Current vitamin B12 guidelines may not be sufficient to safeguard the brains of older adults, according to a new study led by the University of California, San Francisco (UCSF). Researchers found that healthy older individuals, even those with B12 levels considered normal, exhibited signs of subtle neurological and cognitive problems.
The study, published inAnnals of Neurology, focused on older adults without dementia or mild cognitive impairment. Lower levels of active B12 in these participants were linked to slower thinking, reduced visual processing speed, and increased white matter injury in the brain. White matter consists of nerve fibers crucial for brain communication.
Revisiting B12 Deficiency Definitions
Ari J.
Green, MD, senior author from UCSF's Departments of Neurology and Ophthalmology and the Weill Institute for Neurosciences, noted the findings raise questions about existing B12 guidelines. The current minimum threshold for deficiency might not detect early functional changes in the nervous system. Green suggested that previous studies defining healthy B12 amounts may have overlooked subtle functional issues that affect individuals without overt symptoms. He emphasized that revisiting the definition of B12 deficiency to include functional biomarkers could lead to earlier intervention and potentially prevent cognitive decline.
Researchers enrolled 231 healthy participants through UCSF's Brain Aging Network for Cognitive Health (BrANCH) study. Their average age was 71. Participants' average blood B12 level was 414.8 pmol/L, significantly above the U.S. minimum cutoff of 148 pmol/L. The team focused on the biologically active form of the vitamin, believing it better reflects the body's usable B12.
Cognitive Impact of Lower B12
After accounting for factors like age, sex, education, and cardiovascular risk, the study found that participants with lower active B12 displayed slower processing speed on cognitive tests. This effect became more pronounced with increasing age. They also showed delayed responses to visual stimuli, indicating slower visual processing and less efficient brain signaling. MRI scans revealed another concern: lower active B12 correlated with a higher volume of white matter lesions, areas of brain injury associated with cognitive decline, dementia, and stroke risk.
Older adults may be particularly vulnerable to lower B12 levels because absorption efficiency can decrease with age.
Certain medications, digestive conditions, and diets low in animal products also elevate the risk of low B12. Co-first author Alexandra Beaudry-Richard, MSc, suggested that low but technically normal B12 levels could have broader cognitive effects than previously understood, potentially impacting a larger segment of the population. She recommended clinicians consider supplementation for older patients with neurological symptoms, even if their B12 levels fall within normal limits.
Subsequent research has added further context. A 2025 review 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 need for improved biomarkers and brain imaging for earlier detection. A 2025 systematic review and meta-analysis of randomized trials found a small benefit in global cognitive function among older adults who supplemented with B vitamins, including B6, B9, or B12. However, a separate 2025 Mendelian randomization study found no clear evidence that genetically higher total serum B12 levels protect the general population from psychiatric disorders or cognitive impairment.
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