A celiac patient has chronic diarrhea and iron deficiency. In celiac disease, gluten triggers immune destruction of jejunal villi — the absorptive surface flattens, so nutrients (iron, vitamins, calories) pass unabsorbed: diarrhea, weight loss, anemia. Blood antibodies (tTG) diagnose it; a strict lifelong gluten-free diet lets villi regrow and absorption recover. The villi are the lesson — absorption needs surface, and surface lost is nutrition lost.