It was deemed more effective than gruelling gym workouts such as strength training.
It is already known that regular physical activity boosts blood flow to the brain.
But the latest study, in the Journal of the American Geriatrics Society, suggested aerobic exercise offers the best protection.
Cardiologist Gregory Panza said: “Findings suggest exercise may delay the decline in cognitive function that occurs in individuals who are at risk of or have Alzheimer’s.
“Aerobic exercise possibly has the most favourable effect.” His team from Connecticut University in the US pooled data from 19 studies of 1,145 adults in their 70s.
Improvements in cognitive function were three times greater among those who did aerobic exercise alone, compared to those who combined it with strength training.
The World Health Organisation also says people aged 65 or over should do at least 150 minutes of aerobic physical activity a week, including walking, dancing, gardening or cycling