Vitamin B12 is best known for helping the body to form DNA, red blood cells and healthy nerve tissue. However, research suggests that simply meeting the current minimum standard is not always enough, especially in older people. A UCSF-led study found that healthy older people with lower vitamin B12 levels, even when those levels were still within the accepted normal range, showed signs of subtle neurological and cognitive problems. The findings raise a provocative question: Some people may be told their B12 status is fine while their brains are already showing early signs of overload.
“Normal” B12 Levels Do Not Always Mean Optimal
The study, published in the Annals of Neurology, looked at older adults who did not have dementia or mild cognitive impairment. Even in this relatively healthy group, lower levels of active B12 were associated with slower thought processes, slower visual processing and more significant damage to the white matter of the brain. The white matter consists of nerve fibers that enable communication between different parts of the brain.
The work was led by senior author Ari J. Green, MD, of the UCSF Departments of Neurology and Ophthalmology and the Weill Institute for Neurosciences. Green and his colleagues said the findings pointed to a potential weakness in the current B12 guidelines. The minimum threshold used to define a deficiency may not capture early functional changes in the nervous system.
“Previous studies that defined healthy levels of B12 may have missed subtle functional manifestations of high or low levels that can affect people without causing obvious symptoms,” Green said, pointing out that a clear deficiency of the vitamin is often associated with a certain type of anemia. “Revising the definition of B12 deficiency to include functional biomarkers could lead to earlier intervention and prevention of cognitive decline.”
Brain Scans Showed a Disturbing Pattern
The researchers recruited 231 healthy participants as part of the Brain Aging Network for Cognitive Health (BrANCH) study at UCSF. The participants were 71 years old on average, and none suffered from dementia or mild cognitive impairment. Their average B12 blood level was 414.8 pmol/l, well above the US minimum limit of 148 pmol/l. Rather than relying solely on total B12, the researchers focused on the biologically active form of the vitamin, which may better reflect how much B12 the body can actually utilize.
After adjusting for age, gender, education level and cardiovascular risk factors, the team found that participants with lower levels of active B12 had slower processing speed on cognitive tests. The effect was more pronounced with increasing age. They also showed delayed responses to visual stimuli, indicating slower visual processing and reduced efficiency of signal transmission in the brain. MRI scans provided another warning sign. Participants with lower active B12 levels had a higher volume of white matter lesions, areas of brain damage associated with cognitive decline, dementia and an increased risk of stroke.
Why Older Adults May be More Vulnerable
The study focused specifically on older adults, as the body is often unable to absorb and utilize vitamin B12 as efficiently as it does in younger years. To absorb the vitamin, the body needs, among other things, sufficient stomach acid and the so-called intrinsic factor – a special protein in the stomach that enables the transport of B12 in the intestine. Both processes can be impaired in old age. As a result, even a balanced diet can gradually lead to an undersupply without any clear symptoms appearing immediately.
In addition, certain medications can increase the risk of low B12 levels. These include gastric acid blockers or the diabetes medication metformin, which can impair the absorption of the vitamin in the digestive tract. Chronic digestive disorders, intestinal inflammation or diets low in animal foods – the most important natural sources of B12 – can also contribute to a reduced supply of the vitamin.
Co-first author Alexandra Beaudry-Richard, MSc, explained that the findings suggest that even low levels of B12 that are still officially considered “normal” could have a greater impact on brain health than previously thought. In her view, these levels “could affect cognitive abilities more than we previously thought and affect a much larger proportion of the population than we realize.” Beaudry-Richard is a researcher at the University of California, San Francisco Department of Neurology and the Department of Microbiology and Immunology at the University of Ottawa.
The researchers therefore emphasize that the current threshold values for vitamin B12 deficiency may be set too low to detect early neurological changes in good time. While classic symptoms of severe deficiency – such as anemia, numbness or severe memory problems – often appear late, more subtle changes in the brain could begin years earlier. These include slower thought processes, concentration problems or a reduced processing speed of information.
“In addition to redefining B12 deficiency, doctors should consider supplementation in older patients with neurological symptoms, even if their levels are within the normal range,” said Beaudry-Richard. In the long term, the scientists hope that further research will better explain the biological mechanisms behind so-called B12 insufficiency. If it is confirmed that even slightly reduced active B12 levels increase the risk of cognitive decline, early detection and treatment could help to better prevent or at least slow down age-related mental decline.
Recent Findings Provide Important Additional Information
Research published after and around the UCSF study has provided nuance rather than a simple answer. A comprehensive review from 2025 concluded that B12 deficiency remains a modifiable risk factor for neurological and cognitive problems, particularly in high-risk groups such as older adults and vegetarians. The review also highlighted the growing importance of better biomarkers and brain imaging techniques for earlier detection of problems.
A systematic review and meta-analysis of randomized trials from 2025 found that supplementation with B vitamins, including B6, B9 or B12, had very little benefit on overall cognitive function in older adults. The authors categorized the adjusted analysis as very safe, but the effect was small, suggesting that supplementation is not a dramatic boost to the brain for everyone.
Another study from 2025, which used Mendelian randomization, found no clear evidence that genetically higher total B12 serum levels protect the general population from psychiatric disorders or cognitive impairment. However, the authors pointed out an important limitation: Their analysis was based on total B12 serum levels, not the bioactive form measured in the UCSF study.
Overall, the more recent findings support a more cautious assessment. B12 is undoubtedly essential for the nervous system, and a deficiency should not be ignored. But simply increasing B12 levels in everyone may not be the solution. The more pressing question is whether current tests are missing people whose brains are already impaired despite “normal” results.
An Avoidable Risk that Should be Taken Seriously
While the results of the University of California, San Francisco study do not prove that low active vitamin B12 levels directly cause cognitive decline, they do suggest that current cut-offs for “normal” B12 status may not be sufficient to reliably assess brain health. In particular, the biologically active portion of the vitamin may be more important than the current total B12 level in the blood.
For doctors, this could mean taking a closer look at older patients with neurological complaints – even if laboratory values are still within the normal range. Symptoms such as concentration problems, slowed thinking or memory deficits could possibly be linked to a functional B12 deficiency earlier than previously assumed.
The study also emphasizes that a “normal” test result does not always mean an optimal supply. The researchers therefore hope that future studies will clarify whether early detection and treatment of low active B12 levels can help to better prevent age-related cognitive decline.