Scientists from Oxford University said their two-year clinical trial was the largest to date into the effect of B vitamins on so-called "mild cognitive impairment" - a major risk factor for Alzheimer's disease and other forms of dementia.
Experts commenting on the findings said they were important and called for larger, longer full-scale clinical trials to see if the safety and effectiveness of B vitamins in the prevention of neurodegenerative conditions could be confirmed.
Researchers conducted a two-year trial with 168 volunteers with mild cognitive impairment, characterized by slight problems with memory loss, language or other mental functions.
Study participants were given either a vitamin pill containing very high doses of folic acid, vitamin B6 and vitamin B12, or a placebo dummy pill.
These B vitamins are known to control levels of an amino acid called homocysteine in the blood, and high blood levels of homocysteine are linked to an increased risk of developing Alzheimer's disease.
The researchers note that vitamins in the study were given in extremely high doses, up to 300 times the recommended daily intake of B12, four times daily advised folate levels and 15 times the recommended amount of B6.
Brain scans were taken at the beginning and the end of the trial to monitor the rate of brain shrinkage.
The results showed that on average the brains of those taking the vitamin treatment shrank at a rate of 0.76 percent a year, while those taking the dummy pill had an average brain shrinkage of 1.08 percent.
People who had the highest levels of homocysteine at the start of the trial benefited the most from the treatment, with their brains shrinking at half the rate of those on the placebo.
Although the trial was not designed to measure cognitive ability, the researchers found those people who had lowest rates of shrinkage had the highest scores in mental tests.
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