Working in a mentally stimulating job may delay or reduce the risk of developing dementia, a study has found.

Research found those employed in "high stimulation" jobs such as doctors and chief executives were less likely to develop the disease in their 70s than those who employed as drivers, agricultural labourers, booking clerks and cashiers.

In a large-scale trial involving 107,895 participants, researchers measured how "cognitively stimulating" their occupations were at an average age of 45 and followed them for about 17 years.

A total of 1,143 people developed dementia and after adjusting for lifestyle factors, there were 4.8 cases per 10,000 people each year in the high stimulation jobs compared with 7.3 in the jobs considered least stimulating.

Jobs were deemed to be stimulating if they include demanding tasks and came with a high level of control over the work.

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The study, which involved University College London,  found that higher cognitive stimulation was associated with lower levels of plasma proteins that increase dementia risk in old age.

Cognitive decline characterises midlife and old age and it has been suggested that cognitive stimulation might be one measure to slow this neurodegenerative process. However, trial evidence on this possibility is said to be inconclusive.

The study found that higher cognitive stimulation in childhood, as indicated by higher educational attainment, and adulthood based on work were associated with lower dementia risk and the link was strongest for Alzheimer's Disease.

Of the 107, 896 participants, 45 080 (41.8%) were men and 62 816 (58.2%) were women, with an average age of 44.

The incidence at age 75 in the high stimulation group (3.1%) was already observed at age 74 in the low stimulation group and and the incidence at age 80 (8.8%) was already observed at age 78.3, respectively.

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The association between higher cognitive stimulation and lower dementia risk was slightly weaker than that between education and dementia but the study authors said the association was "statistically significant" even after taking other risk factors into consideration such as smoking and inactivity.

The link did not differ between men and women or those younger and older than 60 but there was an indication that the cognitive stimulation-dementia association was stronger for Alzheimer’s disease than for other dementias.

Professor Mika Kvimaki, from University College London, said: "Our observational findings support the hypothesis that mental stimulation in adulthood may postpone the onset of dementia.

"The levels of dementia at age 80 seen in people who experienced high levels of stimulation was observed at age 78.3 in those who had low mental stimulation."

Lower dementia incidence was observed even when 10 years or more separated the assessment of cognitive stimulation and the dementia diagnosis.

It follows the publication of  Scottish research which found footballers who play in defence were five times more likely to develop dementia.

Professor Willie Stewart, the consultant neuropathologist who led the University of Glasgow study, suggested footballs should carry a health warning, such was the strength of the new data.

The ground-breaking research has already shown that footballers were three-and-a-half times more likely to die of neurodegenerative diseases including dementia and had a five-fold risk of Alzheimer's. However, it did not reach any conclusions as to why this was the case.

The latest findings showed that the risk for footballers was dependent on the position played. Goalkeepers had the same risk as the average person while defenders - who head the ball most frequently -  had a five-fold risk and the risk was lowest in forwards (2.79).