How are FUT2 and Müdigkeit related?

The relationship between FUT2 and fatigue is indirect, primarily mediated through vitamin B12 metabolism. FUT2 (fucosyltransferase 2) controls the secretion of blood group antigens in body fluids. Certain variants (e.g., 'non-secretor' status) are associated in GWAS with lower vitamin B12 levels. B12 deficiency can cause fatigue, weakness, and neurological symptoms. However, the effect is modest and modulated by diet, gut microbiome, and other genes. Evidence comes from population genetic studies (GWAS), not from randomized controlled trials directly linking FUT2 to fatigue. For individual diagnostics, the utility is limited because fatigue is multifactorial. A blood test for B12 is more informative than a genetic test alone. Caveat: No clinical recommendation without medical evaluation.