Prevention is the most powerful medicine in the fight against dementia and Alzheimer's disease, a leading international researcher says.

'Prevention is very important, more than any drugs we have or plan to develop,' said Serge Gauthier, director of the Alzheimer's Disease Research Unit at McGill University in Canada.

Dr Gauthier, who is in Australia for a series of seminars to coincide with Dementia Awareness Week, said the majority of dementia cases in those aged over 85 were preventable.

Small strokes were key contributors to dementia in this group, he said.

Making lifestyle changes such as losing weight, quitting smoking and reducing alcohol consumption could reduce the risk of strokes, Dr Gauthier said.

'This is where we can have an impact right now,' he told AAP.

'If you can prevent small strokes, you probably delay dementia by 10 years.'

However, another component of dementia and Alzheimer's is the build-up of naturally occurring ameloid proteins in the brain.

Dr Gauthier said the role of ameloids is still unknown but it is thought the proteins may help protect against infection.

But in larger quantities the proteins can become problematic.

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Dementia expert says prevention is best