Gluten is a small plant protein and is similar to members of the group known as lectins.
Gluten is the specific protein that sufferers of celiac disease must eliminate in order to live.
Celiac disease is a digestive disorder due to severe gluten sensitivity.
Gluten is a protein found in barley, buckwheat, rye, oats, and wheat and many people are allergic to it.
Celiac disease is an auto-immune disease in which the lining of the small intestine becomes damaged when gluten is consumed.
Those fully affected must live their lives in constant fear of gluten. There are many others, who are not quite as sensitive to it. For them they deal with chronic fatigue, joint pain, loss of balance and other neurological or auto-immune disorders. These people may benefit by eliminating gluten consumption. There are other proteins found in grains, rice and legumes. They are called lectins and their removal from the diet is one of the big benefits of the Paleo diet.
Many consumers of gluten-free foods claim to feel better because of this and this may have to do with their inability to completely manage the protein but not be outwardly sick because of it.
Gluten is not found in fruits, nuts, and seeds.
The following is paraphrased from wikipedia as substitute for gluten.
Corn, potatoes, rice, and tapioca are generally considered suitable for gluten-free diets. Other starches include include amaranth, arrowroot, millet, montina, lupin, quinoa, sorghum (jowar), taro, teff, chia seed, and yam. Various types of bean, soybean, and nut flours are sometimes used in gluten-free products to add protein and dietary fiber.
Almond flour is a low-carbohydrate alternative to flour, with a low glycemic index. In spite of its name, buckwheat is not related to wheat; pure buckwheat is considered acceptable for a gluten-free diet, although many commercial buckwheat products are actually mixtures of wheat and buckwheat flours, and thus not acceptable. Gram flour, derived from chickpeas, is also gluten-free.
People wishing to follow a completely gluten free diet must also take into consideration the ingredients of any over-the-counter or prescription medications and vitamins. Also, cosmetics such as lipstick, lip balms, and lip gloss may contain gluten and need to be investigated before use. Glues used on envelopes may also contain gluten.
A gluten-free diet allows for fresh fruits, vegetables, meats and many dairy products. The diet allows rice, corn, soy, potato, tapioca, beans, sorghum, quinoa, millet, buckwheat, arrowroot, amaranth, teff, Montina and nut flours and prohibits the ingestion of wheat, barley, rye and related components, including triticale, durum, graham, kamut, semolina, spelt, malt, malt flavouring or malt vinegar.
In the US, the FDA considers foods containing less than or equal to 20 ppm to be gluten-free, but there is no regulation or law in the US for labeling foods as 'gluten free.' The finding of a current study indicates that some inherently gluten-free grains, seed, and flours not labeled gluten-free are contaminated with gluten. The consumption of these products can lead to inadvertent gluten intake. The use of highly sensitive assays is mandatory to certify gluten-free food products. The European Union, World Health Organization, and Codex Alimentarius require reliable measurement of the wheat prolamins, gliadins rather than all-wheat proteins.
There is still no general agreement on the analytical method used to measure gluten in ingredients and food products. The official limits described in the Codex Draft are 20 ppm for foodstuffs naturally gluten-free and 200 ppm for foodstuffs rendered gluten-free. The ELISA method was designed to detect w-gliadins but it suffered from the setback that it lacked sensitivity for barley prolamins.
Restaurants with gluten free menus obviously recognize the importance of providing such a service, but be aware that the variety of procedures used in kitchens and grills may allow cross contamination to occur. This especially is the case in buffets where utensils are typically used in different food bowls by customers.