Evidence check: LCT in the context of fatigue

The link between LCT variants (lactase persistence) and fatigue is indirect and weakly supported by evidence. LCT gene variants determine whether adults produce lactase and can digest lactose. In lactose intolerance, consuming dairy can cause bloating, diarrhea, and abdominal pain – these symptoms may secondarily lead to fatigue due to nutrient malabsorption or disrupted sleep. However, there is no strong evidence that LCT variants directly cause fatigue. Most studies are mechanistic or observational, without clear clinical actionability. A genetic test for LCT is useful for diagnosing lactose intolerance, but insufficient to explain fatigue alone. Other causes like iron deficiency, thyroid issues, or sleep disorders should be investigated first. Evidence: human-moderate (for lactose intolerance), but inadequate for fatigue as a primary symptom.