Exercise can improve mental abilities in the elderly, and go some way to preventing mental decline, say researchers.
In a recent study, scientists found mental abilities, or cognitive functioning, improved with exercise.
The reason for the link is unclear, but the researchers believe it could be linked to the fact exercise improves the body's ability to pump blood and blood's oxygen-carrying capacity.
The findings were a side benefit of research comparing the effects of exercise and drugs in fighting depression.
Improvements were seen "above and beyond" what was expected after the depression had lifted, say researchers.
James Blumenthal, a psychologist at the Duke University Medical Center, in Durham, North Carolina, said one of the key findings was that exercise had beneficial effects on functions controlled by specific areas of the brain.
Memory, planning, organisation and juggling different tasks all improved under the study.
But attention, concentration and psychomotor skills, controlled by different parts of the brain, were not affected.
The research, named Smile (Standard Medical Intervention and Long Term Exercise) involved 156 patients aged between 50 and 77, who had a major depressive disorder.
They were randomly assigned to three groups.
One did 30 minutes of aerobic exercise - using an exercise bike, walking or jogging - three times a week, the second group took an anti-depressant, and the third a combination of the two.
After 16 weeks, all three groups showed the same degree of improvement in standard measures of depression, leading the researchers to conclude exercise was as effective as medication.
In addition to testing levels of depression, the scientists also measured mental processes, comparing the 42 people in the exercise group with the 42 in the medication group
They found exercise appeared to benefit some areas of cognitive function. Patients with milder depression showed the most improvement.
Dr Blumenthal, who led the research, published in the Journal of Ageing and Physical Activity, said: "The implications are that exercise might be able to offset some of the mental declines that we often associate with the ageing process.
"Further studies are warranted not only to clarify specific mental processes that are improved by exercise, but to better understand the underlying mechanisms of these improvements."
He added: "It is thought that one of the reasons why the elderly - especially those with coronary artery disease or hypertension - tend to suffer some degree of cognitive decline is in part due to a reduction in blood flow to the brain."
"So it may be that just as exercise improves muscle tone and function, it may have similar effects on the brain."
A spokeswoman for Age Concern said "Exercise is extremely beneficial to older people, both in terms of getting out and about and meeting new people, and obviously the health related benefits of keeping mobile and better fitness in general."