For millions who wished they could prevent Alzheimer's disease by taking B vitamins, Oxford University researchers have bad news.

 

"Our study draws a line under the debate: B vitamins do not reduce cognitive decline as we age. Taking folic acid and vitamin B-12 is sadly not going to prevent Alzheimer's disease," said professor Robert Clarke, who led the work.

 

 High levels in the blood of a compound called homocysteine have been found in people with Alzheimer's disease, and people with higher levels of homocysteine have been shown to be at increased risk of Alzheimer's disease.

 

 Taking folic acid and vitamin B-12 are known to lower levels of homocysteine in the body, so this gave rise to the 'homocysteine hypothesis' that taking B vitamins could reduce the risk of Alzheimer's disease.

 

 But the new study shows that B vitamins are not effective enough to slow mental decline.  The researchers brought together data from 11 randomised clinical trials involving 22,000 people which compared the effect of B vitamins on cognitive function in older people against placebo.

 

 Participants receiving B vitamins did see a reduction in the levels of homocysteine in their blood by around a quarter.  However, this had no effect on their mental abilities.  The findings appeared in the American Journal of Clinical Nutrition.