Exercise may help safeguard your brain as you age, and a new study suggests how this might happen.

Previous research has shown that physical activity helps protect brain cells. This paper indicates it may do that through lower levels of insulin and body fat.

"These results may help us to understand how physical activity affects brain health, which may guide us in developing strategies to prevent or delay age-related decline in memory and thinking skills," said study co-author Géraldine Poisnel, from Inserm Research Center in Caen, France.

The study included 134 people, average age 69, who had no memory problems. They completed questionnaires about their physical activity over the past year.

Researchers also gathered information on the participants' body mass index (BMI -- an estimate of body fat based on weight and height), insulin levels, cholesterol, blood pressure and other health factors.

Brain scans showed that participants with the highest levels of physical activity had a higher total volume of gray matter in their brains (about 550,000 cubic millimeters on average) than people with the least amount of physical activity (about 540,000 cubic millimeters).

When the researchers focused only on areas of the brain that would be affected by Alzheimer's disease, they found similar results.

Those participants who were most active also had a higher average rate of glucose metabolism in the brain than those who were least active. Reduced glucose metabolism in the brain can be seen in people with dementia, according to the study.

The results were published online April 13 in the journal Neurology.

Higher levels of physical activity were not associated with how much amyloid plaque people had in their brains. Amyloid plaque is a marker for Alzheimer's disease.

"Older adults who are physically active gain cardiovascular benefits, which may result in greater structural brain integrity," Poisnel said in a journal news release.

The study does not prove that exercise protects brain volume, it only shows an association, the authors said, noting further research is needed.

Still, "maintaining a lower BMI through physical activity could help prevent disturbed insulin metabolism that is often seen in aging, thus promoting brain health," Poisnel said.