Exercise helps you stay fit, hale and hearty, and researchers say it may also help you stave off dementia and Alzheimer's disease. Now they have a better understanding of the hidden benefits that aid the brain.

Older folks who are more physically active have higher levels of a protein that promotes better communication between the brain's synapses, a new study reports.

"Synapses are these critical communicating junctions between nerve cells," said lead researcher Kaitlin Casaletto, an assistant professor of neurology at the University of California, San Francisco. "They're what sends the message from one nerve cell to another, and I think of them as where the magic happens when it comes to cognition. All of our thinking and memory occurs as a result of these synaptic communications."

Even people in their 80s and 90s whose brains were riddled with amyloid plaques and tau tangles – the toxic hallmarks of Alzheimer's – contained higher levels of these synaptic proteins if they were more active, researchers found.

The synaptic proteins are involved with the production and release of neurotransmitters, the biochemicals in the brain that promote communication between nerve cells, Casaletto said.

"We think having more of these proteins suggests that maybe you have more synapses and/or maybe your synapses are working a little bit better," Casaletto said. "You're getting more of that juice into the brain and facilitating communications."

This study involved more than 400 elderly people who agreed to wear monitors that tracked their amount of daily physical activity. They were all participants in the Rush University Memory and Aging Project, a long-term effort to identify the causes of Alzheimer's and dementia.

Researchers already had discovered that people within this group who had greater levels of physical activity also tended to have better cognitive performance and a reduced risk of dementia, Casaletto said.

The next step was to figure out why that was. Brain autopsies provided answers.

"We followed these adults until death. They donated their brains to the autopsy program, and we were able to look at the brain tissue after death," Casaletto said. The average age of death among participants was 90.

Autopsies revealed higher levels of synaptic proteins in the brains of those who, while they lived, tended to move around more in their day-to-day lives, researchers said.