People who have celiac disease have a disorder that makes their bodies react to gluten. The precise cause of celiac disease isn't known. When they eat gluten, an immune system reaction to the protein gradually damages the villi in the small intestine. When the villi are damaged, the body can't absorb the vitamins, minerals, and other nutrients. People with celiac disease are therefore at risk of malnutrition and can develop anaemia or osteoporosis.