How are LCT and Müdigkeit related?

The relationship between the LCT gene and fatigue is indirect and based on mechanistic reasoning. The LCT gene encodes lactase, the enzyme needed to digest lactose (milk sugar). Variants such as rs4988235 determine whether a person continues to produce lactase after childhood (lactase persistence) or not (lactase non-persistence). In lactase non-persistence, consuming dairy leads to undigested lactose in the gut, which is fermented by bacteria, causing bloating, diarrhea, and abdominal pain. These gastrointestinal symptoms can lead to discomfort and subsequently fatigue, especially if chronic or if they impair nutrient absorption. However, there is no strong clinical evidence directly linking LCT variants to fatigue. Fatigue is a non-specific symptom with many causes. Lactase non-persistence may also reduce calcium and vitamin D intake, indirectly contributing to fatigue. The evidence is therefore mechanistic. A DNA test can identify the variant, but dietary changes (low-lactose) should only be made if symptoms are present. Caveat: Not all fatigue is due to lactose intolerance.