A gluten-free diet is a diet that excludes gluten, a protein composite found in wheat and related grains, including barley and rye. Gluten causes health problems in sufferers of celiac disease (CD) and some cases of wheat allergy. For those diagnosed with celiac disease, a strict gluten-free diet constitutes the only effective treatment to date. There is ongoing research and debate on non-celiac gluten sensitivity. Some people believe that there are health benefits to gluten-free eating for the general population, but there is no published experimental evidence to support such claims.
Rationale behind adoption of the diet
Celiac disease is an autoimmune disease that attacks the small intestine due to the presence of gluten, for which a gluten-free diet is the only medically-accepted treatment. The disease affects an estimated 1% of adults worldwide and appears to be on the increase, but because of the rare occurrence of symptoms, it is believed only 5-10 percent of cases are diagnosed. The amount of tolerable gluten varies among people with celiac disease. Although there is no evidence to suggest a single definitive threshold, a daily gluten intake of less than 10 mg is unlikely to cause significant histological abnormalities.
Non-celiac gluten sensitivity
Non-celiac gluten sensitivity (NCGS) is described as a condition of multiple symptoms (including neurological and intestinal) that improves when switching to a gluten-free diet, after celiac disease and wheat allergy are excluded. The ingestion of gliadin (a component of gluten) is responsible for symptoms in celiac disease and, at least in some cases, in NCGS. Much recent research on NCGS has aimed at determining which agents trigger a response in NCGS patients: to which extent gluten, FODMAPs, ATIs or other substances are involved. In particular, it has been found that some self-reported NCGS patients did not have a gluten-sensitivity and their symptoms improved on a low FODMAPs diet. Self-reported NCGS individuals would notice a reduction in symptoms by switching to either a gluten-free diet or a low FODMAPs diet, which both eliminate wheat. Consequently, there is a discussion on whether the term should be non-celiac's gluten sensitivity or non-celiac wheat sensitivity. In NCGS, both wheat and gluten may contribute to different symptoms that can occur in the same individuals.
A low-gluten or gluten free diet has also been suggested for athletes such as distance runners, an estimated 20 to 50 percent of whom suffer from gastrointestinal issues, and the risk of inflammation from gluten or wheat. 
As a fad diet
Gluten-free fad diets are popular and endorsed by celebrities such as Miley Cyrus. The book Wheat Belly which refers to wheat as a "chronic poison" became a New York Times bestseller within a month of publication in 2011. People buy gluten-free food "because they think it will help them lose weight, because they seem to feel better or because they mistakenly believe they are sensitive to gluten." However the gluten-free diet is not recommended as a means to eat healthier or to lose weight. Neither should it be undertaken to diagnose one's own symptoms, because tests for celiac disease are reliable only if the patient has been consuming gluten.
Evidence of the diet's efficacy as an autism treatment is poor. Studies, including one by the University of Rochester, found that the popular autism diet does not demonstrate behavioral improvement and fails to show any genuine benefit to children diagnosed with autism who do not also have a known digestive condition which benefits from a gluten-free diet.
The diet includes naturally gluten-free food, such as meat, fish, nuts, legumes, fruit, vegetables, potatoes, pseudocereals (in particular amaranth, buckwheat, chia seed, quinoa), only certain cereal grains (corn, rice, sorghum), minor cereals (including fonio, Job's tears, millet, teff, called "minor" cereals as they are "less common and are only grown in a few small regions of the world"), some other plant products (arrowroot, canary seed (alpiste seed), mesquite flour), and products made from these gluten-free foods, such as breads and gluten-free beer. Gluten-free bread may be less fluffy, so additives are used to compensate, such as corn starch, eggs, xanthum gum, guar gum, and hydroxypropyl methylcellulose.
Some vineyards use flour paste to caulk the oak barrels, but tests have not detected the presence of gluten in the wine. Gluten may be used as a clarifying agent in wine, some of which might remain in the product.
Some cereal grains, although gluten-free in themselves, may contain gluten by cross-contamination during processing steps or transport; this includes oats. Some processed foods may contain gluten, so they would need specific labeling, such as gluten-free ice-cream, ketchup, chicken bouillon, corn cereal, ice cream toppings, malt flavoring, and chocolate. And some non-foodstuffs may contain gluten as an excipient or binding agent, such as medications and vitamin supplements, especially those in tablet form. People with gluten intolerance may require special compounding of their medication.
