Evidence check: FUT2 in the context of fatigue

The FUT2 variant (e.g., rs601338) affects ABH antigen secretion and is associated in GWAS with lower vitamin B12 levels. B12 deficiency can cause fatigue, but the effect of FUT2 alone is small and not directly tested for fatigue. Evidence is indirect: FUT2 explains only part of B12 variance, and B12 deficiency has many causes (diet, absorption, medications). A genetic test does not provide a clinical diagnosis; for fatigue, standard medical workup (CBC, B12, ferritin, thyroid) should take priority. Supplementation without confirmed deficiency is not recommended. Caveat: most studies are European; transferability to other ethnicities unclear. Evidence grade: human-moderate (GWAS for B12, but no direct fatigue study).