As surprising as it might sound, "gluten" is not any specific substance. This word is widely misused, and a correct understanding of "gluten" is important for sorting out different types of unwanted reactions to grains.
Gluten is quite literally the Latin word for glue. This word origin makes good sense because gluten is associated with the stickiness of dough as well as its sturdy and malleable elastic structure. Gluten helps bread hold together.
While wheat flour contains a small amount of fat (approximately 4-7% of its total calories), the vast majority of its macronutrients consist of carbohydrates (80-85%), followed by proteins (12-15%). When water is added to wheat flour, the mixture can be kneaded into dough. During the course of kneading, a round ball of wheat dough can be created. If this dough ball is rinsed under running water (or kneaded while being placed in a bowl of water), many—but not all—of the starches can get washed away. What remains is what bakers originally referred to as "gluten." In this practical dough-making and rinsing context, gluten was most valued for its protein content because the proteins in gluten were the key to good structure in bread baking. However, the rinsed dough ball referred to as "gluten" was also known to contain small amounts of fats and carbohydrates, as well as both vitamins and minerals. In today's marketplace, however the term "gluten" is seldom used to refer to this rinsed ball of wheat dough. Most of the references to "gluten" that you will see on the Internet or in research studies refer only to the proteins found in gluten.
About 80-85% of the proteins present in a wheat dough ball consist of glutenins and gliadins. These two families of proteins are collectively referred to as "gluten proteins." It is possible to experience an adverse reaction to many specific gliadin proteins and to some glutenin proteins as well. All of these reactions are typically referred to as reactions to "gluten," even though the reactions can take place in different ways.
While once extremely popular, the term "gluten grains" is being used less and less by researchers since grains differ in their glutenin and gliadin composition. These differences include variation in the total amount of glutenin and gliadin proteins found in various grains as well as differing amounts of specific individual glutenins and gliadins known to occur in various grains. Wheat, rye, barley, and spelt (or crossbred hybrids from these four grains, for example, triticale made from wheat and rye) are all considered gluten grains for the purpose of food labeling by the U.S. Food and Drug Administration (FDA). Some researchers consider wheat to be the only true gluten grain since wheat contains its own unique mixture of glutenin and gliadin proteins.
Oats are somewhat controversial in this context. Rather than containing glutenin and gliadin proteins, oats contain proteins called avenins. For this reason, oats are correctly described as a gluten-free grain. However, when the avenin proteins in oats get broken down into smaller parts (called peptides), these smaller molecules can sometimes be quite similar to the peptide breakdown products from wheat and may sometimes trigger unwanted reactions. Research findings, however, show that the majority of persons with adverse reactions to wheat (even individuals who have been diagnosed with celiac disease) can include oats in their meal plan without experiencing an adverse reaction. One additional caveat here: for wheat-sensitive persons, the purchase of oats from a reliable manufacturer is important since oats can become contaminated with wheat residues when processed on the same machinery.