If you've noticed that you react badly to gluten -- with symptoms like diarrhea, stomach upset, abdominal pain, and bloating -- see a doctor to get tested for celiac disease. Do that before you start any gluten-free diet. If you have celiac disease and you eat gluten, the lining of your small intestine becomes inflamed and gets damaged, making it harder for your body to absorb nutrients. That can lead to malnutrition and weight loss. Getting diagnosed involves taking a blood test.