of the University of Texas Southwestern Medical Center. The earlier study looked at 21,758 healthy men who had undergone testing at the Cooper Clinic starting in 1998, and found that “high” levels of physical activity (more on that definition shortly) were indeed associated with elevated levels of calcium indicating stiffer and less healthy arteries, but didn’t seem to lead to any increase in death from heart disease or any other cause after a decade of follow-up. In other words, the risk factors that signal trouble in the average person don’t necessarily have the same meaning in highly fit people.
So what does a “high” level of physical activity mean? The researchers defined it as more than 3,000 MET-minutes per week of exercise. A MET (or “metabolic equivalent of task”) is a measure of how hard you’re exercising, in multiples of your basal metabolic rate. Let’s say you typically burn about 70 calories per hour when you’re just lying around on the sofa. If you go for a run at 10:00/mile pace, that’s equivalent to an intensity of about 10 METs for the average person, which means you’re burning about 700 calories per hour. If you run for 60 minutes at that pace, you’ll have accumulated 600 MET-minutes of exercise (60 minutes times 10 METs). Put all that together, and you find that 3,000 MET-minutes works out to the equivalent of five hours of running at a 10:00-mile pace, or about 30 miles a week of running at that pace.
In most circles, that’s a lot of exercise. But when he read the paper, Burfoot noticed that there were a substantial number of subjects in the cohort who were racking up even bigger numbers. He asked Levine what the data revealed about these super-high exercisers—and the result was last week’s conference presentation, which compares 2,088 men and women with high physical activity levels (more than 3,000 MET-minutes per week) to 66 men and women with “extraordinary” physical activity levels (more than 10,000 MET-minutes per week). Burfoot, for his curiosity, is listed as the seventh author of the study, and wrote an account of the findings in the Washington Post.
(As an aside, it’s interesting that some earlier studies start by defining anything greater than 60 to 90 minutes of exercise per day as “excessive endurance exercise” (EEE), a definition that more or less assumes its own conclusions. The fact that the new study instead uses the term “extraordinary” offers us a telling hint of where the researchers’ sympathies lie.)
Of these 66 extraordinary exercisers, 12 were women. They had an average age of 53.2, exercised on average 35.1 hours and accumulated 13,921 MET-minutes per week, and had been exercising for an average of 28.5 years. That data allows us to determine that their average exercise intensity was 6.6 METs, which equates to jogging or cycling at a self-selected light effort. Since the data is based on survey responses, we don’t really know exactly what these people were doing, let alone how on earth they managed to find time for all this exercise. Clearly most of them weren’t running for five hours a day, so it seems likely that there are some long-course triathletes and cyclists in the group.
After an average of 10 years of follow-up, two of the 66 extraordinary exercisers had died. Neither of the deaths were related to heart disease or other cardiovascular conditions. Overall, the results for the extraordinary group looked pretty much identical to the plain old high exercise group. They had similar BMI, VO2max, cholesterol, coronary artery calcification, and so on. The sample size is small, and the findings will get more rigorous as time passes and more subjects from all groups pass away. But for now, the lack of any obvious impact of exercising for 35 hours a week is a useful data point on a part of the graph with very little previous data.
If you’re a glass-half-empty kind of person, the takeaway here might be that ramping up your exercise to multiple hours per day doesn’t do anything more for your health beyond what you’d get from a much lower dose. Some of the previous data suggests that simply meeting the standard recommendations for physical activity—30 minutes of moderate exercise five times a week—will give you most of the available benefits. But on the glass-half-full side, it looks like going to extremes is not going to kill you either.
As for the challenge Burfoot issued back in 2011, we’re still waiting. “Our paper, based on far more data than previous papers, is essential to counter the idea that the streets are littered with the bodies of dead runners,” Levine tells Burfoot in his Washington Post article. “We studied the most strenuous exercisers ever, and there were no heart deaths in 10 years of follow-up.”
My book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.