Scientists have long known that severe B12 deficiency can cause neurological problems, but the exact threshold for deficiency-related damage remains unclear. A recent study found that older adults with B12 levels in the lower end of the normal range showed signs of neurological dysfunction.

Researchers measured B12 levels and conducted brain scans on 231 healthy older adults. Participants underwent tests to evaluate brain function, including visual processing speed and cognitive performance. The researchers also assessed blood biomarkers associated with nerve damage and brain health.

They found that lower B12 levels, particularly the active form of the vitamin, were associated with slower visual processing, cognitive decline, and increased signs of white matter damage in the brain. Surprisingly, high levels of the inactive form of B12 correlated with increased tau protein, a marker of neurodegeneration.

These findings suggest that current B12 guidelines don’t fully capture what the brain needs to function correctly and that even “normal” B12 levels could contribute to neurological changes. They also highlight the role of adequate nutrition throughout the lifespan and support the “micronutrient triage theory"—the idea that the body prioritizes micronutrient utilization for survival over those used for long-term health. Learn more about micronutrient triage theory in this episode featuring Dr. Bruce Ames.