A gluten-free diet excludes the protein gluten, found in grains such as wheat, barley and rye. A 100% gluten-free lifestyle is considered absolutely necessary for people with gluten intolerance, also known as Celiac Disease, because they suffer an immune reaction every time they eat foods containing gluten; if they continue to consume foods containing gluten, studies have shown their chance of gastrointestinal cancer may be elevated 40 to 100 times compared to the general population.