OBJECTIVE: Vitamin B12 (B12) plays a critical role in fatty- and amino-acid metabolism and nucleotide synthesis. While the association between B12 deficiency and neurological dysfunction is well-known, the exact threshold for adequacy remains undefined in terms of functional impairment and evidence of injury. The objective was to assess whether B12 levels within the current normal range in a cohort of healthy older adults may be associated with measurable evidence of neurological injury or dysfunction. METHODS: We enrolled 231 healthy elderly volunteers (median age 71.2 years old) with a median B12 blood concentration of 414.8 pmol/L (as measured by automated chemiluminescence assay). We performed multifocal visual evoked potential testing, processing speed testing, and magnetic resonance imaging to assess neurological status. Moreover, we measured serum biomarkers of neuroaxonal injury, astrocyte involvement, and amyloid pathology. RESULTS: Low (log-transformed) B12, especially decreased holo-transcobalamin, was associated with visual evoked potential latency delay (estimate = -0.04
p = 0.023), processing speed impairment (in an age-dependent manner
standardized β = -2.39
p = 0.006), and larger volumes of white matter hyperintensities on MRI (β = -0.21
p = 0.039). Remarkably, high levels of holo-haptocorrin (biologically inactive fraction of B12) correlated with serum levels of Tau, a biomarker of neurodegeneration (β = 0.22, p = 0.015). INTERPRETATION: Healthy older subjects exhibit neurological changes at both ends of the measurable "normal" B12 spectrum. These findings challenge our current understanding of optimal serum B12 levels and suggest revisiting how we establish appropriate nutritional recommendations. ANN NEUROL 2025.