Exercise has previously been linked to possible benefits in staving off dementia, but a new look at the topic suggests the earlier the better.

The prevalence of cognitive impairment was significantly lower in women aged 65 and older who reported they were physically active as teens than in those who were inactive in their teen years, the study found.

'Physical activity can be fun and engaging, and we have to convince people of that in order to prevent some of these diseases of old age.'— Researcher Laura Middleton

"If we want to optimally prevent dementia, it's important to start physical activity as early in life as possible," said principal investigator Laura Middleton of the Heart and Stroke Foundation Centre for Stroke Recovery at Sunnybrook Health Sciences Centre in Toronto.

"More and more people are starting to recognize physical activity as one of the most promising means to prevent cognitive impairment and dementia. And what this study adds is that it's not only important in mid and late life — that we really have to start as early as possible."

The study was published Wednesday appears in the July issue of the Journal of the American Geriatrics Society.

Middleton worked on the project while she was at the University of California in San Francisco, and used data from the Study of Osteoporotic Fractures. She analyzed the responses of 9,704 women in four U.S. cities: Baltimore, Minneapolis, Portland, Ore., and Monongahela Valley, Pa.

The women reported on their participation in regular physical activity as teens, at ages 30 and 50, and in late life. Their cognition was evaluated using a standardized test, and those who performed well below the average were classified as cognitively impaired, Middleton said.

The researchers looked at the relationship between physical activity at each age, and cognitive impairment in late life.

"People who were active at teen age had a greater reduction in their risk of cognitive impairment compared to any of the other ages," Middleton said.

The prevalence of cognitive impairment in the women who'd been physically active in their teen years was 8.5 per cent, compared to 16.7 per cent among those who weren't physically active as teens.

For those active versus inactive at age 30, prevalence of cognitive impairment at age 65 or older was 8.9 per cent compared with 12 per cent, and at age 50, 8.5 per cent versus 13.1 per cent.

But even women who were inactive as teenagers and became active in later life had a lower risk of cognitive impairment than those who remained inactive, the study found.

Other lifestyle factors?

Louis Bherer, director of clinical research at the Montreal University Geriatric Institute, said most people in the field agree that physical activity is probably the most promising strategy to protect against cognitive decline.

The novelty of this study is the idea that even physical activity as a teen might protect against cognitive decline in older adults, which he says is kind of surprising.

"Although I would love to believe in it," said Bherer, professor of psychology at University of Quebec at Montreal.

"I would love to be able to tell my kids, you know, if you exercise right now you protect your brain against cognitive decline. But there's some limits of the study."

He's concerned about how cognition was measured, and also noted that participants were self-reporting their physical activity and it's common for people to overrate themselves. In addition, there are no guarantees that people who are 65 or 75 years old remember well their levels of activity from decades earlier, he suggested.

"I think it's far-fetched a little bit to conclude that physical exercise at teen age protects your brain against decline," Bherer said.

"There's so many things that can happen from your teen age to retirement," he said, noting that lifestyle factors come into play along with nutrition, social networking and cognitive stimulation.

But he said it's not harmful to suggest that people engage in physical activity. And Middleton said that from a public health perspective, people need to be persuaded to exercise.

"Physical activity can be fun and engaging, and we have to convince people of that in order to prevent some of these diseases of old age."