For the study, 107 people between the ages of 61 and 87 underwent brain scans, memory testing and physical exams.
Researchers also collected blood samples to check vitamin B12 levels. Brain scans and memory tests were also performed again five years later.
The study found that people who had higher vitamin B12 levels were six times less likely to experience brain shrinkage compared with those who had lower levels of the vitamin in their blood.
None of the people in the study had vitamin B12 deficiency.
"Many factors that affect brain health are thought to be out of our control, but this study suggests that simply adjusting our diets to consume more vitamin B12 through eating meat, fish, fortified cereals or milk may be something we can easily adjust to prevent brain shrinkage and so perhaps save our memory," said study author Anna Vogiatzoglou, MSc, with the University of Oxford in the United Kingdom.
"Research shows that vitamin B12 deficiency is a public health problem, especially among the elderly, so more vitamin B12 intake could help reverse this problem.
"Without carrying out a clinical trial, we acknowledge that it is still not known whether B12 supplementation would actually make a difference in elderly persons at risk for brain shrinkage."
"Previous research on the vitamin has had mixed results and few studies have been done specifically with brain scans in elderly populations. We tested for vitamin B12 levels in a unique, more accurate way by looking at two certain markers for it in the blood," said Vogiatzoglou.
Vogiatzoglou says the study did not look at whether taking vitamin B12 supplements would have the same effect on memory.