Scientists found that the vitamin B12 levels doctors call normal are quietly damaging the aging brain
Your doctor looked at your B12 level, saw a number above the cutoff, and told you everything was fine. A study from UCSF published in the Annals of Neurology is now saying that conversation may have missed something important.
Researchers enrolled 231 healthy older adults with an average age of 71, none of whom had dementia or mild cognitive impairment. Their average blood B12 level was 414.8 pmol/L, nearly three times the US minimum cutoff of 148 pmol/L. By every standard clinical measure, this was a healthy group with normal B12 levels. What the researchers found when they looked more carefully is that within that normal range, the people with lower B12 were already showing measurable brain damage and cognitive slowing compared to those with higher levels. The clinical threshold that defines deficiency was not protecting them. It was just not flagging the problem.
The Number Your Doctor Checks Is the Wrong Number
The first thing to understand about B12 testing is that the standard blood test does not measure what the brain actually uses. Total serum B12 measures both the active and inactive fractions of the vitamin together, but only the active fraction, called holotranscobalamin, can actually be used by the body’s cells. The inactive fraction, which makes up the majority of what circulates in the blood, is bound to a protein called haptocorrin and cannot be delivered to tissues that need it.
Most B12 tests measure total serum B12. This means a person can have a perfectly normal total B12 reading while their active B12, the fraction that actually reaches the brain and keeps neurons functioning, is quietly running low. Ari Green, the senior author of the study and chief of neuroimmunology at UCSF, said that the effects were strongest when looking at the active fraction of B12, which is not routinely checked, rather than just total blood B12 levels.
This distinction matters because the clinical cutoff for B12 deficiency was established using total serum B12, not active B12. The threshold that determines whether a doctor intervenes or reassures was built on a measurement that is not capturing what the brain actually has access to.
What the Brain Scans Showed
The UCSF team measured neurological status using three separate methods: multifocal visual evoked potential testing to assess how quickly the visual system transmits signals, processing speed assessments through cognitive testing, and MRI to examine the structural state of the brain’s white matter.
Even in this relatively healthy group, lower levels of active B12 were linked to slower thinking, slower visual processing, and more visible injury in the brain’s white matter. White matter is the network of nerve fibers that allows different brain regions to communicate with each other. Damage to white matter creates communication bottlenecks that show up as slower processing speeds, delayed responses, and eventually more significant cognitive impairment as the damage accumulates.
The relationship between lower active B12 and these neurological signs was present across participants whose B12 levels were all considered normal by standard clinical definitions. The researchers were not comparing B12-deficient people to healthy people. They were looking at gradations within the normal range and finding that those gradations carried measurable brain consequences.
There was also a compounding effect with age. The cognitive impact of lower active B12 was amplified by older age, meaning the same level of B12 that produces minimal effects in a 65-year-old produces more significant effects in a 75-year-old. As the brain ages, its tolerance for nutritional insufficiency narrows, and the margin between adequate and inadequate B12 shrinks in ways the current clinical framework does not account for.
Why the Cutoff Was Set Too Low
The current US minimum cutoff of 148 pmol/L for total serum B12 was established primarily to catch cases of megaloblastic anemia and severe neurological symptoms, the most obvious and dramatic manifestations of B12 deficiency. It was not designed to identify the subtler, slower neurological deterioration that the UCSF study is now documenting.
“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 Green. The cutoff was calibrated for a different clinical question than the one the brain research is now asking. It answers the question of when B12 is low enough to cause obvious anemia. It does not answer the question of when B12 is low enough to cause measurable cognitive slowing and white matter injury in the absence of any symptoms a patient would notice or report.
The practical consequence is that a significant number of older adults are being told their B12 is fine when their brain is already registering the insufficiency through slower signal transmission, reduced processing speed, and accumulating white matter damage. The gap between what the test confirms and what the brain is experiencing can be years wide.
The Groups Most at Risk
B12 absorption is heavily dependent on a protein called intrinsic factor, which is produced by the stomach lining and required for B12 to be absorbed in the small intestine. As people age, stomach acid production declines and intrinsic factor availability decreases, making B12 absorption progressively less efficient regardless of dietary intake. B12 deficiency affects around 6% of adults under 60 and approximately 20% of adults over 60, but these figures are based on the total serum B12 cutoff that the UCSF research suggests is set too low. The proportion of older adults with functionally insufficient active B12 is likely higher.
People taking metformin for type 2 diabetes face an additional risk, as the medication is known to reduce B12 absorption over time. People following vegan or vegetarian diets, where B12 comes almost exclusively from fortified foods rather than natural animal sources, face consistent dietary risk that may not translate into dramatically low total serum B12 numbers but could still manifest as reduced active B12 availability.
People over 50 who take proton pump inhibitors for acid reflux face a third pathway. These medications reduce stomach acid, which impairs the release of B12 from food, making dietary B12 less bioavailable regardless of how much meat, eggs, or dairy a person consumes.
What the Researchers Say Should Change
The UCSF team made their clinical recommendations explicit. “Revisiting the definition of B12 deficiency to incorporate functional biomarkers could lead to earlier intervention and prevention of cognitive decline.”
Specifically, they suggest that testing active B12, or holotranscobalamin, alongside or instead of total serum B12 would give clinicians a more accurate picture of what the brain actually has access to. They also recommend that clinicians consider B12 supplementation in older patients with neurological symptoms even when their total serum B12 levels appear normal, because the normal label may be masking a functionally relevant insufficiency.
Co-first author Alexandra Beaudry-Richard said that lower B12 levels 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 research does not establish that raising B12 above the lower end of the normal range will reverse existing cognitive slowing or repair white matter damage that has already occurred. That question requires longitudinal intervention trials that have not yet been completed. What it does establish is that the current threshold used to reassure patients is not the threshold at which the brain stops being affected.
For older adults who have been told their B12 is fine, the UCSF findings are a reason to ask a more specific question: fine by which measure, and fine for what?
Source:
Abdelhak, A., Beaudry-Richard, A., Green, A.J., et al. Vitamin B12 Levels Association with Functional and Structural Biomarkers of Central Nervous System Injury in Older Adults. Annals of Neurology, 2025; 97(6): 1190. DOI: 10.1002/ana.27200 https://www.ucsf.edu/news/2025/02/429491/healthy-vitamin-b12-levels-not-enough-ward-neuro-decline
https://onlinelibrary.wiley.com/doi/full/10.1002/ana.27200
