Marathons and Myocardium: Cardiac Biomarkers Rise in Amateur Runners Tackling Longer Distances

For less-experienced athletes, cardiac stress is more severe when running a marathon than when competing in shorter races, according to new research looking at acute changes in biomarker levels.

Senior author Juan Del Coso, PhD (Camilo José Cela University, Madrid, Spain), told TCTMD that his group has been researching long-distance runners since 2011, an effort that involved being at water stations along the route and at the finish line of several marathons. “In these investigations, we observed that a high proportion of amateur marathoners do not [fully] ‘run the race,’ and we started to speculate that this was because of inappropriate training,” he said.

Those earlier studies showed a link between lack of training and more skeletal-muscle damage, leading the researchers to wonder if this phenomenon also extended to the heart, Del Coso explained. The idea wasn’t that running the distance of a marathon is inherently problematic for amateur runners, he said, but rather that excessive cardiac strain could occur when athletes complete the race without enough training.

Del Coso along with lead investigator Beatriz Lara, PhD (Camilo José Cela University), and colleagues outlined their latest findings in a research letter published online today in Circulation.

The biomarker increases “suggest that the strain imposed on the myocardium by competing in a full marathon is much greater compared with competing in shorter distances such as the half-marathon or 10-km races,” they write. “Although the release of cardiac troponins after the exercise may not be indicative of any cardiovascular dysfunction, the higher concentration of cardiac troponins after the marathon reflects a superior cardiac stress at this running distance.”

What I would encourage doctors to tell their patients is that we know from solid scientific evidence there is a whole host of benefits to exercise in many different avenues of the body, not just the heart. Keith Diaz

Lara et al looked at 63 healthy but nonprofessional runners who were of similar age (mean 37 years) and had similar body sizes (mean 67 kg) and amount of running experience (mean 3.3 years). Their 10-year CV risk was 2.8% on average. Divided into thirds, 21 runners competed in a 10-km race, 21 ran a half marathon, and 21 ran a full marathon; training volumes increased along with distance, from 23.9 ± 9.0 km/week to 34.8 ± 8.0 km/week and 40.6 ± 16.4 km/week across the three groups.

The amount of time it took for individuals to complete each race varied by distance, as did body mass loss (an indication of dehydration). Yet all three groups reported the same level of exertion on the Borg scale.

Serum concentrations of cardiac troponins I and T rose among all runners. But only marathon runners saw significant increases in NT-proBNP and creatine kinase-MB concentrations, the investigators report, noting that similar upticks were seen for serum myoglobin and creatinine kinase-MM.

To TCTMD, Del Coso pointed out that the increases in biomarkers weren’t seen across the board for marathon runners. “Interestingly, athletes with similar low running experience and low training background presented much lower values of cardiac stress, because they decided to compete in shorter distances,” he said.

Could It Be Benign?

Keith Diaz, PhD (Columbia University Irving Medical Center, New York, NY), an exercise physiologist who commented on the findings for TCTMD, said that it’s been known for a while that physical activity can spur the release of cardiac biomarkers. “What’s interesting . . . is the study design is pretty well done and it’s elegant. I think that’s what’s important about it—it answered a question in a very good way that other studies hadn’t previously done,” showing that the biomarker rises grew as distance and exertion increased, he observed.

The trouble is that it’s unclear what the biomarker increase means, Diaz said. Thus, it’s too early for physicians to apply these results when counseling patients.

“There’s the possibility that when we exercise our body releases these cardiac troponins as a way to facilitate adaptation. So this could be the signal [from] the body to tell the heart get stronger and bigger,” rather than a marker of injury, he explained.

“Where this creates confusion is that these markers are used also for diagnostic purposes to determine if a person had a heart attack or if they have some type of heart injury,” Diaz continued. “The challenge is we don’t really know the mechanism as to how exercise facilitates the release of the troponins. We know what happens when the cell starts dying because you’re having a heart attack [but] we’re not sure how exercise causes the release. It could be through a totally benign, nonharmful, reversible process that’s completely healthy and normal.”

Del Coso agreed that these increases may not in fact be negative.

“Cardiac strain is a potent stimulus for adaptation, and this is the base of cardiac modelling produced by training,” he said, cautioning, however, that the risk-benefit ratio comes down to the magnitude of the strain. “We believe that the cardiac strain produced during 10-km races and 21-km races could be more beneficial in terms of magnitude, than the strain obtained during a marathon, at least in our sample of low [training]-low experience amateur marathoners.”

What to Tell Patients

Marathons have a popular appeal that many other sports do not, according to Del Coso. “[L]ots of people are attracted by the historic perspective of the marathon,” as well as the physiological and mental challenges, he said. Additionally, the races are now “mass events where anyone can register but without any filtering to determine if they are really prepared to run the race.”

Clinicians can advise patients on how to complete their marathon safely, Del Coso suggested. Even for healthy people, it’s important not to just give general information but to provide individualized guidance. Training principles are key, he added. “I would say that exercise intensity should be prioritized over training volume, because it can produce faster and greater adaptations.”

For novice runners, a race shorter than a marathon may be best, Del Coso continued. Those who want to sign up for a marathon should understand that appropriate training is necessary even for people with years of running experience. “Specifically, marathoners should train at least 5 days per week and distances over 80 km/week to increase their likelihood of succeeding during the race,” he advised.

“We just don’t know enough yet about what these markers mean in the context of exercise, so [there’s] nothing to be concerned about,” Diaz commented. “But what I would encourage doctors to tell their patients is that we know from solid scientific evidence there is a whole host of benefits to exercise in many different avenues of the body, not just the heart. To just focus on one aspect while ignoring the rest would be naive of us.”

In comparison to these benefits, the risk posed by marathons on the whole is “so slight,” he added. Rather, the decision on whether to participate in a marathon should be tailored to patients’ history and risk factors.

The paper notes that the incidence of cardiac arrest is approximately one in 100,000 marathon finishers, though “a high proportion of all exercise-induced cardiac events occurs during marathon competitions, particularly in male runners > 35 years of age.”