Evidence check: B12 in the context of fatigue

The question about vitamin B12 in the context of fatigue is clinically relevant, but the provided context contains no specific information on B12 or its biomarkers. B12 deficiency can indeed cause fatigue, weakness, and neurological symptoms. However, direct-to-consumer (DTC) DNA tests that analyze genetic variants such as MTHFR, FUT2, or TCN1 to assess B12 risk have limited scientific support. Most of these variants have small effect sizes and are overshadowed by dietary intake, absorption, and other factors. A blood test for holotranscobalamin (active B12) or methylmalonic acid (MMA) is clinically more informative. Without concrete lab values or medical evaluation, the evidence for genetically driven B12-related fatigue remains unclear. Consumers should not rely solely on DNA results; persistent fatigue warrants a doctor's visit and targeted blood work.