Vitamin B12 gets a lot of press for its role in making DNA, red blood cells, and healthy nerve tissue. But a new study from UCSF suggests that simply having the government's stamp of approval on your B12 levels might not be enough - especially if you're over 70 and your brain is already quietly screaming for help.
The study, published in Annals of Neurology, found that healthy older adults with lower vitamin B12 levels - even when those levels were technically within the accepted normal range - showed signs of subtle neurological and cognitive problems. In other words, your blood work might give you a thumbs-up while your brain's white matter is slowly waving a white flag.
Researchers enrolled 231 healthy participants through the Brain Aging Network for Cognitive Health (BrANCH) study at UCSF. The participants had an average age of 71, and none had dementia or mild cognitive impairment. Their average blood B12 level was 414.8 pmol/L, a comfortable distance above the U.S. minimum cutoff of 148 pmol/L. But instead of relying only 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 use.
After adjusting for age, sex, education, and cardiovascular risk factors, the team found that participants with lower active B12 had slower processing speed on cognitive tests. The effect was stronger with older age. They also had delayed responses to visual stimuli, pointing to slower visual processing and reduced brain signaling efficiency. MRI scans added another warning sign: participants with lower active B12 had a higher volume of white matter lesions - areas of brain injury that have been linked to cognitive decline, dementia, and stroke risk.
"Previous studies that defined healthy amounts of B12 may have missed subtle functional manifestations of high or low levels that can affect people without causing overt symptoms," said senior author Ari J. Green, MD, of the UCSF Departments of Neurology and Ophthalmology and the Weill Institute for Neurosciences. "Revisiting the definition of B12 deficiency to incorporate functional biomarkers could lead to earlier intervention and prevention of cognitive decline."
Co-first author Alexandra Beaudry-Richard, MSc, currently completing her doctorate at UCSF and the University of Ottawa, said the findings suggest that low but technically normal B12 could have broader effects than previously recognized. "In addition to redefining B12 deficiency, clinicians should consider supplementation in older patients with neurological symptoms even if their levels are within normal limits," she said. "Ultimately, we need to invest in more research about the underlying biology of B12 insufficiency, since it may be a preventable cause of cognitive decline."
Of course, the scientific community hasn't exactly rushed to a consensus. A 2025 comprehensive review concluded that B12 deficiency remains a modifiable risk factor for neurological and cognitive problems, especially in high-risk groups such as older adults and vegetarians. A 2025 systematic review and meta-analysis of randomized trials found that supplementation with B vitamins produced a very small benefit in global cognitive function among older adults - the kind of benefit that sounds promising until you realize it's about as dramatic as a lukewarm cup of tea. Another 2025 study using Mendelian randomization found no clear evidence that genetically higher total serum B12 levels protect the general population from psychiatric disorders or cognitive impairment, though it used total serum B12 rather than the bioactive form.
So what's the takeaway? B12 is clearly essential for the nervous system, and deficiency shouldn't be ignored. But the UCSF findings don't prove that lower active B12 directly causes cognitive decline, and they don't mean every older adult should start popping supplements without medical guidance. They do, however, suggest that the current definition of B12 deficiency may be too blunt for brain health. For clinicians, the study points to the possible value of looking beyond total B12, especially when older patients have neurological symptoms. For patients, it highlights a practical message: a 'normal' lab result may not always tell the full story, particularly when subtle changes in memory, thinking speed, or vision are already appearing.