According to Dutch researchers, taking vitamin B12 and folic acid supplements does not seem to cut the risk of developing dementia in healthy people.
In one of the largest studies to date, there was no difference in memory test scores between those who had taken the supplements for two years and those who were given a placebo.
The new research was published in the Neurology journal.
However, other researchers say longer trials were needed to be sure.
B vitamins have been linked to Alzheimer’s for some years, and scientists know that higher levels of a body chemical called homocysteine can raise the risk of both strokes and dementia.
That, along with studies linking low vitamin B12 and folic acid intake with poor memory,has prompted scientists to view the supplements as a way to ward off dementia.
Yet in the study of almost 3,000 people – with an average age of 74 – who took 400 micrograms of folic acid and 500 micrograms of vitamin B12 or a placebo every day, researchers found no evidence of a protective effect.
All those taking part in the trial had high blood levels of homocysteine, which did drop more in those taking the supplements.
On four different tests of memory and thinking skills taken at the start and end of the study, there was no beneficial effect of the supplements on performance.
The researchers did note that the supplements might slightly slow the rate of decline but concluded the small difference they detected could just have been down to chance.
Study leader Dr. Rosalie Dhonukshe-Rutten, from Wageningen University in the Netherlands, said: “Since homocysteine levels can be lowered with folic acid and vitamin B12 supplements, the hope has been that taking these vitamins could also reduce the risk of memory loss and Alzheimer’s disease.
“While the homocysteine levels decreased by more in the group taking the B vitamins than in the group taking the placebo, unfortunately there was no difference between the two groups in the scores on the thinking and memory tests.”
The researchers stressed the research cannot be extrapolated to people who already had cognitive problems and earlier research had suggested they may benefit.