Myths vs facts: LCT and fatigue

The claim that LCT gene variants (lactase persistence/non-persistence) directly cause fatigue is a myth. LCT variants affect lactase production and lactose digestion. Lactose intolerance typically causes gastrointestinal symptoms like bloating, diarrhea, and cramps, not systemic fatigue. Some individuals report tiredness after consuming dairy, but this is likely due to malabsorption, disrupted sleep from discomfort, or a nocebo effect. Evidence for a direct causal link between LCT genotype and fatigue is weak (mechanistic/observational). Commercial DNA tests that market fatigue as a consequence of lactose intolerance often overstate the science. If you experience persistent fatigue, consult a physician to rule out common causes like iron deficiency, thyroid issues, or poor sleep. Safety note: For suspected lactose intolerance, a hydrogen breath test or elimination diet is more clinically useful than genetic testing alone.