Nov 13, 2007
Studies published in US journals suggested that a "Mediterranean diet" or long-term beta-carotene supplements could ward off the illness.
Both contain anti-oxidants, which could protect the brain from damage.
The Alzheimer's Society said that most people could cut their risk by eating a healthy diet.
The first study, in the journal Neurology, looked at the diets of more than 8,000 healthy men and women aged over 65.
They found that those who regularly ate omega-3 oils, found in some cooking oils and certain types of fish, were far less likely to develop dementia over the following four year period.
People who ate fish at least once a week had a 40% lower risk of dementia, while eating fruit and vegetables once a day reduced the risk by 35%.
However, eating other types of cooking oils containing omega-6 - such as sunflower oil - rather than omega-3 doubled the risk.
Dr Pascale Barberger-Gateau, from the French National Institute for Health and Medical Research in Bordeaux, said: "These results could have considerable implications for public health."
The second study looked at the effects of beta-carotene supplements over an average of 18 years.
Beta-carotene is the chemical which gives carrots their colour, and is thought to have beneficial anti-oxidant properties.
The 4,000 volunteers took either a beta-carotene pill or a "placebo" pill with no active ingredients every other day.
Those who took the beta-carotene scored significantly higher on mental tests - particularly on "verbal memory".
Doing badly on these tests is believed to be a sign that the person has a higher risk of dementia in later life.
The reason why any of these foods or supplements might ward off dementia is unclear.
But many experts suggest that anti-oxidants can slow down the damage to the body's cells - including brain cells - which is normally accumulated over a lifetime.
And there is also debate over whether vitamin supplements are the best way to get anti-oxidants into the body.
In an accompanying editorial, Dr Kristine Yaffe of the University of California at San Francisco said that other studies into beta-carotene supplements had shown contrasting results, and the overall picture did not justify their use to boost memory.
One study has even suggested that taking beta-carotene long-term could be harmful.
Susanne Sorensen, the Director of Research for the Alzheimer's Society, said that it was possible that those who were prepared to take a pill so reliably over a long period might be more careful about their diet than the average person, accounting for some of the difference.
However, she said people should realise that there were steps they could take - such as adopting a so-called "Mediterranean diet" - to cut their risk of the illness.
She said: "The majority of the population can reduce their risk by eating a diet rich in vegetables, fruit and fish."