I have read a few articles that claim that being able to speak second a language has various benefits to the mind, including preventing or delaying Alzheimer's disease,
Is this really true?
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First, I want to stress the fact that there is difference between delaying and preventing. The articles I have read online claim that bilingualism may delay Alzheimer's progression.
Those claims are actually true, since there is some evidence online suggesting that bilingualism may (and not will) slow Alzheimer's Progression.
Here is the abstract:
Objectives: There is strong epidemiologic evidence to suggest that older adults who maintain an active lifestyle in terms of social, mental, and physical engagement are protected to some degree against the onset of dementia. Such factors are said to contribute to cognitive reserve, which acts to compensate for the accumulation of amyloid and other brain pathologies. We present evidence that lifelong bilingualism is a further factor contributing to cognitive reserve.
Methods: Data were collected from 211 consecutive patients diagnosed with probable Alzheimer disease (AD). Patients' age at onset of cognitive impairment was recorded, as was information on occupational history, education, and language history, including fluency in English and any other languages. Following this procedure, 102 patients were classified as bilingual and 109 as monolingual.
Results: We found that the bilingual patients had been diagnosed 4.3 years later and had reported the onset of symptoms 5.1 years later than the monolingual patients. The groups were equivalent on measures of cognitive and occupational level, there was no apparent effect of immigration status, and the monolingual patients had received more formal education. There were no gender differences.
Conclusions: The present data confirm results from an earlier study, and thus we conclude that lifelong bilingualism confers protection against the onset of AD. The effect does not appear to be attributable to such possible confounding factors as education, occupational status, or immigration. Bilingualism thus appears to contribute to cognitive reserve, which acts to compensate for the effects of accumulated neuropathology.
Source: Craik FIM, Bialystok E, Freedman M. Delaying the onset of Alzheimer disease: Bilingualism as a form of cognitive reserve. Neurology. 2010;75(19):1726-1729. doi:10.1212/WNL.0b013e3181fc2a1c.
Another study here found that:
Bilingualism appears to contribute to increased CR, thereby delaying the onset of AD and requiring the presence of greater amounts of neuropathology before the disease is manifest.
With no doubt, more research is actually needed before it's completely understood how cognitive reserve works to delay the onset of Alzheimer's disease.
So answering your question:
Does bilingualism prevent Alzheimer's disease?
Not necessarily. There is a link between bilingualism and Alzheimer's disease, but it has not been proven that bilingualism prevents Alzheimer's disease.