Additionally, a gluten-free diet may exclude oats. Medical practitioners are divided on whether oats are an allergen to celiac disease sufferers or if they are cross-contaminated in milling facilities by other allergens.

The term gluten-free is generally used to indicate a supposed harmless level of gluten rather than a complete absence. The exact level at which gluten is harmless is uncertain and controversial. A recent systematic review tentatively concluded that consumption of less than 10 mg of gluten per day is unlikely to cause histological abnormalities, although it noted that few reliable studies had been done. Regulation of the label gluten-free varies widely by country. In the United States, the FDA issued regulations in 2007 limiting the use of "gluten-free" in food products to those with less than 20 ppm of gluten. The current international Codex Alimentarius standard allows for 20 ppm of gluten in so-called "gluten-free" foods.

Casein ( /ˈkeɪs.ɪn/ or /ˈkeɪˌsiːn/, from Latin caseus, "cheese") is the name for a family of related phosphoproteins (αS1, αS2, β, κ). These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 20% and 45% of the proteins in human milk.^[1] Casein has a wide variety of uses, from being a major component of cheese, to use as a food additive, to a binder for safety matches.^[2] As a food source, casein supplies amino acids; carbohydrates; and two inorganic elements, calcium and phosphorus.