Gluten is a protein found naturally in grains that is used in many foods as a stabilizer or thickener. Many vitamin companies also use gluten as a filler. In some people, the gluten molecule appears to trigger a reaction, with symptoms ranging from bloating, gas, and abdominal cramping to joint pain and "brain fog." Gluten is never used as a filler in Metagenics products, most of which are gluten-free.