Sedentary adults may improve their memory as soon as six weeks after taking up aerobic exercise, a small brain imaging study suggests.

Cardiovascular fitness and cognitive performance such as attention seem to improve after six months or more of aerobic exercise in previous aging studies.

Now researchers in Texas have found signs of increased regional blood flow in the brain of 37 sedentary adults with an average age of 64 who were randomized to physical training or a control group who had the training after a waiting period.

Participants in an exercise and brain imaging study too part in three, 60-minute sessions on treadmills or exercise bikes. (Tom Uhlman/Associated Press) (The Associated Press)

They found a higher resting cerebral blood flow in the brain's anterior cingulate region in the physical training group compared with controls. The anterior cingulate region is associated with better memory functions. The size of this brain region was also larger in another study of "successful cognitive agers" over the age of 80 compared to middle-aged or elderly controls.

"A relatively rapid health benefit across brain, memory and fitness in sedentary adults soon after starting to exercise, some gains starting as early as six weeks, could motivate adults to start exercising regularly," the study's lead author, Sandra Bond Champman of the Center for BrainHealth in Dallas and her co-authors concluded in Monday's issue of the journal Frontiers in Aging Neuroscience.

"The current findings shed new light on ways exercise promotes cognitive/brain health in aging."

The participants all had a physical exam and screening for dementia, early cognitive impairment, depression and IQ before the study began. A noninvasive type of MRI was used to measure brain blood flow before, half way through the 6-week training sessions and at 12 weeks.

Chapman said that in another research study, complex mental training increased whole brain blood flow as well as blood flow across key areas.

"Combining physical and mental exercise may be the best health measures to improve overall cognitive brain health," she said in a release.

The researchers acknowledged that the study had a small sample of participants. It is possible that the benefits were from social engagement rather than the physical training alone.

In March, a Canadian report suggested that more than one in seven cases of Alzheimer's disease could be prevented if people who are physically inactive started getting regular doses of exercise.

The new study was funded by the U.S. National Institute of Health, the T. Boone Pickens Foundation, the Lyda Hill Foundation and Dee Wyly Distinguished University Endowment.