Older adults who burn calories in moderate physical activities like jogging, swimming, gardening and dancing are likely to have lower risk of developing Alzheimer's disease, says a study.

Exercise-associated calorie-burn can increase the level of grey matter volume in key brain areas which are responsible for memory, enabling the active adults to protect their brain from cognitive decline.

"Grey matter houses all of the neurons in your brain, so its volume can reflect neuronal health," said study lead author Cyrus Raji from University of California, Los Angeles.

The findings showed that people with highest calorie expenditure had larger grey matter volumes in key areas on initial brain scans and were half as likely to have developed Alzheimer's disease five years later.

Also, the individuals who burned the most calories had larger grey matter volumes in the frontal, temporal and parietal lobes of the brain -- areas that are associated with memory, learning and performing complex cognitive tasks.

"Rather than wait for memory loss, we might consider putting the patient on an exercise programme and then rescan later to see if there are any changes in the brain," Raji noted.

For the study, published in the Journal of Alzheimer's Disease, the team examined the data obtained over five years from nearly 876 people 65 year or older participating in the multicenter Cardiovascular Health Study.

All participants had brain scans and periodic cognitive assessments. They also were surveyed about how frequently they engaged in physical activities, to assess their calorie expenditure or energy output per week.