Staying active throughout adulthood could help stave off dementia though even smaller bouts of exercise may help, British research suggests.

A long-term study found that people who exercise as they age are more likely to have good brain health than those who take up an activity for shorter periods of time but then give it up.

However, even taking up exercise in your 60s is better than doing nothing at all for improving cognitive function, the research suggested.

The study, led by a team at University College London (UCL), examined data from 1417 people (53 per cent women) who filled in surveys about how much exercise they did.

The surveys were carried out five times throughout adulthood, when people were aged 36, 43, 53, 60-64 and 69.

People were categorised as either not being active (no physical activity in a month), moderately active (once to four times per month) and most active (five or more times per month).

Cognitive tests, plus those looking at processing speed and memory, were carried out once people hit the age of 69.

The study found people who were physically active at least one to four times a month in all five separate surveys performed the best on the tests.

This effect was bigger than for people who exercised frequently (more than five times a month) during at least one survey period, but who did not necessarily keep this up across multiple stages of life.

"Being physically active at all time points in adulthood was associated with higher cognitive performance and verbal memory scores at age 69," the authors concluded.

"Notably, the effect sizes were similar across all adult ages, and for those who were either moderately or most physically active, suggesting that being physically active at any time in adulthood, even if participating as little as once per month, is linked with higher cognition.

"However, most effects were observed in those maintaining physical activity across adulthood."

Lead author Sarah-Naomi James said the study suggested engaging in any leisure-time physical activity, at any point in adult life, had a positive effect on cognition.

"This seems to be the case even at light levels of activity, between once to four times a month," Dr James said.

"What's more, people who have never been active before, and then start to be active in their 60s, also appear to have better cognitive function than those who were never active."

The study, published in the Journal of Neurology, Neurosurgery and Psychiatry, found that some of the link between exercise and brain health was explained by education and socio-economic background, though the effect remained significant even when these were taken into account.

Medical authorities say there is no certain way to prevent all types of dementia, but exercise, eating a healthy, balanced diet, maintaining a healthy weight, keeping blood pressure under control and stopping smoking can help.