At UCSF, researchers have uncovered a subtle but troubling disconnect: healthy older adults with vitamin B12 levels that doctors would call "normal" are already showing signs of cognitive decline and brain injury. The discovery, published in the Annals of Neurology, suggests that our current understanding of what constitutes adequate B12 may be dangerously incomplete.
The study matters because vitamin B12 is essential for building nerve tissue and maintaining brain function, yet millions of older adults may be walking around with what appears to be sufficient B12 while their brains are quietly deteriorating. In a time when dementia prevention has become a public health priority, this research hints at a preventable risk factor that current guidelines may be overlooking entirely.
Led by Dr. Ari J. Green of the UCSF Departments of Neurology and Ophthalmology and the Weill Institute for Neurosciences, the team enrolled 231 cognitively healthy participants through the Brain Aging Network for Cognitive Health study, with an average age of 71. Rather than relying on total B12 measurements, the researchers focused on biologically active B12—the form the body can actually use. The participants' average blood B12 level was 414.8 pmol/L, nearly three times higher than the U.S. minimum cutoff of 148 pmol/L. Yet even among this group with seemingly adequate levels, those with lower active B12 showed measurable problems.
The cognitive tests revealed slower processing speed, and the effect grew more pronounced with age. Participants with lower active B12 also had delayed responses to visual stimuli, suggesting their brains were working less efficiently at fundamental tasks. But the most striking finding came from MRI scans. Those with lower active B12 had a higher volume of white matter lesions—areas of brain injury linked to cognitive decline, dementia, and stroke risk. White matter consists of the nerve fibers that allow different brain regions to communicate, making these lesions particularly concerning.
Why older adults are especially vulnerable comes down to biology. Absorption of B12 becomes less efficient with age, and certain medications and digestive conditions compound the problem. Even diet plays a role: people eating fewer animal-based foods are at higher risk. Co-first author Alexandra Beaudry-Richard, a doctoral student at UCSF, emphasized that the implications extend far beyond a small at-risk group. Low but technically normal B12 could "impact cognition to a greater extent than what we previously thought, and may affect a much larger proportion of the population than we realize."
The findings raise a specific call to action. Green and his team argue that current B12 guidelines should be revisited to incorporate what they call "functional biomarkers"—measurable signs that B12 levels are affecting brain health, not just abstract numbers. Beaudry-Richard went further, suggesting clinicians should consider B12 supplementation in older patients with neurological symptoms even if their B12 levels fall within normal ranges. Recent reviews published in 2025 have confirmed that B12 deficiency remains a modifiable risk factor for cognitive problems, particularly in older adults and vegetarians, though research on supplementation effects shows mixed results.
The path forward isn't about panic or mass supplementation, but rather about precision. Investing in better biomarkers and earlier detection through brain imaging could identify people at risk before damage accumulates. For older adults concerned about their cognitive health, this research suggests it's worth asking doctors not just whether B12 is "normal," but whether it's optimal.