An inexpensive supplement will halt the progress of Alzheimer's disease, scientists hope, after a trial suggested that patients stopped deteriorating while taking it.

Resveratrol is a naturally occurring compound found in red grapes, raspberries, dark chocolate and red wine.

Although quantities in food are tiny, high-dose supplements are available over the counter in most health food shops for just a few pounds and purer forms can be bought from pharmacists.

Scientists enrolled 119 people with mild to moderate Alzheimer's in a trial in which they were given 1g of high-grade resveratrol twice a day for 12 months while a control group received a placebo.

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Normally, as Alzheimer's progresses, the level of a protein called Abeta40 decreases in the blood. But those taking the supplement showed no alteration, suggesting the disease had stabilised. They were also found to have improved at dressing themselves, cooking and using public transport.

Sufferers taking the placebo continued to show decreased levels of Abeta40 and did not improve in cognitive tests. Although researchers warn that the trial is too small to advise people to start taking the supplement, they say the results are "very interesting" and further work should be done.

"Number one, we found that resveratrol was safe in older people with Alzheimer's disease and number two, it looked like it may have had a beneficial effect on biomarkers and disease progression," said Dr Scott Turner, director of the memory disorders programme at Georgetown University Medical Centre, in Washington DC.

The researchers chose resveratrol because it activates proteins called sirtuins, which are also switched on by calorie restriction. Animal studies have found that most age-related diseases, including dementia, can be prevented or delayed by cutting calories to two thirds of the recommended level.

Resveratrol is also released by plants in response to injury and is thought to have antioxidant properties.

Dr Doug Brown, director of research at the Alzheimer's Society, said: "While this is an interesting study, it did not investigate whether resveratrol has any effect on memory or improving other symptoms of Alzheimer's disease.

"Before we can say this has potential as a treatment option we need to see larger and longer trials in people that specifically look at whether it can improve the lives of those with the condition".

The research was published in the journal Neurology.

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