Gluten is a group of protein in cereals, especially wheat. Bran, usually referred to as wheat chaff, is what remains after wheat is ground into flour. After the human body ingests the bran shell, it cannot be completely decomposed into amino acids in the intestine, and some small fragments containing several amino acids are retained, which are called "peptides". Polypeptide can trigger the immune response of human body. A few people are allergic to gluten, and these people should choose a gluten-free diet.