That's according to researchers from the University of Oxford in England.
The scientists studied 107 people between the ages of 61 and 87 without memory or thinking problems. The average age of participants was 73, and 54% were women.
The researchers collected blood samples to check the levels of vitamin B12, a nutrient found in meat, fish, and milk. Participants underwent yearly brain scans using magnetic resonance imaging (MRI), memory testing, and physical exams.
None of the people in the study had a vitamin B12 deficiency.
When the researchers compared the results, they 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.
The researchers write that they were unable to investigate whether lower vitamin B12 could cause cognitive impairment by its effect on brain size.
"Many factors that affect brain health are thought to be out of our control, but this study suggests that simply adjusting our diets to get 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," says study author Anna Vogiatzoglou, MSc, with the University of Oxford.
Since the researchers did not look at whether taking vitamin B12 supplements would have an effect, it remains unknown whether they could make a difference in elderly persons at risk for brain shrinkage.
"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," Vogiatzoglou says.