Researchers have found a direct link between low levels of two vitamins - B12 and folic acid - and the onset of Alzheimer's disease.

Alzheimer's is a degenerative disease of the brain, which destroys memory and eventually all other mental functions.

Now the Oxford Project to Investigate Memory and Ageing (Optima) says it may be caused by a vitamin-deficient diet, New Scientist magazine reports.

Optima was launched in 1988 and over the past 10 years has collected blood samples from hundreds of patients.

Analysis results

When the group analysed the samples they found that those which had below normal levels of vitamin B12 came from patients who were four times as likely to develop Alzheimer's.

Changes in diet could prevent Alzheimer's

Those with similarly low levels of folate - a form of folic acid - were three times as likely to develop the disease.

One of Optima's interests is the relationship between a harmful amino acid called homocysteine and levels of folates.

People who have the highest quantities of the amino acid in their blood stream are four-and-a-half times as likely to develop Alzheimer's, the group found.

Dr Margaret Rayman, a specialist in nutrition at Surrey University, said: "Both B12 and folate are required to ensure that homocysteine doesn't build up in the bloodstream.

"If it does, there is evidence that there is some relationship with Alzheimer's."

The Optima group's findings were originally published in the Archives of Neurology, and lend weight to calls for folic acid supplements to be added to basic foodstuffs such as bread.

This has happened in the US since the beginning of 1998.

Blanket supplements

Dr Rayman said that folic acid supplements would be useful, but they would need to be provided along with other vitamins.

She said there were dangers associated with just taking folate supplements because high levels of folic acid could hide deficiencies of B12.

She added that B12 or folate deficiency could both cause anaemia.

Dr Rayman said: "You would get over the symptoms of anaemia with folic acid, but there could be an underlying B12 deficiency which was masked by the folic acid."

She added that even when B12 deficiency was not bad enough to cause anaemia itself, it could still cause irreversible damage to the nervous system.

"People wouldn't know that they were B12 deficient and there could be nerve damage going on.