Reporting on emerging science can sometimes feel like watching live coverage of an ultramarathon. Sure, there is the occasional dramatic movement, but for long periods of time nothing seems to happen. Beneath the surface, however, the action continues. Tiredness mounts, blisters begin to form, an aid station is missed… the evidence gradually accumulates, and only later do we realize when the outcome was decided.
In that spirit, I have a couple of mid-race updates on a topic of long-standing interest: the potential ill effects of too much endurance exercise. I have been reporting on this controversy for more than a decade and summarized the current state of the evidence most recently last summer. It would be nice, of course, if we now had final evidence on whether training for marathons or ultramarathons could harm the heart. Instead, it has become clear that the perfect study is nearly impossible to design, because you simply can’t randomize people to spend a few decades running marathons or lounging on the couch. Still, the steady trickle of incremental evidence continues, and two new studies fill in some important gaps.
rigid arteries
First, published in the British Journal of Sports Medicineexplores the links between exercise and atherosclerosis, the accumulation of plaques that narrow and harden the arteries. One way to test for atherosclerosis is to get a coronary artery calcium score (CAC), which uses a CT scan to assess the amount of calcium present in the arteries of the heart. Recent evidence suggests that master endurance athletes tend to have higher CAC scores than non-athletes, perhaps due to wear and tear from years of pumping all that blood during exercise. That’s not good, because high CAC scores reliably predict elevated risk of serious and potentially fatal heart problems in the general population.
The good news is that endurance athletes tend to have different plates compared to non-athletes. Athletes have plates that are smooth, hard, and unlikely to break; non-athletes have softer plaques that are more likely to break away from the arterial wall and block blood flow. So there is a theoretical argument that high CAC scores should not be considered as much of a problem in athletes as they are in others. But no one has shown that this is how it works in the real world.
This is where the new study comes into play. A group led by Pin-Ming Liu of Sun Yat-sen University in China analyzed data from a long-term study whose subjects underwent a baseline CAC test in 2000 or 2001, a follow-up CAC test five or ten years later, and they filled out questionnaires about their exercise habits on at least three different occasions during the study. These repeated measures are crucial, because they can distinguish between those whose CAC scores are high (perhaps simply due to bad genetic luck) and those whose scores are rising (presumably due to some lifestyle factor such as exercise).
They looked at three groups totaling about 2,500 subjects: those who consistently exercised less than the recommended amount; those that consistently met or slightly exceeded recommendations; and those that averaged at least three times the recommendations. In this case, the recommended amount of exercise, according to public health advice, is 150 minutes a week of moderate exercise or 75 minutes a week of vigorous exercise, with activities like running counting as vigorous.
There were two key takeaways. First, the group that exercised more was more likely to have an increased CAC score on their second test, consistent with previous studies. Second, despite their higher CAC scores, the heavily exercised group was not more likely to have adverse cardiac events during study follow-up. This is also consistent with the idea that exercise promotes plaque formation, but plaques do not carry the same risks as plaques in sedentary people.
This is far from the last word on the subject, in part because only a handful of subjects had exercise levels comparable to an elite endurance athlete. But it’s an encouraging sign that CAC scores mean something different in those who exercise than in those who don’t.
the j-curve
Discussions about CAC scores and other risk factors sometimes feel a bit abstract. The study many of us yearn for is much simpler: Take a group of people, find out how much exercise they get, and wait to see who dies first. Many such studies have been done, but their results are difficult to interpret because there are many other differences, beyond exercise habits, between those who choose to run 100 miles a week and those who choose not to run at all.
Despite those caveats, there were two such studies, one from the Cooper Clinic in Texas Y the other from copenhagen, who claimed to see an “inverse J curve” in the relationship between amount of exercise and mortality risk. Doing a little exercise produced a dramatic decrease in your chances of dying prematurely; doing more produced a modest additional increase; but doing too much bent the curve up and started to increase your risk again.
Numerous other studies have tested the same idea and have been unable to find evidence that more exercise, beyond a certain point, increases the risk of premature death. But given the inaccuracies inherent in this kind of observational data, it’s hard to know which study to trust (especially when you really want a particular conclusion), so I wouldn’t normally report on yet another study that finds too much exercise isn’t bad for you. you after all.
However, this one has an interesting twist. It is published in Circulation, by a group led by Dong Hoon Lee of the Harvard T. H. Chan School of Public Health, and follows 116,221 adults from the Nurses’ Health Study and the Health Professionals Follow-up Study, which began in the 1980s. Over the course of 30 years, there were more than 47,000 deaths among the subjects, which means you’re not jumping to conclusions based on small numbers. (The Copenhagen study I mentioned earlier suggested that “strenuous” running increases the risk of premature death based on just two deaths in that group.)
The crucial detail is that the subjects of the new study were asked about their exercise habits every two years, instead of just once at the beginning of the study. This allowed the researchers to divide subjects into groups based on their average levels of exercise over the course of the study, rather than relying on a single snapshot of exercise habits to infer someone’s health up to 30 years later.
The main result is that those who got 150 to 300 minutes a week of vigorous exercise like running (or, roughly equivalent, 300 to 600 minutes a week of moderate exercise like walking) were about half as likely to die during the study. Even after adjusting for other secondary benefits of exercise, such as a lower body mass index, their risk was still a quarter lower. Keep in mind that 300 minutes a week is five hours of running, not a heavy-duty ultramarathon training program, but it’s still a substantial amount of exercise.
As for those who worked more than five hours a week, the benefits remained more or less the same. At least, they did if you use the average levels of physical activity over the course of the study. When the researchers ran the analysis again using only the first exercise questionnaire from the 1980s, the inverse J-curve reappeared. There are several problems with relying on a single measure of exercise habits, the researchers say, including the risk of reverse causality: Declining health before your baseline assessment could prompt you to exercise more, leading to false impression that exercise causes ill health. . This is the way almost all previous studies on exercise and mortality have been done, so the new results may finally explain why some studies have found that J.
It is still too early to declare that years of serious resistance training have no effect on the heart. In fact, it’s clear that training changes the heart, that’s the point, and it wouldn’t be surprising if those changes sometimes end up having negative effects. But epidemiological evidence continues to accumulate that the overall effects on longevity are positive or, at worst, neutral. And that doesn’t even take into account how much fun it is.
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