Unless great care is taken, a gluten-free diet can lack the vitamins, minerals, and fiber which are found in wheat, barley, rye, kamut, and other gluten-containing whole grains and may be too high in fat and calories. Processed gluten-free foods are often higher in salt, sugar, glycemic index, transfats and other processed fats. Although the lack of vitamins, minerals and fiber can be mitigated through the consumption of brown rice and quinoa, many practitioners of the diet do not consume the recommended number of grain servings per day. Many gluten-free products are not fortified or enriched by such nutrients as folate, iron, and fiber as traditional breads and cereals have been during the last century.
Advances towards higher nutrition-content gluten-free bakery products, improved for example in terms of fiber content and glycemic index, have been made by using not exclusively corn starch or other starches to substitute for flour. In this aim, for example the dietary fiber inulin (which acts as a prebiotic) or quinoa or amaranth wholemeal have been as substitute for part of the flour. Such substitution has been found to also yield improved crust and texture of bread.
Regulation and labels
Regulation of the label gluten-free varies by country. Most countries derive key provisions of their gluten-free labeling regulations from the Codex Alimentarius international standards for food labeling has a standard relating to the labeling of products as gluten-free. It only applies to foods that would normally contain gluten. Gluten-free is defined as 20 ppm (= 20 mg/kg) or less. It categorizes gluten-free food as:
- Food that is gluten-free by composition
- Food that has become gluten-free through special processing.
- Reduced gluten content, food which includes food products with between 20 and 100 ppm of gluten. Reduced gluten content is left up to individual nations to more specifically define.
The Codex Standard suggests the enzyme-linked Immunoassay (ELISA) R5 Mendez method for indicating the presence of gluten, but allows for other relevant methods, such as DNA. The Codex Standard specifies that the gluten-free claim must appear in the immediate proximity of the name of the product, to ensure visibility.
There is no general agreement on the analytical method used to measure gluten in ingredients and food products. The ELISA method was designed to detect w-gliadins, but it suffered from the setback that it lacked sensitivity for barley prolamins. 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.
The Australian government recommends that:
- food labeled gluten-free include no detectable gluten (<5ppm ), oats or their products, cereals containing gluten that have been malted or their products
- food labeled low gluten claims such that the level of 20 mg gluten per 100 g of the food
All food products must be clearly labelled whether they contain gluten or they are gluten-free.
Health Canada considers that foods containing levels of gluten not exceeding 20 ppm as a result of contamination, meet the health and safety intent of section B.24.018 of the Food and Drug Regulations when a gluten-free claim is made. Any intentionally added gluten, even at low levels must be declared on the packaging and a gluten-free claim would be considered false and misleading. Labels for all food products sold in Canada must clearly identify the presence of gluten if it is present at a level greater than 10 ppm.
- gluten-free: 20 ppm or less of gluten
- very low gluten foodstuffs: 20-100ppm gluten.
In the United Kingdom, only cereals must be labelled; labelling of other products is voluntary.
Until 2013 anyone could use the gluten-free claim with no repercussion. In 2008, Wellshire Farms chicken nuggets labeled gluten-free were purchased and samples were sent to a food allergy laboratory where they were found to contain gluten. After this was reported in the Chicago Tribune, the products continued to be sold. The manufacturer has since replaced the batter used in its chicken nuggets. The U.S. first addressed gluten-free labeling in the 2004 Food Allergen Labeling and Consumer Protection Act (FALCPA). The FDA issued their Final Rule on August 5, 2013. When a food producer voluntarily chooses to use a gluten-free claim for a product, the food bearing the claim in its labeling may not contain:
- an ingredient that is a gluten-containing grain
- an ingredient that is derived from a gluten-containing grain that has not been processed to remove gluten
- an ingredient that is derived from a gluten-containing grain, that has been processed to remove gluten but results in the presence of 20 ppm or more gluten in the food. Any food product claiming to be gluten-free and also bearing the term "wheat" in its ingredient list or in a separate "Contains wheat" statement, must also include the language "*the wheat has been processed to allow this food to meet the FDA requirements for gluten-free foods," in close proximity to the ingredient statement.
- Any food product that inherently does not contain gluten may use a gluten-free label where any unavoidable presence of gluten in the food bearing the claim in its labeling is below 20 ppm gluten.
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