Exercise is good for you, but is there a point at which more exercise is either no longer beneficial or perhaps even harmful? Two new studies now help clarify -- albeit not resolve -- the issue.
The studies offer evidence that people who exercise for long periods and with great intensity appear to have an elevated risk for atherosclerosis. But, reassuringly, the atherosclerotic lesions that develop in intense exercisers are more likely to be characterized as stable.
Earlier studies found that people who exercised a lot were more likely to have indications of atherosclerosis than more moderate exercisers, but the association was clouded because some of the high-volume exercisers were already at elevated risk for cardiovascular disease. The new studies looked at populations of low-risk older athletes.
Ahmed Merghani (St. Georges, University of London) and colleagues performed a battery of cardiovascular tests on 152 master athletes and 92 matched controls (70% men). All the participants had a low Framingham 10-year risk score.
Although most athletes and controls had normal coronary artery calcium scores (CAC) -- 60% and 63%, respectively -- male athletes were more likely to have high calcium scores and evidence of coronary plaque. Most (72%) of the plaques in male athletes were calcified, but "their stable nature could mitigate the risk of plaque rupture and acute myocardial infarction," the authors wrote.
By contrast, most (61.5%) of the plaques in controls had a "mixed morphology," thus indicating that they might be at higher risk for an event. Seven men among the athletes, but none in the controls, had a pattern of late gadolinium enhancement on MRI consistent with a prior MI.
Vincent Aengevaeren (Radboud University, the Netherlands) and colleagues studied 284 men with a wide range of lifelong exercise volumes. Overall, 53% of the men had an abnormal calcium score (greater than zero). 68% of men at the highest activity level (greater than 2,000 MET-minutes per week) had an abnormal calcium score, compared with 43% in the lowest activity level group (less than 1,000 MET-minutes per week).
As in the first study, plaque type differed among the activity levels. Thirty-eight percent of men in the most active group had calcified plaques compared with 16% in the least active group. "These observations may explain the increased longevity typical of endurance athletes despite the presence of more coronary atherosclerotic plaque in the most active participants," concluded the authors.
Aaron Baggish (Massachusetts General Hospital) and Benjamin Levine (University of Texas Southwestern) wrote that "the dose-response relationship between exercise and health outcomes, in particular, at exercise doses that exceed current recommendations, remains incompletely understood." The two new studies "represent important contributions to the sports cardiology literature but raise as many questions as they answer."
The observational nature of the two studies "is well suited for documenting associations but is not capable of establishing cause-and-effect relationships between exposures and clinical phenotypes," they wrote, citing the potential role for unadjusted confounding factors.
Even more important, they wrote, "it is important to acknowledge the complete absence of clinical outcomes data in athletes with CAC." Although CAC in less active populations has been linked to adverse outcomes, "it is possible that the presence of CAC among dedicated lifelong endurance athletes may very well represent a clinically benign phenotype."
The editorialists said that physicians evaluating athletic patients should rely on conventional risk factors and do not need to obtain a CAC score except "in limited situations" where it may "provide adjunctive information."
Asked to comment, Michael Joyner (Mayo Clinic) said that the papers "provide important insight to inform the 'too much exercise' debate. The key point is that while some high volume habitual exercisers have high coronary calcium scores, they also appear to have more stable plaque. Stable plaque plus larger coronary arteries that dilate more in high volume exercisers should be highly protective against coronary events."