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An adult gets bloating and diarrhea after drinking milk. This is lactose intolerance — the intestinal enzyme lactase (which splits lactose into glucose + galactose) declines after childhood in most of the world. Undigested lactose reaches the colon, where bacteria ferment it (gas, bloating) and its osmotic pull draws water (diarrhea). Lactose-free milk (pre-digested) or fermented products (yogurt — bacteria already ate the lactose) are tolerated. It is enzyme deficiency, not milk allergy — the immune system is uninvolved.