ARTICLE IN BRIEF
Investigators reported that vigorous exercise lowers the risk of developing a silent brain infarct, but found no benefit from moderate or light exercise, and no effect for any level of activity on white matter hyperintensity volume.
Vigorous exercise lowers the risk of developing a silent brain infarct (SBI), a prospective, observational study of elderly New Yorkers has found.
The study, carried out among residents of northern Manhattan, previously linked moderate-to-vigorous physical exercise to a reduced risk of symptomatic stroke. The new findings, using MRI to detect asymptomatic strokes, found no benefit from moderate or light exercise, and no effect for any level of activity on white matter hyperintensity volume (WMHV).
Despite the somewhat contradictory findings, the new paper, published June 14 in Neurology, should encourage neurologists to redouble their efforts to educate patients about the risks of inactivity, a stroke specialist not involved with the study, said.
“Older patients worry about the risk of having a stroke,” said Jeffrey L. Saver, MD, professor of neurology at the University of California-Los Angeles, and director of the UCLA Stroke Center, who was not involved with the study. “If they're told by a physician that it's proven that vigorous exercise may prevent both clinical stroke and silent stroke, that can change behavior.”
The first author of the Neurology paper conceded that the lack of a dose-response gradient for varying levels of exercise was potentially puzzling.
“We have a couple of ideas why that may be the case,” said Joshua Z Willey, MD, assistant professor of neurology at Columbia University School of Medicine. “One possibility is that we did not have sufficient numbers of participants in the study. Another is that sometimes you do need to perform a certain level of physical activity to gain a benefit in terms of disease outcomes. It may not be sufficient to do a little bit of exercise.”
Even so, Dr. Willey added, “There are still multiple other health benefits to exercising. We don't want our findings to discourage people from performing mild-intensity activities.”
The Northern Manhattan Study is a population-based prospective cohort of 3,298 participants recruited between 1993 and 2001, and followed annually by telephone. Participants were invited to participate in the MRI sub-study beginning in 2003. An additional 199 household members (but not first-degree relatives) were recruited into the MRI cohort for a total of 1,290 participants. The mean age at the time of MRI was 70 years. Sixty percent were women; 65 percent were Hispanic, 17 percent non-Hispanic black, and 15 percent non-Hispanic white.
Measures of leisure-time physical activity were collected only during the baseline interview, using an in-person questionnaire to record the duration and frequency of activities for the two weeks prior to the interview. For each activity, the participant was asked the duration of activity, and the times they engaged in this same activity, and if this level of activity was typical of other weeks. If the duration of activity was less than 10 minutes, it was coded as “no activity.” This same measure also correlated with body-mass index (BMI), activities of daily living scores, and activity scores on a quality of well being scale. Questionnaires were correlated with compendia of physical activity to allow for categorization of total physical activity using metabolic equivalents (MET).
Physical activity was categorized by quartiles of the MET score, ranging from none to light (between 1 and 5.5 on the scale, representing activities including golfing or bowling), moderate (5.5-8, e.g., bicycling or swimming), or heavy (>8 METs, including racquetball).
The MRIs, obtained a mean of six years after the physical activity assessment, revealed that 197 participants, or 16 percent, had evidence of SBI. In fully adjusted models, compared to those who did not engage in physical activity, those in the upper quartile of MET scores were almost half as likely to have SBI (adjusted OR 0.6, 95%CI 0.4-0.9). No association was seen between physical activity levels and WMHV.
A weakness of the study is that only a single measure of physical activity and of potential confounders were taken at baseline, said Majaz Moonis, MD, director of the Stroke Prevention Clinic at the University of Massachusetts Memorial Medical Center in Worcester, who was not involved with the study.
“Someone who didn't exercise at baseline, or who had badly controlled diabetes, or whose hypertension was poorly managed, might have become more active and better controlled as the years went by,” he said. “Or they might have been active and healthy at baseline, and then become sedentary and less healthy. You need to have concurrent values of these factors for the findings to be reliable.”
Dr. Moonis also called the lack of a dose-response gradient for exercise levels, or of any association between exercise and WMHV, “puzzling.”
Dr. Saver agreed that the single measure of activity at the study's baseline was a weakness, but emphasized, “that would actually tend to bias the study toward failing to find association between exercise and MRI findings. So the fact that they did find an association with silent brain infarct suggests it is a robust relationship. That may be why they didn't find a relationship between more moderate exercise, and with white matter hyperintensities.”
Other studies have also found a lack of association between exercise levels and subsequent findings of WMHV on MRI, including the National Heart, Lung, and Blood Institute Twin Study. “The explanations for the lack of association between physical activity and WMHV could be due to the heterogeneity of pathology underlying white matter hyperintensities,” the authors of the new paper noted. “While evidence links a heavy burden of white matter hyperintensities to numerous vascular risk factors, retinal vascular changes, subcortical cerebral infarction and intracerebral hemorrhage, the pathological basis remains poorly characterized, with only small series providing definite pathological correlation with MRI.”
Likewise, other papers, including a 2003 meta-analysis published in Stroke, have also found that only vigorous exercise is associated with a reduced risk of ischemic stroke, whereas less intense activities were not.