The more proficient you become, the better, but "every little bit helps," said Ellen Bialystok, a psychology professor at York University in Toronto.
Bialystok studied 450 Alzheimer's patients, all of whom showed the same degree of impairment at the time of diagnosis. Half are bilingual - they've spoken two languages regularly for most of their lives. The rest are monolingual.
The bilingual patients had Alzheimer's symptoms and were diagnosed between four and five years later than the patients who spoke only one language, she told the annual meeting of the American Association for the Advancement of Science.
Being bilingual does nothing to prevent Alzheimer's disease from striking. But once the disease does begin its silent attack, those years of robust executive control provide a buffer so that symptoms don't become apparent as quickly, Bialystok said.