Alzheimer's disease progresses more rapidly in highly educated people, research suggests.
Education triggers new nerve connections in the brain
It is thought high levels of education may ward off Alzheimer's by helping the brain better tolerate damaging changes.
But the latest study, involving 312 Alzheimer's patients, suggests once accumulated damage reaches a critical level, decline is relatively swift.
The study, by New York's Columbia University, features in the Journal of Neurology Neurosurgery and Psychiatry.
The researchers monitored 312 people aged 65 and older who had been diagnosed with Alzheimer's for five years.
Each patient underwent a battery of tests to assess their neurological function.
Overall mental agility declined every year among all the patients.
But each additional year of education equated to an additional 0.3% deterioration per year.
The level of this drop-off was particularly evident in the speed of thought processes and memory.
It was independent of age, mental ability at diagnosis, or other factors likely to affect brain function, such as depression and vascular disease.
The researchers said one possible explanation is what has been dubbed the "cognitive reserve" theory.
This holds that highly educated people either have a greater number of nerve connections in their brains, or the nerve connections that they have are more efficient.
Therefore, when the damaging changes associated with Alzheimer's - such as the deposition of toxic protein clumps - start to take place, educated people are better placed to resist their effect at first.
However, the subsequent impact is likely to be greater than it would be in less educated brains, because of the higher levels of accumulated damage.
Harriet Millward, of the Alzheimer's Research Trust, praised the study for following a large group of people, and assessing them in a community, rather than a clinic setting.
However, she said it was difficult to draw firm conclusions, as there were many factors which could determine the progression of the disease.
"These effects are subtle and not easy to test so, despite the thoroughness of the study, further evidence is needed before we can make any conclusions," she said.
"A key question remains whether these effects are truly due to years of schooling or due to other factors related to education, such as wealth, occupation or lifestyle."
Susanne Sorensen, head of research at the Alzheimer's Society, said the study suggested that a decision by the NHS drug watchdog, the National Institute for Health and Clinical Excellence (Nice) to deny drugs to people who score above a certain threshold on a test of mental aptitude might discriminate against the highly educated.
"We are calling for Nice to revaluate their guidance to allow people access to Alzheimer's drugs from the earliest stages of diagnosis. This will give people treatment at a time it is most valued."