The BBC's Cathy Killick reports on the research

The effect is particularly pronounced in older people. The team at the University of Illinois found that an improvement of only five to seven percent in cardio-respiratory fitness led to an improvement of up to 15% in mental tests.

Different parts of the brain age more quickly than others, resulting in the blood flow to these areas being reduced.

Worst affected are the frontal and prefrontal regions responsible for "executive control functions" such as planning, using working memory, making decisions and picking out relevant information from irrelevant distractions.

Couch potatoes

Professor Arthur Kramer explains the findings

Professor Arthur Kramer and his team devised an experiment to find if increasing the blood flow in the brain by improving cardiovascular fitness could reverse the ageing effects. They recruited 214 volunteers aged from 60 to 75.

"We chose couch potatoes," Professor Kramer told News Online. "Since this was an exercise intervention it was important for us to select healthy older adults - but adults who hadn't been involved in any physical exercise for at least the last 5 to 10 years. Indeed most of our subjects hadn't done any formal exercise for more like 30 or 40 years."

The recruits were first subjected to a battery of psychological tests and had their level of aerobic fitness measured using the VO2max scale - a standard test of physical fitness.

More brawn does not mean more brain

For the next six months, half the volunteers took long walks three times a week. The other half did stretching and toning exercises using weights.

When they were then retested, the walkers had improved significantly in the mental tests, as well as being fitter. The non-walkers may have built up brawn, but that did nothing for their brains.

The results confirm the importance of fitness to our mental wellbeing.

"The lack of exercise over a lifetime has cumulative effects both in terms of physical function and mental function," said Professor Kramer. "It goes to show it's never too late to exercise."

The research is published in the journal Nature