Keeping one's brain active, trying not to become depressed and eating a diet rich in fruit and vegetables are the best ways to ward off developing dementia, a study of almost 1,500 volunteers has found.

If younger people were to follow such advice, millions worldwide could avoid or postpone the debilitating condition in old age, the research suggests.

The combined effects would far outstrip the theoretical possibility of eliminating a gene known to increase the chance of dementia, according to the study, published today in the British Medical Journal.

Dr Karen Ritchie, a neuropsychologist at the French National Institute of Medical Research, led a team who assessed how the cognitive ability of 1,433 pensioners in Montpellier changed over a seven-year period.

They asked them a series of questions about their lifestyle, medical history and educational background, as well as carrying out reading tests.

Their results indicate that how much intellectual exercise a person takes has an enormous effect on their likelihood of developing dementia.

Those with lower reading scores were 18 per cent more likely to develop "mild cognitive impairment or dementia" - the former widely seen as a forerunner of the latter.

Those with depression were 10 per cent more likely to develop it; while those who ate fruit and vegetables less than twice a day were 6.5 per cent more likely to do so.

Having diabetes was also a significant factor, leading to a five per cent higher risk than those without.

By comparison, possessing a particular gene associated with dementia increased the risk by seven per cent.

With 820,000 currently living with dementia in Britain, a number forecast to double by 2050, the report's authors point out that "even small reductions in incidence, or delaying the age of onset, are likely to have significant effects on prevalence and the enormous associated public health burden".

While studies have identified such risk factors before, they have not quantified their relative impact.

Although the bare statistics indicated intellectual activity was the most important factor, the study noted it was difficult to pry apart how much a good reading score was based on genetics and how much on upbringing.

In the absence of an answer, "the public health message can only be to encourage literacy at all ages irrespective of innate ability," the academics concluded.

Similarly, while depression was strongly linked to developing dementia, "the causal relation ... remains unclear". If it was an early sign of dementia rather than a direct trigger, they warned, treating it would not necessarily offer protection.

Getting people to eat more fruit and vegetables was also difficult, they accepted.

With such problems they concluded the most practical short-term solution was tackling diabetes, which previous studies have confirmed to be a causal factor.

Professor Clive Ballard, director of research at the Alzheimer's Society, said:
"Effective prevention of diabetes, depression and heart disease could potentially improve the lives of millions of people affected by this cruel condition and reduce the billions spent on dementia care each year.

"With the numbers of people with dementia rising quickly it is important that everyone manages their own risk and that as a society we invest in further dementia research that will one day help us find a cure."
Rebecca Wood, Chief Executive of the Alzheimer's Research Trust, added:
"What is painfully evident from the study is the gaping hole that remains in our understanding and ability to diagnose or treat dementia effectively, a hole that can only be filled by more research."