Gluten is a natural protein component of the cereals: wheat, rye, oats, barley, spelt, and unripe spelt grain. Rice, maize, millet, buckwheat, quinoa, and amaranth are gluten-free. The last two were cultivated by the Aztecs and Incas.

Gluten is a composite of the proteins, glutenin and prolamin (triggers celiac disease). It is called gliadin in wheat, secalin in rye, hordein in barley, and avenin in oats.

Celiac disease, also known as sprue, causes an intolerance to gluten. The only possible treatment is the avoidance of gluten, through a lifelong gluten-free diet.