Vitamin B12 is best known for helping the body produce DNA, red blood cells, and healthy nerve tissue. However, research shows that meeting current minimum standards may not always be enough, especially for older adults.
A UCSF-led study found that healthy older adults with low vitamin B12 levels show signs of subtle neurological and cognitive problems, even if their vitamin B12 levels are within acceptable normal ranges. This finding raises provocative possibilities. Some people may be told that their vitamin B12 status is fine, even though their brain is already showing early signs of stress.
“Standard” B12 is not always the best choice
This research neurological chroniclestargeted elderly people without dementia or mild cognitive impairment. Even in this relatively healthy group, lower levels of active vitamin B12 were associated with slower thinking, slower visual processing, and more pronounced brain white matter damage. White matter is made up of nerve fibers that allow different parts of the brain to communicate.
The study was led by senior author Ali J. Green, MD, of the UCSF Department of Neurology and Ophthalmology and the Weill Neuroscience Institute. Green et al. said the results draw attention to potential weaknesses in the current B12 guidelines. The minimum thresholds used to define deficiency may not capture early functional changes in the nervous system.
“Previous studies defining healthy amounts of vitamin B12 may have missed subtle functional manifestations of high or low levels that can affect people without causing obvious symptoms,” Professor Green said, noting that apparent deficiencies in the vitamin are commonly associated with a type of anemia. “Rethinking the definition of B12 deficiency to incorporate functional biomarkers may lead to early intervention and prevention of cognitive decline.”
Brain scan reveals problematic pattern
Researchers enrolled 231 healthy participants through UCSF’s Brain Aging Network for Cognitive Health (BrANCH) study. The average age of the participants was 71 years, and none had dementia or mild cognitive impairment.
Their average blood B12 level was 414.8 pmol/L, well above the U.S. minimum cutoff of 148 pmol/L. Rather than relying solely on the total amount of vitamin B12, the researchers focused on the biologically active form of the vitamin. This may be a better reflection of the amount of vitamin B12 your body can actually use.
After adjusting for age, gender, education, and cardiovascular risk factors, the researchers found that participants with lower levels of active vitamin B12 were slower on cognitive tests. The effect became stronger with increasing age. They also showed slower responses to visual stimuli, slower visual processing, and less efficient brain signaling.
The MRI scan added another warning sign. Participants with lower active vitamin B12 levels had higher amounts of white matter lesions, areas of brain damage associated with risk of cognitive decline, dementia, and stroke.
Why older people may be more vulnerable
This study focused on older adults, who may be particularly susceptible to declines in vitamin B12 because absorption efficiency may decrease with age. Some medications, digestive disorders, and a diet low in animal products can also increase your risk of low vitamin B12.
Co-lead author Alexandra Beaudry-Richard, Ph.D., MSc, said the study results suggest that low but technically normal vitamin B12 levels may have a broader impact than previously realized. These levels “may affect cognition to a greater extent than we previously thought, and may affect a much larger population than we realize.” Beaudry-Richard is currently pursuing a research and medical doctorate in the UCSF Department of Neurology and the University of Ottawa’s Department of Microbiology and Immunology.
“In addition to redefining vitamin B12 deficiency, clinicians should consider supplementation in elderly patients with neurological symptoms, even if their vitamin B12 levels are within the normal range,” he said. “Ultimately, vitamin B12 deficiency may be a preventable cause of cognitive decline, so we need to invest in more research into its underlying biology.”
New evidence adds important context
Studies published before and after the UCSF study added nuance rather than a simple answer. A comprehensive review in 2025 concluded that vitamin B12 deficiency remains a modifiable risk factor for neurological and cognitive problems, especially in high-risk groups such as the elderly and vegetarians. The review also highlighted the growing importance of better biomarkers and brain imaging for early detection of problems.
A 2025 systematic review and meta-analysis of randomized trials found that supplementation with B vitamins, including B6, B9, or B12, had very modest benefits on overall cognitive function in older adults. The authors rated the clean analysis as having a high degree of certainty, but the effect was small, suggesting that supplements may not produce dramatic brain improvements for everyone.
Another 2025 study using Mendelian randomization found no clear evidence that genetically high total serum B12 levels protect the general population from psychiatric or cognitive impairment. However, the authors noted important limitations. Their analysis used total serum B12, rather than the bioactive form measured in the UCSF study.
New evidence taken together supports a more cautious message. Vitamin B12 is clearly essential for the nervous system and deficiency should not be ignored. But simply increasing everyone’s vitamin B12 levels may not be the answer. A more pressing question is whether current tests are missing people whose brains are already affected despite “normal” results.
Preventable risks worth taking seriously
The UCSF findings do not prove that a decline in active B12 directly causes cognitive decline, nor do they mean that all older adults should start taking supplements without medical guidance. However, they suggest that the current definition of vitamin B12 deficiency may be too explicit for brain health.
This study suggests the value for clinicians to look beyond total vitamin B12, especially in elderly patients with neurological symptoms. For patients, this highlights a practical message. “Normal” test results don’t always tell the whole story, especially if subtle changes in memory, thinking speed, or vision are already occurring.
Authors: Co-first author is Ahmed Abdelhak, MD, of the UCSF Department of Neurology and the Weill Institute for Neuroscience.
Funding and disclosure: Westridge Foundation and Canadian Institutes of Health Research. There are no conflicts of interest to report.
