Vitamin B12 is an important nutrient that helps the body make red blood cells, maintain healthy nerves, and produce DNA.
Doctors have long used blood tests to check whether people have enough of this vitamin. If the test result falls above a certain level, patients are usually told that their vitamin B12 status is normal.
But a new study suggests the story may be more complicated, especially for older adults.
Researchers at the University of California, San Francisco found that some older people with vitamin B12 levels still considered “normal” may already be showing signs of brain and nerve problems.
The findings raise concerns that current medical guidelines may miss subtle warning signs linked to declining brain health.
The study was published in the journal Annals of Neurology.
Vitamin B12 is found mainly in animal-based foods such as meat, fish, eggs, and dairy products. Some people also get it through supplements or fortified foods. The body needs vitamin B12 to keep nerves working properly and to support healthy brain function.
Severe vitamin B12 deficiency is already known to cause serious health problems. People with very low levels can develop anemia, nerve damage, numbness, memory issues, balance problems, and fatigue. Doctors have traditionally focused on identifying these more advanced cases.
However, the new research suggests that problems may begin earlier than previously thought.
The UCSF research team studied 231 healthy older adults with an average age of 71. None of the participants had dementia or mild cognitive impairment at the beginning of the study.
The researchers looked not only at total vitamin B12 levels but also at the “active” form of vitamin B12, which may better reflect how much of the nutrient the body can actually use.
Interestingly, the participants’ average vitamin B12 levels were far above the minimum cutoff currently used in the United States to define deficiency. By current medical standards, most participants would be considered healthy.
Despite this, the researchers discovered important differences among participants with lower active B12 levels.
People with lower active vitamin B12 performed more slowly on tests measuring thinking speed and visual processing. Their brains also reacted more slowly to visual signals, suggesting reduced communication efficiency in the nervous system.
Brain scans added another important finding. Participants with lower active B12 had more white matter lesions in the brain. White matter helps different regions of the brain communicate with each other. Damage in these areas has been linked to stroke, dementia, and cognitive decline.
The effects appeared stronger as people became older.
Senior author Dr. Ari J. Green from the UCSF Departments of Neurology and Ophthalmology said the findings suggest that current definitions of vitamin B12 deficiency may not fully capture early changes affecting the brain and nervous system.
According to the researchers, some people may experience subtle neurological stress long before obvious symptoms appear.
This may be especially important for older adults because the body often becomes less efficient at absorbing vitamin B12 with age. Certain medications, digestive disorders, and diets low in animal products may also increase the risk of lower vitamin B12 levels.
The researchers believe doctors may eventually need to look beyond standard vitamin B12 blood tests. Measuring the biologically active form of the vitamin and using other functional markers may help identify people at risk earlier.
Co-first author Alexandra Beaudry-Richard said clinicians may want to consider vitamin B12 supplementation in some older adults with neurological symptoms even when standard test results appear normal.
At the same time, the researchers stressed that the study does not prove lower active B12 directly causes cognitive decline. The findings only show an association between lower active B12 and signs of brain changes.
Recent studies have added more context to the discussion. A large 2025 review concluded that vitamin B12 deficiency remains an important and potentially preventable risk factor for cognitive and neurological problems, especially among older adults and vegetarians.
Another 2025 analysis found that supplements containing B vitamins produced only very small improvements in overall cognitive function among older adults. This suggests that simply taking large amounts of vitamin B12 may not dramatically improve brain health for everyone.
Researchers also noted that one recent genetic study found no clear evidence that higher total blood vitamin B12 protects the general population from cognitive decline or psychiatric disorders. However, that study focused on total B12 rather than the active form examined in the UCSF research.
Together, these findings suggest that the issue may not be as simple as “more vitamin B12 is always better.” Instead, scientists are becoming increasingly interested in how the body processes and uses vitamin B12, especially in aging brains.
The UCSF study highlights an important possibility: some older adults may have “normal” blood test results while subtle brain changes are already occurring in the background.
From a practical point of view, the study may encourage doctors to pay closer attention when older patients report mild memory problems, slower thinking, balance issues, or changes in vision, even if standard vitamin B12 tests appear acceptable.
The research also reminds people that healthy aging involves many interconnected factors, including nutrition, brain health, nerve function, exercise, sleep, and overall medical care.
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