Researchers from Cambridge University found that the longer someone spends in school and at university, the lower their risk of developing the condition.
For every year in education the odds of being diagnosed with Alzheimer’s later in life drops 11%, they calculated. Scientists believe complex thinking creates "cognitive reserve" that helps to ward off dementia.
This creates a greater number of connections between brain cells, so when the wiring of the brain comes under attack from Alzheimer’s, the brain has "back-up" networks to use instead.
The scientists write in the British Medical Journal: “This implies that an individual with more cognitive reserve, for instance from higher education or intelligence, uses more efficient processing pathways and can sustain more Alzheimer’s (disease) before the initial clinical signs and symptoms emerge."
Alzheimer’s is the most common form of dementia and affects 500000 people in Britain, a number expected to rise as the elderly population grows. With no effective treatment, doctors are examining how people can ward off the disease instead.
Previous studies have found links between education and lower risks of dementia, but critics say educated people are likely to be from richer backgrounds and have all the health benefits of a privileged youth.
They say socially deprived people are more likely to smoke, have poor physical health and do less exercise, and so already be more at risk from dementia.
The new study, which is based on 54000 people, adds huge weight to the theory that education makes a difference despite someone’s start in life. The researchers analysed the genetic make-up of participants and used this information to rule out the impact of pre-existing conditions and other confounding factors.
Analysing 246 genetic variants, they subtracted the factors they thought would unduly influence their results and found education differences alone still had a huge impact on dementia risk.
Lead author Professor Hugh Markus, of the department of clinical neurosciences at Cambridge, said: "Many studies have shown that certain risk factors are more common in people with Alzheimer’s disease, but determining whether these factors actually cause Alzheimer’s is more difficult."
The study looked at each individual’s DNA and compared genes associated with environmental risk factors - for example, genes that make people more likely to smoke or drink - to see which were associated with Alzheimer’s.
If a gene is associated with both, then it provides strong evidence that this risk factor really does cause the disease.
The study found the strongest association with genetic variants that predict higher educational attainment. Co-author Dr Susanna Larsson said: “This provides further strong evidence that education is associated with a reduced risk of Alzheimer’s."