Researchers have found that taking vitamin B12 and folic acid supplements may not reduce the risk of memory and thinking problems at all.
The study involved people with high blood levels of homocysteine, an amino acid.
High levels of homocysteine have been linked to memory loss and Alzheimer's disease.
"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," said Rosalie Dhonukshe-Rutten from Wageningen University, the Netherlands.
For the study, 2,919 people with average age 74 took either a tablet with 400 microgram of folic acid and 500 microgram of vitamin B12 or a placebo every day for two years.
Tests of memory and thinking skills were performed at the beginning and end of the study. All of the participants had high blood levels of homocysteine.
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"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," Dhonukshe-Rutten pointed out.
The study appeared in the journal Neurology.