Folic acid, the vitamin prescribed to pregnant women, could be a brain saver, a study has found.

People who took high dose supplements of folic acid did significantly better in tests of memory and cognitive performance than those given a placebo, researchers report.

The vitamin is found in green leafy vegetables, beans and liver but few people in the UK and northern Europe obtain high enough levels from diet alone. Average intake is around 200 micrograms a day, half the recommended amount.

Folic acid plays a crucial role in the development of the embryo and some countries such as the US now fortify flour with the vitamin. As a result the incidence of spina bifida and similar birth defects has declined. Low folate levels have also been linked with poor cognitive performance, but research has failed to show a benefit among people given supplements.

Now scientists from the University of Wageningen in the Netherlands have demonstrated with the use of more sensitive tests of cognitive performance that high dose folic acid supplements taken over a long period slow the effect of ageing on the brain.

A group of 818 participants, aged 50 to 70, were given a daily dose of 800 micrograms of folic acid, equivalent to four times the average daily intake, for three years. Tests showed their performance on memory and speed of thinking tasks was comparable to that of people five years younger. The results are published in The Lancet.

All the participants had raised levels of homocysteine, a naturally occurring enzyme which is over-produced in response to low folate levels. High concentrations of homocysteine increase the risk of heart disease and have been associated with atrophy of the hippocampus, an area of the brain important for memory consolidation.

Other cognitive functions, such as word fluency, were unaffected by the supplements, suggesting that the effect of folic acid might be restricted to basic aspects of speed and information processing.

The researchers say: "We have shown that three-year folic acid supplementation improves performance on tests that measure information processing speed and memory, domains that are known to decline with age... Trials similar to our own should be repeated in other populations to provide greater insight into the clinical relevance of folic acid supplementation."