Celiac disease is an autoimmune condition in which eating gluten (a type of protein present in whole grains) causes the immune system to attack the cells of the small intestine. It makes it hard for those dealing with this condition to absorb the food completely, as a result, they lose weight or usually remain thin. Other symptoms of this condition include bloating, chronic diarrhea, constipation, nausea and vomiting, and abdominal pain. Avoiding gluten can help maintain a healthy weight.