Is it bad for your brain if you don't get enough exercise for years on end?

Years of too little physical activity increases the risk of cognitive decline, dementia and Alzheimer's disease. This is the finding of multiple systematic analyses of observational research in older adults. The association is clear and consistent, although evidence for direct causality is harder to establish in humans.

Not only too little exercise, but also prolonged uninterrupted sitting plays a role. In more than two thousand dementia-free older adults, longer consecutive periods of sitting were associated with lower scores on memory, verbal fluency and overall cognition. This association held even among people who otherwise got sufficient physical activity. Brain scans showed that changes in brain structure explain part of this association, suggesting that sedentary behaviour can affect cognition through the brain itself.

There is an important nuance: what you do while sitting may matter. Sitting in a cognitively active way, such as reading or doing puzzles, appears to be less harmful than sitting passively in front of the television. The evidence for this, however, is still limited and not strong enough for firm conclusions.

The good news is that exercise demonstrably helps. Regular physical activity improves learning, memory and brain plasticity, both in younger people and in older adults. Animal and human research points to a concrete mechanism: exercise stimulates the production of a growth factor that supports brain cells and their connections. Even light activity in place of sitting was sufficient for better scores on memory and verbal fluency. The optimal amount and type of exercise has not yet been precisely established, but both aerobic training and strength training have shown positive effects.