Does exercise really slow brain aging? A new study complicates the narrative

Exercise is often thought to protect the aging brain, but a new study suggests the relationship might actually work the other way around. Researchers at Jena University Hospital in Germany analyzed data from a large group of adults in Leipzig and found no strong evidence that exercise leads to better cognitive performance or healthier brain structures. Instead, it appears that having a healthy brain may make people more likely to stay active.

The study, published in eLife, examined 2,576 participants' self-reported physical activity levels, as well as objective data from 227 participants who wore accelerometers to track their movements. Brain structure was measured using MRI scans, which assessed hippocampal volume and a Brain Age Gap Estimate (BrainAGE), a metric that estimates a person's brain age based on brain structure.

Participants also completed cognitive tests, such as the Trail Making Test, which measures mental processing speed and executive function. A smaller subset of participants had follow-up visits several years later, providing longitudinal data for analysis. Researchers adjusted their statistical models to account for factors like age, smoking history, alcohol intake, and socioeconomic status.

The baseline data showed no significant positive effect of physical activity on hippocampal volume or BrainAGE. Interestingly, higher self-reported physical activity was actually associated with slightly worse executive function scores. Objective accelerometer data also showed no significant relationship between movement and brain health measures in the general adult sample.

However, the researchers did find an age-related bias in self-reported exercise habits. Older adults, particularly those over 60, tended to overestimate their physical activity levels compared to the objective accelerometer data. When focusing on the 30- to 60-year-old group, higher accelerometer-measured activity was linked to a younger-appearing brain. This effect did not hold for older adults over 60, who made up the majority of the sample.

More importantly, the longitudinal data provided evidence for reverse causation. Participants with older BrainAGE at the study's start tended to report lower physical capacity at the follow-up visit. Researchers suggest that underlying age-related issues, such as cellular energy problems or low-grade inflammation, might silently deteriorate brain health and simultaneously cause physical fatigue, leading to reduced activity levels.