On Tuesday, the journal Circulation published two new studies 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?
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.
And that is, indeed, what the two new studies in Circulation suggest.
One study, from a group in Britain, compared 152 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.
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