Is MTHFR useful for weight loss?

The question of whether MTHFR is useful for weight loss is often promoted by direct-to-consumer genetic tests, but the scientific evidence is weak. MTHFR (methylenetetrahydrofolate reductase) is an enzyme involved in folate metabolism. Certain variants (e.g., C677T) can slightly reduce enzyme activity, potentially raising homocysteine levels. Some marketers claim these variants slow metabolism and hinder weight loss. However, large meta-analyses show no consistent association between MTHFR polymorphisms and body weight or fat mass. Any effects are very small and clinically irrelevant. Weight loss is primarily determined by calorie balance, physical activity, and behavior. In Germany, MTHFR testing is regulated under the Genetic Diagnostics Act (GenDG) and requires physician involvement for medical interpretation. Supplementation with methylfolate is indicated only for confirmed deficiency, not for weight loss. The evidence is largely marketing-driven. Consumers should not rely on such genetic tests for weight management.