Does running really harden your arteries?

On Tuesday, the journal Circulation published three articles—two new studies plus an editorial—that answer some longstanding questions and raise new ones about the link between high levels of running and the health of your coronary arteries. The news is mostly reassuring for runners, but it’s worth taking some time to understand what the new data tell us.

For many decades, the relationship between exercise and heart health was presumed to be simple: the more you do, the less likely you are to suffer heart problems. But in the last decade, that orthodoxy has been questioned, in part because we now have, for the first time, a generation of adults reaching retirement age who have been running (and doing other forms of endurance exercise) for nearly their entire adult lives. Being “active” is one thing; but if you’re running multiple marathons a year for four decades, does that eventually take a toll on your ticker?

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For a general overview of this issue, check out my in-depth feature in Runner’s World from last fall. What the new studies in Circulation deal with is one particular aspect of the debate, related to the likelihood of developing blockages in the arteries leading to your heart.

In 2008, contrary to conventional wisdom, a study in Germany found that a group of marathon runners had higher “coronary artery calcium” (CAC) scores than a matched group of controls. CAC scores, which use CT scans to assess how much calcium has accumulated in your coronary arteries, quantify the degree to which artery-hardening plaques have built up in your blood vessels.

There were some questions about the German study, though. For example, the subjects had all completed at least five marathoners in the previous three years, but their earlier exercise history wasn’t considered, so it wasn’t clear whether the marathoners had been long-term athletes or more recent converts who “got religion” after a dissolute youth. More than half of the marathoners were former smokers, which tended to hint at the latter possibility. Still, it flagged the issue and sparked the launch of several other studies.

In the years since, as more data has emerged, the idea that I’ve heard from numerous sports cardiologists is that high levels of vigorous endurance exercise over many years may indeed increase coronary artery calcification, but that the pattern of plaques that form in athletes tends to be a healthier and more stable form of plaque, less likely to causes serious problems like a heart attack.

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And that is, indeed, what the two new studies in Circulation suggest.

One study, from a group in Britain, compared 152 masters athletes (average age 54, 70 percent male) to 92 non-athlete controls. The athletes and non-athletes were equally likely to have elevated CAC scores, with roughly 40 percent in each group.

The bad news for athletes is that those who did have elevated CAC scores tended to have higher (i.e., worse) scores, and the higher scores were associated with the number of years of training.

The good news is that about 70 percent of the plaques in male athletes were hard, calcium-rich plaques, whereas about only 30 percent of the plaques in the controls were calcific. These hard plaques do narrow your arteries, but they’re much less likely to break off and cause a complete blockage that triggers a heart attack. (For more on this, see my description of this data from when it was first presented as a conference abstract a few years ago.)

The second study, from a group in the Netherlands, looked at 284 middle-aged men who competed in recreational sports. About half the participants were runners or cyclists, but the cohort included lots of different sports, including soccer and tennis.

The subjects completed detailed questionnaires reporting their lifelong exercise histories, and were divided into three groups based on their typical weekly exercise dose averaged over their adult lives. About half the athletes had non-zero CAC scores, but the prevalence was higher in the highest training group (68 percent) than the lowest training group (43 percent). And once again, the highest training group was more likely to have stable calcific plaques than the lowest training group.

In the comparison above, the lowest training group averaged less than 1,000 MET-minutes per week of exercise through their lives after the age of 12, which is equivalent to the higher end of public health guidelines for how much exercise everyone should get. (“MET” stands for metabolic equivalent of task, and is a way to measure the energy cost of differing activities.) The highest training group was getting above 2,000 MET-minutes per week, which is roughly equivalent to running 20 miles a week at a pace of 10:00 per mile.

This study also broke down the data by intensity of exercise, and found that intensities above 9 METs seemed to be associated with higher CAC score. For comparison, running at 10:00-mile pace burns about 10 METs in the average person, though habitual runners probably don’t have to work as hard (relatively speaking) to maintain a given pace.

For now, we don’t really have enough data to draw any conclusion about specific intensities or patterns of exercise and their link to arterial health. Still, taken together, these studies strengthen the picture that long-term endurance training is associated with more plaques but of a less dangerous type. So what message, in practical terms, should we take from this?

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The accompanying editorial is by Aaron Baggish, of Massachusetts General Hospital, and Ben Levine, of the University of Texas Southwestern—both leading researchers in this area as well as sports cardiologists who are in the trenches dealing with masters athletes. They make three key points in their discussion.

The first is the old correlation-versus-causation caveat. Does endurance exercise really cause plaque build-up? There are some plausible hypotheses—that the turbulent, high-pressure blood flow through arteries during exercise contributes to plaque formation, or that hormones associated with intense exercise play a role.

But these studies don’t prove that link—and more than half the athletes in the two studies had no evidence of any calcification at all, despite their decades of hard training. So clearly training on its own isn’t enough to directly cause plaques.

So the second point Baggish and Levine raise is the potential role of unmeasured confounding variables. Do hardcore lifelong endurance athletes—or at least some of them—tend to eat poorly, without realizing the hidden consequences because their training keeps them thin? Do they overuse anti-inflammatory medications? Are they chronically stressed out? We don’t know.

The third point is the most important: the absence of data on clinical outcomes. In the average sedentary person, we know that high CAC scores are associated with an elevated risk of future cardiovascular events. And we know that prolonged endurance exercise seems to be associated with higher CAC scores in some people. But we don’t have any data linking high CAC scores in athletes with elevated risk of cardiovascular events. Both the British and Dutch studies offer convincing evidence that plaques in athletes are “different” from those in other people—so we still need to figure out what they mean in practice.

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For Baggish and Levine, who deal with lots of endurance athletes in their clinics, CAC scores simply aren’t a high priority compared to more conventional and well-understood risk factors like cholesterol levels and blood pressure (which can be managed with lifestyle changes and drugs), and exercise capacity testing (which can be managed with, well, exercise). When they see patients with high CAC scores, they don’t currently recommend reducing training—instead, they discuss the current state of knowledge about risk factors, and offer guidance on some of the other ways of lowering risk.

That seems to me like a pretty good place to leave things. Obviously we all want better information and more certainty, and hopefully further research will get us there. But until then, it’s not clear what CAC scores really mean in endurance athletes.

In fact, it reminds me of a spoof article in the Canadian Medical Association Journal a few years ago: “Cigarette smoking: an underused tool in high-performance endurance training.” Numerous studies show that smokers have higher lung volume and higher hemoglobin levels—factors associated with endurance performance, but also with respiratory problems. So should runners smoke in order to get faster? Of course not: the meaning of a measurement depends on who it’s measured in.

This article first appeared on Runner's World.