Normal B12 levels may still harm aging brains
Meeting current vitamin B12 guidelines might not be enough to protect the aging brain. Researchers at UC San Francisco found measurable cognitive decline in older adults whose B12 levels were considered normal.
Participants with lower but technically normal levels of active B12 processed visual information more slowly. They also showed more damage to white matter, the network of fiber pathways that connects brain regions. These changes appeared even without a clinical deficiency diagnosis.
Vitamin B12 supports the production of myelin, the protective coating around nerve fibers. Without sufficient B12, those fibers deteriorate. The result is slower processing and poorer cognitive performance, detectable even when standard tests show no deficiency.
Active B12 matters more than total B12
A key finding in the study is the distinction between total B12 and active B12 in the blood. The total includes a bound form that the body cannot use. The active fraction, called holotranscobalamin, reflects what is actually available to cells. Standard blood tests typically measure total B12, meaning functional shortfalls often go undetected.
Older adults face heightened risk. B12 absorption declines with age, partly because the stomach produces less acid. Vegetarians and vegans are also vulnerable, since B12 is found almost exclusively in animal products.
A case for revised guidelines
The researchers call for higher recommended daily intakes and routine testing of active B12 in older adults. Current guidelines were largely set to prevent anemia, not to protect brain function specifically. These thresholds do not appear to be the same.
Whether raising B12 intake in people with low-normal levels actually prevents or reverses brain damage still needs to be shown in intervention trials. But the findings already provide grounds for reconsidering existing cut-off values, especially for aging populations.