Getting up from your desk and going for a brisk walk may keep your mind agile later in life, say US researchers.
Exercise may induce brain changes
They have found clear evidence that an aerobic exericise programme - even a fairly gentle one - may boost performance in key areas of the brain.
Their report, published in the Proceedings of the National Academy of Sciences, predicts that exercise could improve "decision-making" and "focus".
Stretching and toning exercise did not yield the same level of benefits.
Other studies have also suggested that keeping physically active could have a benefit on mind power.
It is possible that improved blood flow to the brain may be partly responsible, or that exercise stimulates the release of chemicals that influence brain cell growth and activity.
The research team, led by Professor Arthur Kramer from the Beckman Institute in Illinois, scanned the brains of volunteers using a magnetic resonance scanner.
He then split them up and put some on a cardiovascular fitness programme, while the others were sent on non-aerobic stretching sessions.
The aerobic exercise was not particularly severe, involving gradually increasing periods of walking over three months, followed by 45 minutes of brisk walking a day for the final three months.
After six months, the volunteers were scanned again and given mental tests.
The scans showed distinct changes in brain function in two areas - the middle frontal and superior parietal regions.
These areas have been linked to the ability to keep the mind on a particular task, and spatial attention in general.
When the aerobic exercisers were given mental tests, they improved their scores by 11%, while the volunteers in the other group did slightly worse.
The academics were able to predict how well each volunteer would do in their test by interpreting the activity in their brain scans.
Professor Kramer said: "The brain circuits that underlie our ability to think - in this case to attend selectively to information in the environment - can change in a way that is conducive to better performance on tasks as a result of fitness.
"The kinds of tasks that we explored are similar to those encountered in real world situations such as driving a vehicle or any endeavour that requires a person to pay attention despite distractions."
In mice, research has suggested that exercise produces increased levels of a molecule called brain-derived neurotrophin factor - which not only protects the brain, but can increase "connections" between brain cells.
An Age Concern spokeswoman said there were clear benefits for older people from exercise.
"The important thing is to start from where you are and build your way up gradually.
"If you're enjoying good health and don't suffer from heart problems, pains in the chest or joint and bone problems, you can build more activity into your life without consulting your GP.
"However, if you've got any concerns, or you've been inactive for a long time, it's best to check with your doctor before embarking on new activities."