Vigorous physical activity may exacerbate the association between air pollution and changes in the brain, while air pollution itself can attenuate the benefits of exercise, new imaging research suggests.
Results from an epidemiologic cohort study showed the association between nitrogen dioxide (NO2) and white matter hyperintensity volume (WMHV) was stronger in participants who had 30 or more minutes of vigorous physical activity per week than in those who had fewer minutes of such activity.
In addition, air pollution appeared to neutralize the beneficial effect of vigorous physical activity on WMHV.
However, the findings did not indicate that participants who engaged in physical activity had worse outcomes than those who did not, researchers note.
“We just saw that they didn’t experience the same benefits if they were in high-pollution areas,” lead author Melissa A. Furlong, PhD, assistant professor of environmental health sciences at the University of Arizona in Tucson, told Medscape Medical News.
Neurologists should encourage patients to exercise in areas away from highways and factories, she added.
The findings were published online December 8 in Neurology.
Interaction Not Well Understood
Physical activity has been associated previously with enhanced cognition and brain volumes, as well as reduced risk for dementia, while air pollution has been linked to smaller brain volumes, worse cognition, and increased risk for dementia. The interaction of physical activity and air pollution is not well understood.
To examine this interaction, the researchers analyzed data from the UK Biobank, a large epidemiologic cohort with data on neuroimaging, air pollution, and physical activity that includes more than 500,000 participants between ages 40 and 69 years. At baseline, participants provided demographic information in response to a questionnaire.
The current analysis included participants who took part in the UK Biobank’s accelerometer and imaging substudies. Participants in the former substudy wore accelerometers on their dominant wrists for 1 week.
The investigators defined vigorous physical activity as 425 milligravities (mg) or more. Moderate to vigorous physical activity was defined as 100 mg or more.
Participants in the imaging substudy underwent 3-T MRI. The researchers examined total gray matter volume (GMV), total white matter volume (WMV), and WMHV.
Also used was a pan-European Land Use Regression model to estimate annualized air pollution for the 2010 calendar year, focused specifically on nitrogen oxides (NOx), NO2, particulate matter ≤2.5 µm (PM2.5), particulate matter 2.5–10 µm (PM2.5–10), particulate matter ≤10 µm (PM10), and PM2.5–absorbance.
In all, 8600 adults were included in the analysis. Data on WMHV were available for 8016 of these participants. The population’s mean age was 55 years, and 54% were women. Approximately 98% of participants were White, and 92% lived in urban locations.
Results showed increases of one interquartile range (IQR) in NO2 (β = -0.04, P = .01), PM2.5 (β = -0.03, P = .02), and PM2.5absorbance (β = -0.03, P = .037) were inversely associated with GMV. These pollutants did not have a significant influence on WMV, however.
Engaging in vigorous physical activity for 30 minutes or more a week was significantly associated with GMV (β = 0.2, P < .001) and with WMHV (β = -0.23, P < .001). As the amount of vigorous physical activity increased from zero, the magnitude of the association increased.
The adverse effect of NO2 and PM2.5absorbance on WMHV increased as vigorous physical activity increased.
Participants with 30 or more minutes of vigorous physical activity per week had the greatest effects of NO2 (β = 0.06, P = .037) and PM2.5absorbance (β = 0.07, P = .037) on WMHV. The interactions between the other pollutants and physical activity were not significant.
In addition, among patients in the first quartile of exposure to NO2 and PM2.5absorbance, physical activity had a strong negative association with WMHV. Among those in the highest quartile of exposure, however, vigorous physical activity was no longer negatively associated with WMHV.
“There haven’t been any other epidemiological studies of air pollution and physical activity interacting on the brain,” said Furlong.
“When you’re engaging in vigorous physical activity, you increase your respiration rate and you’re breathing a lot more, so your dose of air pollution is likely going to be a lot higher,” she added.
The current results show that the effects of air pollution are stronger among individuals who engage in vigorous physical activity, which is “consistent with the hypothesis that people who are exercising are exposing themselves to more air pollution,” Furlong said.
“Also, we saw that the benefits of physical activity were much stronger than the adverse impacts of air pollution,” she added. “If you can be flexible about the days you exercise, check the Air Quality Index. If it’s particularly high, maybe exercise the next day.”
Commenting for Medscape Medical News, Charles S. DeCarli, MD, director of the Alzheimer’s Disease Research Center at the University of California Davis Health, called it “a great study.”
“It takes epidemiology at its face value and extracts some relatively straightforward results that could impact health,” said DeCarli, who was not involved in the research.
The large population is a notable study strength, and it appears to be widely dispersed, he added. This should provide enough variability in the distribution of pollutants to make it easy to show differences, DeCarli noted.
On the other hand, the method used to perform geocoding is unclear, he said. Also, when researchers download a publicly available data set, as the current investigators did, it is not easy to completely understand the way the data were acquired, he added.
The findings raise the question of whether it is beneficial for individuals to move away from highly polluted areas and exercise, he noted.
“Would it be beneficial even after the exposure?” asked DeCarli. “Could that actually help them recover?” It is not certain whether a move would allow a patient to regain the normal benefits of exercise, he added.
Overall, the study “really points to the fact that these published databases can give us hypothesis-generating ideas,” DeCarli concluded.
The study was funded by the McKnight Brain Research Foundation, the State of Arizona, and Arizona Department of Health Services. Furlong receives support from the National Institutes of Health and the US Centers for Disease Control. DeCarli has reported no relevant financial relationships.
Neurology. Published online December 8, 2021. Abstract
Follow Erik Greb on Twitter: @MedscapeErik.