The study found that most people had jobs with about the same level of mental demands when they were in their 20s.

However, this changed in later years. Those who did not have Alzheimer's went on to do more mentally demanding jobs.

Those who went on to be diagnosed with the disease did not. They were more likely to spend their working lives in physical jobs and the mental demands of their jobs did not change significantly over the decades.


Further research
The researchers, who are based at the Case Western Reserve University in Ohio, said further studies are needed to find out if there really is a link.

However, previous studies have indicated that keeping the brain active can protect against Alzheimer's.

Research published last year suggested that dancing, playing musical instruments, reading and playing board games can all reduce the risks of developing the disease.

Dr Kathleen Smyth, who was involved in this latest study, said mentally demanding jobs may boost brain activity and help it to fight against Alzheimer's.

"It could be that higher levels of mental demands result in increased brain cell activity, which may help maintain a 'reserve' of brain cells that resists the effects of Alzheimer's disease," she said.

However, the researchers said other factors may also be involved. Their study did not look at the socio-economic background of the participants.

"Variations in income, access to healthcare, better nutrition and other factors related to socio-economic status could be responsible in part for our findings," said Dr Smyth.


Some caveats
The UK's Alzheimer's Society welcomed the study. However, Professor Clive Ballard, its director of research, said the findings should be interpreted cautiously.

"Some caution should be used in interpreting studies comparing people who develop Alzheimer's disease with those who do not, as any biases in the matching of groups can strongly influence the results.

"In addition, people with more mentally demanding jobs may have other differences in their lifestyles or medical care, which may explain the apparent differences."

But he added: "Despite these caveats, this is the most rigorous study of its type and lends further support to the theory that keeping your brain active may reduce the risk of developing Alzheimer's disease."