Here are two facts that may or may not be connected. At conference in 2012, researchers presented preliminary data from an analysis of 50,000 patients at the Cooper Clinic in Texas, suggesting that running more than 20 miles per week is as bad for your long-term health as not running at all. And in 2013, participation in running races in the United States peaked at around 19 million before starting a steady decline that has continued to this day.
The question of whether too much aerobic exercise is bad for your heart was hotly debated for several years after that 2012 study. The Cooper Clinic data, when it was finally published in a peer-reviewed journal more than two years later, had been reanalyzed so that the supposed dangers of too much running disappeared. But by then the idea was firmly implanted in the public mind: marathons are dangerous. (I wrote in depth on this dispute, and how the evidence has shifted, in this 2016 feature.) The topic no longer pops up in the headlines as regularly as it did a few years ago, but it’s still lurking in people’s minds.
That’s why I think it’s worth saying a few words about a newly published study that draws some conclusions I once would have considered too painfully obvious to write about. It’s a massive analysis of the survival rates of 122,000 patients who received maximal treadmill testing at the Cleveland Clinic in Ohio between 1991 and 2014, published in JAMA Network Open. The question it asks is simple: do people with higher VO2max—higher aerobic fitness, in others words—live longer?
VO2max refers to the maximum rate at which you can suck oxygen into your lungs, transfer it to your bloodstream, and deliver it to your working muscles. A high value means your heart, lungs, and circulatory system are working well (and probably means you can run or cycle or swim pretty fast too). The most accurate way to measure it is with an exercise test, usually on a treadmill, to the point of exhaustion, while wearing a mask that measures the gases in the air you breathe in and out.
The links between higher aerobic fitness and life expectancy are well-known. In fact, back in 2016 the American Heart Association issued a scientific statement arguing that VO2max should be considered a “vital sign” and measured (or at least estimated) regularly by your doctor. It’s a better predictor of mortality than risk factors like smoking and high blood pressure. But most of the data backing these statements comes from relatively average people, not endurance athletes. What if the dangers of being really, really superfit are being overlooked?
A 2015 study of 37,000 people in Detroit took a preliminary look at this question. Their data showed the usual relationship, with greater VO2max associated with longer survival. They looked specifically at the fittest people in their cohort, and found no hint that the benefits were leveling off. Even a VO2max of 16 METs was better than 15 METs; and while they didn’t have enough subjects at higher fitness to get a statistically significant result, an “exploratory analysis” suggested that values higher than 16 METs were even better. (The MET unit, or “metabolic equivalent of task,” is a multiple of your basal metabolic rate. If your peak value is 16 METs, that means you’re burning 16 times as much energy at the end of your treadmill test as you would if you were just lying quietly on the sofa. As a rough estimate, each MET is roughly 3.5 ml of oxygen per minute per kilogram of body weight, meaning that 16 METs is a VO2max of 56 ml/kg/min.)
The new study, with a much larger cohort, reaches similar conclusions. With an average follow-up period of 8.4 years and a total of 13,637 deaths, greater fitness conferred greater probability of survival right to the very top of the spectrum. Those with “elite” fitness, defined as above the 97.7th percentile relative to their age and sex, were only 20 percent as likely to die during the study as those with low fitness—impressive, but no surprise. But even those with high fitness, in the top quartile, were 29 percent more likely to die during the study compared to the elite group.
In fact, a supplementary analysis found that those results persist even if you define elite as the top 1 percent. Being really fit, which in this study required values comparable to those seen in studies of endurance athletes, was a health advantage rather than a disadvantage.
So how do you reconcile these results with the other studies (for an overview of those studies, see here) that seem to find dangers in too much exercise? The VO2max studies measure fitness objectively, which avoids the problems of self-reported questionnaires about exercise habits. But VO2max isn’t just a result of your exercise habits; it also reflects your genetics. Some people have relatively high fitness even if they don’t exercise much; others only achieve modest levels of fitness despite doing lots of exercise. That means studies of fitness and of exercise habits aren’t measuring the same thing.
I don’t know exactly how to reconcile those two sets of data. I’m inclined to trust the objective VO2max data, but that’s probably at least in part because I like the conclusions. In the end, the link between too much exercise and negative effects on your health is as tenuous as the link I drew at the top of this article, between scare stories about extreme exercise and declining participation in endurance sports like running. Whatever the reason for that decline, it’s bad news for the health of Americans, because in today’s world, too little exercise is way riskier than too much—if such a thing really exists.
My new 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.