(Transcript from World News Australia Radio)
New research from India indicates bilingual people may be able to delay the onset of dementia by years, regardless of whether they have the ability to read.
The study, published in the United States Journal of Neurology, found those who speak two languages developed dementia about four-and-a-half years later than those who speak just one language.
But the report's author says people who speak more than two languages don't gain additional benefits.
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The study looked at the medical records of 648 people diagnosed with dementia at a clinic in the southern Indian city of Hyderabad.
Researchers found those who spoke two languages developed dementia at an average age of 65.6 - four-and-a-half years later than people who spoke just one language.
Dementia is irreversible and refers to more than 100 different diseases characterised by memory loss and confusion.
An estimated 298,000 Australians live with dementia, with that number forecast to triple to around 900,000 by 2050.
The report's author is neurologist Suvarna Alladi from Nizam's Institute of Medical Sciences in Hyderabad.
Dr Alladi says the differences were seen in several types of dementia, including Alzheimer's disease, vascular dementia - which is associated with poor blood flow to the brain - and frontotemporal dementia, which is caused by degeneration of the brain's frontal or temporal lobes.
She says the result is explained by the way switching between languages stimulates the brain.
"Our research gives a hypothesis of why this could be. And we believe it improves certain aspects of attention, cognitive control and executive functioning. So it basically improves cognitive reserve and that is likely to delay dementia onset."
The study is not the first to reach this conclusion.
Two previous smaller studies conducted in the Canadian province of Ontario found a later onset of Alzheimer's disease in bilingual people, but that group were largely immigrants.
Dr Alladi says the new study proves the result more definitively among bilingual people raised in the same country and culture.
She says the location of the study is important because many people in India speak two or three languages at home, work or school in the course of one day.
And she says the study has pinpointed a new and surprising finding: that a person's ability to read had no impact on the study's findings.
"So previous studies have kind of hinted that the effect of bilingualism is due to immigration or education. We were able to show a separate effect of bilingualism, which was not influenced by education. It's heartening to know that even in illiterate people bilingualism protects."
But the researchers found that a person didn't get any additional advantage by speaking three or more languages.
"Since in India we're extremely bilingual and we're constantly switching between languages, we believe that we've reached a level of code-switching, so regardless of whether you speak two, or three, or four languages you've already achieved the benefits that one will achieve."
Dr Alladi says more work needs to be done into whether people can delay dementia by picking up a second language beyond childhood and also into whether bilingual people can cope better with dementia once the symptoms develop.
But she says bilingualism has one key advantage when compared to other preventative measures.
"Keeping oneself socially active, physical exercise, cognitively stimulating activities, high education, complex occupational activities, a good diet; several of these factors have been demonstrated to improve cognitive reserve and and ward off dementia. So bilingualism is yet another kind of activity that could protect. And because it's a lifelong practice, especially in countries like India - it just happens all the time - it seems to be a very kind of natural protection."
