Gluten is a natural protein that occurs in wheat/barley/rye flour, and can also be found in some cosmetic products. It gives elasticity to dough and creates a chewy texture for bread. Some people have an intolerance to gluten, particularly those with a what allergy or those suffering from coeliac disease because the immune system has an adverse reaction to it. A gluten intolerance can cause bloating, constipation, diarrhea, and joint pain.