Vitamin B12 is widely recognized for its crucial role in the production of DNA, red blood cells, and maintaining healthy nerve tissue. However, emerging research suggests that simply meeting the established minimum standards for this vitamin may not suffice, particularly for older adults.
In a study led by UCSF, researchers discovered that healthy older individuals with lower levels of vitamin B12—despite these levels still being classified as normal—exhibited subtle neurological and cognitive impairments. This raises an intriguing question: Are some individuals being informed that their B12 levels are adequate while their brains are silently experiencing strain?
“Normal” B12 May Not Always Mean Optimal
The study, published in the Annals of Neurology, examined older adults who did not suffer from dementia or mild cognitive impairment. Even among this healthier demographic, lower levels of the biologically active form of B12 were associated with slower cognitive processing, diminished visual acuity, and increased injury to the brain’s white matter. White matter encompasses the nerve fibers that facilitate communication between various brain regions.
The research was spearheaded by Ari J. Green, MD, from the UCSF Departments of Neurology and Ophthalmology and the Weill Institute for Neurosciences. Green and his team emphasized that their findings highlight potential shortcomings in current B12 guidelines, specifically the minimum threshold for deficiency, which may not adequately reflect early functional changes in the nervous system.
“Previous studies that established healthy levels of B12 may have overlooked subtle functional symptoms associated with both high and low levels, affecting individuals without manifesting clear symptoms,” remarked Green. He noted that while overt deficiencies of the vitamin are typically linked to a specific type of anemia, redefining B12 deficiency to include functional biomarkers could allow for earlier interventions, potentially preventing cognitive decline.
Brain Scans Revealed a Troubling Pattern
The study included 231 healthy participants through the Brain Aging Network for Cognitive Health (BrANCH) study at UCSF. These participants averaged 71 years of age and had no signs of dementia or mild cognitive impairment.
Their average blood B12 level was 414.8 pmol/L, significantly higher than the U.S. cutoff of 148 pmol/L. The researchers shifted their focus from total B12 to the biologically active form of the vitamin, believing it better reflects the amount that the body can effectively utilize.
After adjusting for various factors such as age, sex, education, and cardiovascular risks, the team found that individuals with lower levels of active B12 demonstrated slower cognitive processing speeds, with effects becoming more pronounced with age. Additionally, these individuals exhibited delayed responses to visual stimuli, suggesting reduced visual processing ability and less efficient brain signaling.
Moreover, MRI scans presented another red flag. Participants with lower levels of active B12 showed an increased volume of white matter lesions, areas of brain damage correlated with cognitive decline, dementia, and elevated stroke risk.
Why Older Adults May Be More Vulnerable
The study concentrated on older adults, a group particularly susceptible to lower B12 levels due to the decreased efficiency of absorption that often accompanies aging. Certain medications, digestive conditions, and diets lacking animal-based foods may further elevate the risk of low B12 levels.
Alexandra Beaudry-Richard, MSc, co-first author, indicated that these findings imply that low but technically normal B12 levels may have more extensive consequences than previously acknowledged. “These levels could impact cognition more significantly than we realize, potentially affecting a broader segment of the population,” she noted while completing her doctorate at the UCSF Department of Neurology and the University of Ottawa’s Department of Microbiology and Immunology.
“In addition to redefining B12 deficiency, clinicians should contemplate supplementation for older patients exhibiting neurological symptoms, even if their levels fall within normal ranges,” she stated. “Ultimately, we need to invest in additional research regarding the underlying biology of B12 insufficiency, as it could be a preventable cause of cognitive decline.”
Newer Evidence Adds Important Context
Subsequent research surrounding the UCSF study has contributed valuable nuance rather than straightforward conclusions. A 2025 comprehensive review affirmed that B12 deficiency remains a modifiable risk factor for neurological and cognitive issues, especially among high-risk groups, including older adults and vegetarians. The review emphasized the increasing importance of improved biomarkers and brain imaging techniques for earlier detection of potential issues.
A 2025 systematic review and meta-analysis of randomized trials revealed that supplementation with B vitamins—including B6, B9, and B12—yielded a minimal improvement in global cognitive function among older adults. While the authors rated their findings with high certainty, the overall effect was small, indicating that supplementation may not provide a significant cognitive boost for everyone.
Additionally, a 2025 study applying Mendelian randomization found no compelling evidence that genetically higher total serum B12 levels safeguard the general population from psychiatric disorders or cognitive decline. However, the authors acknowledged a notable limitation: their analysis utilized total serum B12 rather than the bioactive form highlighted in the UCSF study.
Together, these newer findings advocate for a more nuanced understanding. B12 is undeniably vital for nervous system health, and deficiency should not be overlooked. However, simply increasing B12 intake for all may not be the solution. The pressing question remains whether current testing protocols are overlooking individuals whose cognitive functions are already compromised, despite receiving “normal” test results.
A Preventable Risk Worth Taking Seriously
The UCSF findings do not conclusively establish that lower active B12 directly causes cognitive decline or imply that all older adults should begin supplementation without receiving medical advice. However, they raise concerns that the current criteria for defining B12 deficiency may be overly simplistic when considering brain health.
For healthcare providers, this study underscores the potential benefits of looking beyond total B12 levels, particularly for older patients presenting neurological symptoms. For patients, it conveys a vital message: a “normal” lab result may not fully capture the situation, especially when subtle alterations in memory, cognitive speed, or visual acuity are already in progress.
Authors: Co-first author is Ahmed Abdelhak, MD, PhD, from the UCSF Department of Neurology and the Weill Institute for Neurosciences.
Funding and Disclosures: Westridge Foundation and the Canadian Institutes of Health Research. There are no conflicts of interest to report.
