The longitudinal cohort study followed 1648 participants for 10 years, and found a doubling in holotranscobalamin (holoTC) concentrations was associated with a 30 per cent slower rate of cognitive decline.
The study is published in the American Journal of Clinical Nutrition .
Moreover, increased levels of the amino acid homocysteine doubled the risk of dementia or cognitive impairment, report the researchers, led by Robert Clarke from the University of Oxford.
Previously, epidemiological studies have reported that high levels of the amino acid homocysteine are associated with suspected or confirmed dementia.
Indeed, the Framingham study reported that people with homocysteine levels above 14 micromoles per litre of serum had twice the risk of dementia.
This has led to the hypothesis that by lowering circulating levels of homocysteine by B vitamin supplements, the risk and occurrence of dementia could be reduced.
However, intervention trials have reported mixed results, and the subject remains debatable.
The new study measured serum concentrations of vitamin B-12, holoTC, homocysteine, methylmalonic acid (MMA), and folate of the participants in 1995, and related this to measures of cognitive function assessed on three occasions during the ten year study.
Mental function was tested using the Mini-Mental State Examination.
Clarke and co-workers report that doubling holoTC levels from 50 to 100 picomoles per litre was associated with a 30 per cent slower rate of cognitive decline.
Folate levels did not impact cognitive performance.
Furthermore, a doubling of homocysteine levels, from 10 to 20 micromoles per litre, or MMA levels, from 0.25 to 0.50 micromoles per litre, was associated with a rapid decline of cognitive function of over 50 per cent.
The researchers, called for randomised trials to investigate the potential benefits from vitamin B-12 supplementation for prevention of dementia.
The B-Vitamin Treatment Trialists' Collaboration should soon be better able to address the link between B-vitamins, homocysteine levels, and cognitive function.
The effects of three to seven years of treatment with B vitamins on cognitive function should eventually be available on about 20,000 of the 50,000 participants with previous cardiovascular or renal disease from 12 large homocysteine-lowering trials.
Vitamin B12, also known as cobalamin, is only found in food of animal origin, including meat, milk and eggs.
The recommended daily intake of vitamin B12 in Europe is 2.5 micrograms.
Source: American Journal of Clinical Nutrition November 2007, Volume 86, Number 5, Pages 1384-1391 "Low vitamin B-12 status and risk of cognitive decline in older adults" Authors: Robert Clarke, J. Birks, E. Nexo, P.M. Ueland, J. Schneede, J. Scott, A. Molloy and J. Grimley Evans