Taking B vitamins doesn't slow mental decline nor will it prevent Alzheimer's disease, according to clinical trial data involving 22,000 people.

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 a new analysis carried out by the B-Vitamin Treatment Trialists' Collaboration, an international group of researchers led by the Clinical Trial Service Unit at the University of Oxford. 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. 

When looking at measures of global cognitive function – or scores for specific mental processes such as memory, speed or executive function – there was no difference between those on B vitamins and those receiving placebo to a high degree of accuracy.

"It would have been very nice to have found something different," says Dr Robert Clarke of Oxford University, who led the work. "Our study draws a line under the debate: B vitamins don't reduce cognitive decline as we age. Taking folic acid and vitamin B-12 is sadly not going to prevent Alzheimer's disease."

"Taking supplements like B vitamins doesn't prevent heart disease, stroke or cognitive decline," says Professor Clarke. 'About 25% of the adult population take multi-vitamins, often with the idea that they are also good for the heart or the brain, but the evidence just isn't there. Much better is to eat more fruit and vegetables, avoid too much red meat and too many calories, and have a balanced diet."

Maternal folic acid intake before and during early pregnancy reduces a woman's risk of having a neural tube defect birth defect and those thinking of having a baby are routinely advised to take folic acid supplements. Countries that have adopted mandatory population-wide folic acid fortification programs have also demonstrated reductions in neural-tube defect associated pregnancies without any adverse effects.