Those who have been contemplating learning a second language may want to enroll on a course, as researchers have found that being bilingual may help to delay three types of dementias.

The study, published in the journal Neurology, is also the first of its kind to report a second language advantage in those who are illiterate, say the researchers - who come from both the University of Edinburgh in the UK and Nizam's Institute of Medical Sciences in Hyderabad, India.

For their study, the researchers evaluated 648 people from India who were diagnosed with dementia. The average age of the patients was 66, and a total of 240 had Alzheimer's disease, while 189 had vascular dementia and 116 had frontotemporal dementia.

According to the Alzheimer's Society, frontotemporal dementia is not as common as other forms of dementia, and it is caused by nerve cells in the frontal or temporal lobes of the brain dying, causing the pathways that connect them to change.

The remaining 103 of the patients from India had dementia with Lewy bodies (DLB) and mixed dementia. The American Academy of Neurology notes that DLB is one of the more common types of dementia and is caused by the build-up of Lewy bodies - accumulated bits of protein - inside areas of the brain that control memory and motor control.

The researchers found that people who spoke two languages did not develop dementia until 4.5 years later than those who were monolingual.

Protection: bilingualism 'better develops certain brain areas'

The types of dementias that did not develop until much later in the bilingual patients were Alzheimer's disease, frontotemporal dementia and vascular dementia.

In the sample of patients, 14% were illiterate, which study author Suvarna Alladi says makes their study unique:

"Our study is the first to report an advantage of speaking two languages in people who are unable to read, suggesting that a person's level of education is not a sufficient explanation for this difference."

The difference in dementia onset was also observed in those who could not read, the researchers say, adding that there was no extra benefit in speaking more than two languages.

This effect was separate of other factors, such as education, sex, occupation and rural versus urban living.

Alladi says:

"Speaking more than one language is thought to lead to better development of the areas of the brain that handle executive functions and attention tasks, which may help protect from the onset of dementia."

He adds that their results "offer strong evidence for the protective effect of bilingualism against dementia in a population very different from those studied so far in terms of its ethnicity, culture and patterns of language use."

Thomas Bak, from the University of Edinburgh's School of Philosophy, Psychology and Language Sciences, notes that their findings "suggest that bilingualism might have a stronger influence on dementia than any currently available drugs."

He therefore says that this makes the relationship between cognition and bilingualism a high research priority.