One in three cases of dementia could be prevented by tackling risk factors such as education and depression, a large new international review estimates.

A team of 24 experts in dementia conducted the review on prevention and care.

In Thursday's issue of The Lancet and at this year's Alzheimer's Association International Conference in London, researchers model how nine health and lifestyle factors contribute to about 35 per cent of dementia.

For instance, they said that if everyone reaped the brain stimulation and interaction of staying in school until over the age of 15, the total number of dementia cases could be reduced.

Controllable factors they identified as important in midlife are preserving hearing and treating high blood pressure and obesity.

In late life, they said, controlling depression, smoking and social isolation are important.

Maintaining physical activity and controlling diabetes also help. But they said that overall, 65 per cent of the risk isn't considered potentially modifiable.

Imagine twins, said Prof. Lon Schneider of the Keck School of Medicine of the University of Southern California in Los Angeles. One receives early education and the other does not.

It's thought that the cognitive and other boosts of education help to establish more nerve connections between synapses in the brain, helping to build up a reserve that resists losses as we age.

Dr. Tarek Rajji of the Centre for Addiction and Mental Health in Toronto is working to enhance brain reserves in people 60 and older. (Nicole Ireland/CBC)

"Later in life, they each may be destined to develop dementia but the one with cognitive reserve and with a greater number of synapses will develop dementia later than his twin."

The interventions would not delay, prevent or cure all cases of dementia, the researchers said.

'There's so much we can do'

The incidence of age-specific dementia incidence has declined in Canada, the U.S., the U.K., Sweden and the Netherlands over four decades. But it's increased in China.

"There's so much we can do in terms of our behaviour and our public health policy," Schneider said.

Filling the gap between identifying what's potentially and actually preventable remains a challenge, said Dr. Kenneth Rockwood, one of the study's authors and a professor of geriatric medicine and neurology at Dalhousie University in Halifax.

"An important part of dementia seems to be preventable," Rockwood said in an email from London. "But it's tricky. Poor physical health is an important risk, especially stroke and high blood pressure. So too are impoverished social circumstances."

The systematic look suggests a change in the way we think about dementia, said Dr. Tarek Rajji, chief of geriatric psychiatry at the Centre for Addiction and Mental Health in Toronto.

"We cannot afford to think about dementia just late in life," Rajji said.

(Keck Medicine of University of Southern California)

Rajji was not involved in the review. He's one of the principal investigators of a five-year dementia prevention trial in people 65 and older with a history of depression. Other participants in their 60s have mild cognitive impairment, such as memory problems that aren't severe enough to interfere with living independently.

In Rajji's PACt-MD trial, he aims to test a combination of memory exercises and painless, non-invasive brain stimulation compared with a control in 375 individuals at risk for Alzheimer's disease.

"What we're trying to do is directly trying to enhance cognitive reserve late in life," Rajji said.

'Move a muscle, the brain will follow'

Bonnie Buxton, 77, of Toronto was diagnosed with dementia in October. Buxton has a family history of Alzheimer's disease. She and her husband, Brian Philcox, do crosswords and swim together to keep their brains and bodies active.

"We had a dear friend who used to tell us all the time, 'Move a muscle, the brain will follow,' and I think that really works," said Philcox.

In 2013, a Canadian research review concluded more than one in seven cases of Alzheimer's disease could be prevented if people who are physically inactive started working out regularly.

Since then, other Canadian scientists have looked at the benefits of more intense exercise and walking.