In recent years, with the enthusiastic adoption of accelerometers by the designers of epidemiological studies, it has become clear that even quite modest levels of physical activity correlate strongly with improved health and a slower pace of age-related degeneration. In most human data there is no way to establish which of these is cause and which of these is consequence, but animal studies are quite definitive on the point that exercise produces improvements in health, even if it doesn't appear to extend life span. Physical activity, like all interventions, has a dose-response curve, and there is a sizable difference between being sedentary and being even modestly active. It is still a better idea to be more than just modestly active, of course; research suggests that the recommended levels of exercise, 150 minutes per week, may well be too low.
Considerable evidence suggests that engaging in regular physical activity (PA) may prevent cognitive decline and dementia. Active individuals have lower metabolic and vascular risk factors, and these risk factors may explain these individuals' propensity for healthy brain aging. Even short-term exercise interventions have been shown to prevent hippocampal atrophy in older adults11 and may also improve brain connectivity. Furthermore, cross-sectional epidemiologic studies have established an association of physical inactivity with brain aging. However, further work is needed to pinpoint the optimal dosage of PA needed to promote healthy brain aging.
A growing body of literature has established light-intensity PA as an important factor for improving health outcomes, but in our review of the literature, light-intensity PA has not often been considered separately from total PA for its association with brain structure. Previous studies have identified positive associations of self-reported PA with brain volume, but accelerometry studies often have smaller sample sizes and have focused on examining the association of total PA with brain volume. However, PA variables are associated with one another, so in our analyses, we went a step further and modeled them together to determine what type of PA intensity (low or high) is driving the association of PA with brain volume.
The simplification of PA as a predictor variable has potentially masked more nuanced associations of components of PA with brain health. Compared with previous research, our study provides multiple PA levels and intensities and uses accelerometry-determined intensity thresholds (ie, light-intensity PA and moderate to vigorous PA) in the same statistical models to provide a more sensitive measure of PA doses and examine what type of PA is driving the associations we observe.
The study sample of 2354 participants had a mean age of 53 years, 1276 were women, and 1099 met the PA guidelines. Incremental light-intensity PA was associated with higher total brain volume; each additional hour of light-intensity PA was associated with approximately 1.1 years less brain aging. Among individuals not meeting the PA guidelines, each hour of light-intensity PA and achieving 7500 steps or more per day were associated with higher total brain volume, equivalent to approximately 1.4 to 2.2 years less brain aging. After adjusting for light-intensity PA, neither increasing moderate to vigorous PA levels nor meeting the threshold moderate to vigorous PA level recommended by the PA guidelines were significantly associated with total brain volume.