People with celiac disease cannot tolerate a protein called gluten, which is found in wheat, rye, and barley. The disease damages the small intestine and interferes with absorption of nutrients from food. When a person with celiac disease consumes gluten, he or she may experience a wide range of symptoms in the digestive system or in other parts of the body, such as diarrhea, abdominal pain (or stomach pain), irritability, and depression. The disease is treated by eliminating all gluten from the diet.