Speaking more than one language could help to delay the onset of dementia, researchers claim.
Bilingual people who develop the condition tend do so up to five years later than those who only speak one language, an international team of scientists has found.
Researchers from University of Edinburgh and Nizam's Institute of Medical Sciences in India looked at when dementia was diagnosed in 650 people.
They discovered that those who spoke two or more languages experienced a later onset of Alzheimer's disease, vascular dementia and fronto-temporal dementia.
Switching between the different sounds, words and grammatical structures of languages could act as a natural form of brain training, but further study is needed to fully explain the findings, the researchers said.
Thomas Bak, of University of Edinburgh's school of philosophy, psychology and language sciences, said: "These findings suggest that bilingualism might have a stronger influence on dementia that any currently available drugs. This makes the study of the relationship between bilingualism and cognition one of our highest priorities."
The effect was seen even in patients who never attended school and were classed as illiterate, indicating that it is not caused by differences in formal education, researchers said.
The study, published in the medical journal Neurology, is said to be the largest so far to look at the impact of bilingualism on the onset of dementia, independent of other factors such as education, gender and occupation.