People who are fully bilingual and speak both languages every day for most of their lives can delay the onset of dementia by up to four years compared with those who only know one language, Canadian scientists have revealed.
Researchers said the extra effort involved in using more than one language appeared to boost blood supply to the brain and ensure nerve connections remained healthy -- two factors thought to help fight off dementia.
"We are pretty dazzled by the results," Professor Ellen Bialystok of Toronto's York University said in a statement.
"In the process of using ... two languages, you are engaging parts of your brain, parts of your mind that are active and need that kind of constant exercise and activity, and with that experience (it) stays more robust," she later told CTV television.
The leading cause of dementia among the elderly is Alzheimer's disease, which gradually destroys a person's memory. There is no known cure.
Bialystok's team focused on 184 elderly patients with signs of dementia who attended a Toronto memory clinic between 2002 and 2005. Of the group, 91 spoke only one language while 93 were bilingual.
"The researchers determined that the mean age of onset of dementia symptoms in the monolingual group was 71.4 years, while the bilingual group was 75.5 years," the statement said.
"This difference remained even after considering the possible effect of cultural differences, immigration, formal education, employment and even gender as (influences) in the results," it added.
Bialystok stressed that bilingualism helped delay the start of dementia rather than preventing it altogether.
Psychologist Fergus Craik, another member of the team, said the data showed that being fully bilingual had "a huge protective effect" against the onset of dementia but he added that the study was still a preliminary finding. The team plans more research into the beneficial side-effects of bilingualism.
The Alzheimer Society of Canada described the report as exciting and said it confirmed recent studies that showed that keeping the brain active was a good way to delay the impact of dementia.
"Anything that staves off the time when the risk factor (for dementia) overcomes the defenses is wonderful news," scientific director Jack Diamond told Reuters.
The society estimates that in 2000 - the latest year for which data is available - Canada spent C$5.5 billion (€3.6 billion) taking care of people with Alzheimer's disease.