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

Years of low physical activity significantly increase the risk of dementia, and the association is stronger than you might expect. Among nearly 50,000 British adults aged 60 and over, the findings were striking: people who sit for 10 hours a day already have an 8% higher risk of dementia than average. At 12 hours a day that risk rises to 63%, and people who sit for 15 hours a day have more than three times the risk. This is an observational association, not a proven cause, but the measurements were objective (wrist accelerometer) and participants were followed for an average of nearly seven years.

Not all sitting is equal. A review of the research suggests that cognitively active sitting, such as reading or working, appears less harmful to the brain than passive sitting, such as watching television. This distinction has not yet been worked out in detail in large studies, but it already offers a practical handle: the quality of how you sit matters.

There is also a biological story behind this. Muscles are not passive tissue: during movement they secrete substances that support brain connections. One of those substances, a growth factor that helps brain cells survive and form connections, is produced in smaller amounts when you barely move. At the same time, muscle loss caused by prolonged inactivity likely leads to greater inflammatory activity in the body, which accelerates cognitive decline.

Low levels of physical activity also harm sleep. People who sit a great deal more often sleep poorly, and poor sleep is in turn a well-known risk factor for cognitive problems in the long term. This association has been demonstrated in a sample of more than 22,000 Americans. Among teenagers something similar is at play: more sitting is associated with more depressive feelings and lower life satisfaction, and exercise programmes measurably improve the mental health of young people, although the effect is modest.

The risk factors accumulate during middle age. Inactivity in one's forties and fifties, combined with being overweight, high blood pressure and smoking, is seen as a period in which dementia risk builds up. That does not mean it is too late at sixty, but it does underline that exercising early yields more benefit than exercising late.