Elderly people are set to make up a quarter of the total European population by 2020. The most dramatic demographic changes are in the oldest age group (80 years and over) that is estimated to grow from 21.4 million in 2000 to 35 million in 2025.

The Dutch trial found that middle-aged men and women who took 800mcg of folic acid a day over three years scored significantly better in cognitive tests than peers taking a placebo. Their memory was as good as if they were two to five years younger, said the researchers from Wageningen university in the Netherlands.

"It's the first study to convincingly show that folic acid can slow cognitive decline," said lead author Jane Durga. "We showed that folic acid, not a mix of B vitamins, can do the job."

Previous research has suggested that folic acid, as well as other B vitamins, can reduce levels of homocysteine, an amino acid thought to play a role in the onset of Alzheimer's disease as well as heart disease and stroke.

The subjects in this trial had not been diagnosed with Alzheimer's however and therefore cannot demonstrate that folic acid helped stave off the disease but this is being investigated in other studies.

The current study involved 818 men and post-menopausal women aged 50 to 70 years old who had elevated levels of homocysteine at baseline. They were randomized to receive either folic acid or a placebo for three years.

Blood folate levels for those in the supplement group increased five-fold and plasma total homocysteine concentrations decreased by around 25 per cent by the end of the study.

In several cognitive tests, this group performed significantly better than the placebo group. Memory was equivalent to being 5.4 years younger annd information processing was that of people two years younger. Sensory motor speed was typical of 1.9 years younger.

The research adds to mounting evidence that increasing intake of the B vitamin can offer a variety of health benefits. Folic acid is proven to reduce birth defects and several studies suggest that it may also prevent heart disease and strokes.

A new trial sponsored by the US National Institutes of Health is testing whether very high doses of folic acid and vitamins B6 and B12 can slow the rate of mental decline in people with Alzheimer's. It is expected to be completed in February.

The research was presented at the US-based Alzheimer's Association's first conference on prevention of dementia, running in Washington this week (presentation HT-002).