1.Lactose intolerance 2.Celiac disease 3.Ulcerative colitis 4.Crohn disease It's an immune reaction to eating gluten, a protein found in wheat, barley and rye. If you have this disease, eating gluten triggers an immune response in your small intestine. Over time, this reaction damages your small intestine's lining and prevents absorption of some nutrients (malabsorption). The intestinal damage often causes diarrhea, fatigue, weight loss, bloating and anemia, and can lead to serious complications.