It seems that being able to speak multiple languages, and/or being able to express oneself well, may help to prevent dementia.
Researchers from the University of Waterloo recently conducted a study, published in the Journal of Alzheimer’s Disease, to explore the association between multilingualism and dementia risk.
To do so, they examined 325 Roman Catholic nuns who were members of the Sisters of Notre Dame in the United States. They gathered the data from the Nun Study, which assesses the sisters and their brain health.
After reviewing the material, they found 6% of the nuns who spoke four or more languages developed dementia, compared to 31% of those who spoke only one language.
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The team also evaluated the nuns’ writing and discovered those who could best express their ideas on paper also had a lower dementia risk.
There's more at the link.
When you think about it, I suppose that's logical. One's mind has to work that much harder to translate, not just words, but concepts, between languages; so it stands to reason that doing so often would "stretch" one's mental capabilities, just as physical exercise is a workout for the body's muscles. If physical exercise helps to stave off physical disease or deterioration, why shouldn't mental exercise do the same for the mind?
However, I guess one would need to use that ability frequently if it's to have the desired effect. I learned five languages in my younger days, in varying degrees of fluency: English, Afrikaans, French, Zulu and Southern Sotho. However, I haven't used the latter four (except in passing, very infrequently - the latter two usually to swear when I stub my toe!) since coming to the USA more than two decades ago. I'm not sure whether simply having studied them, once upon a time, would confer any immunity to dementia if I'm not actually using them.
As for writing being an anti-dementia exercise, that's debatable. Again, I think it depends on what you're writing. If you're not actually using your brain to develop concepts and express them, is there any creativity involved? And, if not, would that mental activity confer any sort of health benefit? I'm not sure. I've certainly heard of enough writers who've gone senile, or exhibited other mental problems.
Be that as it may, I guess I'd better write faster, just in case!