Gluten-free refers to a diet that excludes sources of gluten, a protein found in wheat, barley, spelt, kamut, rye and derivatives thereof such as flour, malt extract, wheat starch etc. Gluten gives bread its elasticity and shape, and is used as a stabilizing agent in other foods.
The gluten-free diet can be adopted as a treatment for legitimate medical conditions, most notably celiac disease, but it has also been embraced as a fad diet, weight-loss gimmick, and purported autism cure, mostly by people who don't actually know what gluten is and can't define it when asked. 
 Reasons for adoption
While the diet is used to treat celiac disease and related disorders, some people adopt a gluten-free diet for a different imagined health benefit. Because of the constraints the diet imposes, doing so may be detrimental to their health.
 Celiac disease
Celiac disease (or coeliac outside of the US) is an autoimmune disorder which interferes with digestion. The immune system responds to gluten peptides absorbed in the small intestine, causing inflammation and damage to the gut and a variety of unpleasant symptoms. There are also long-term effects primarily related to malabsorption, including failure to thrive (in children), chronic diarrhoea and digestive problems, osteoporosis, anaemia (generally iron, vitamin D and folic acid), a characteristic form of blistering eczema and a small increase in risk of cancer. Even a small amount of gluten (e.g. a small crumb or dusting of flour) can trigger an attack. Symptoms of being "glutened" range from person to person but typically include lethargy, nausea, vomiting, and diarrhea.
The condition has been recognized since at least the first century CE, and was described by Aretaeus of Cappadocia ("celiac" is of ancient Greek origin). Greater understanding and research has only occurred more recently; the link with wheat was not definitively made until 1940. Like autism, this has led some people to think that celiac is a modern disease. The reality is that even ancient forms of wheat such as spelt and kamut trigger an attack as easily as any modern wheat variety. It is more likely that better recognition of the symptoms accounts for the higher rates of diagnosis. It is still widely considered to be under-diagnosed with an estimated prevalence in the population as high as one percent.
It is not possible to self-diagnose celiac disease, not least because the symptoms are generally indistinguishable from conditions such as irritable bowel syndrome (irritable bowel syndrome). Diagnosis normally consists of a blood test followed by a duodenal biopsy that examines the intestine lining for villous atrophy (erosion of the villi, structures which increase the surface area of the gut wall to improve absorption). In extreme cases the gut wall may become completely smooth. The blood test is very accurate with a false negative rate of less than 5% and a much lower false positive rate, leading some specialists to dispute the need for a (mildly distressing) biopsy in clear-cut cases.
The only current proven treatment is a completely gluten-free diet which allows the intestine to heal, absorb nutrients and function normally. (While the intestine is healing, the patient might also need to avoid dairy products, as the damaged intestines might be unable to cope with milk proteins; but most of the time dairy can be resumed afterward.) However, there are some early vaccine trials that are attempting to desensitize the body's immune response to gluten peptides. There has also been some preliminary evidence that Helminthic therapy might be an effective treatment.
 Wheat allergy or gluten sensitivity
Aside from celiac disease, many people have a less-intense but still real (maybe) sensitivity to gluten. Levels of sensitivity can vary from person to person. Still other people have a wheat allergy, which is not the same as celiac disease or gluten sensitivity. Because of the lifestyle changes a gluten-free diet imposes, and because advanced cases of celiac sometimes need additional immunosuppressant or steroid treatments, it's important to be examined by a competent medical professional when someone has concerns about their intake of gluten.
Not all sensitivites to gluten are real, though. What may actually be going on are so-called FODMAPs (Fermentable Oligo-Di-Monosaccharides and Polyols), short-chain carbohydrates akin to the ones in beans that are digested by gut bacteria and cause flatulence; although they generally aren't terribly digestible, some people react to the fermentation products worse than others, producing gastrointestinal symptoms akin to irritable bowel syndrome. This however is not nearly as damaging as celiac.
An "elimination" low-FODMAP diet, includes no wheat, so that could explain why many people mistakenly think that they are gluten-intolerant when in fact it has nothing to do with gluten and everything to do with FODMAPs.
Interestingly, one component of FODMAPs, polyols, can be properly digested by 0% of the human population — so in a sense, everyone is FODMAP-intolerant to a greater or lesser degree.
A correlation exists between patients suffering from autoimmune hypothyroidism and celiac disease. This association is on the order of a tenfold increase in the odds of having celiac disease if you have Hashimoto's thyroiditis, as compared with the incidence of celiac disease in the general population. One possible mechanism is a similarity between gliadin, a protein component of gluten, and one or more thyroid tissues.
This, of course, leads to inevitable overreaction, such as claiming that all hypothyroid patients should go gluten-free, whether they've been tested for celiac disease or not.
 Autism quackery
Gluten-free diets are sometimes promoted by anti-vaccination types such as Jenny McCarthy as a means to treat or "cure" autism. There is no known basis for this assertion, and in fact a gluten-free diet can make you worse off if you aren't somehow making up for lost nutrients.
A study presented at the International Meeting for Autism Research showed that 18% of children with autism, Asperger disorder, or PDD-NOS had been put on a "special diet" advocated by alternative medicine, and among those, 47-50% had been put on a gluten-free diet. Families discontinued the use of special diets at a rate of only 5-10% per year, indicating a strong adherence to the idea despite a lack of results.
 Weight loss quackery
Diet woo for weight loss has jumped on this one! Here is the reason you're fat: it's gluten. Few medical experts or professional dieticians have ever found any reason to suggest a gluten-free diet aids in weight loss, but seemingly it fits so well into the "low-carb" "paleo diet" world. Early in the trend, going "gluten-free" was likely to lead to weight loss, because it necessarily cut out a number of foods which are calorie dense, such as pastas, breads, and other baked goods. However, since the trend has grown, it is now easy to find gluten-free varieties of these items, as well as a large number of gluten-free junk food items, thus making it just as easy to be obese without gluten as it is with.
 Athletics quackery
A 2015 study of 1,000 competitive athletes in Australia found that 41% of those surveyed were following a gluten-free diet. The prevailing belief was that eating gluten-free would reduce the symptoms of digestive disease, which affects athletes disproportionately. A follow-up study in which athletes followed a gluten-free diet combined with either a gluten-free or glutinous "energy bar" found that the gluten-free diet had no effect on their overall health whatsoever.
A number of gluten-free advocates also buy into GMO hysteria and claim that the presence of gluten in food is essentially food adulteration for profit; there are numerous related memes among such people, including that GMO wheat has increased gluten (there is no GMO wheat on the market as of 2014), or that gluten is an expendable filler, or such things. Monsanto pops up a lot in the discussion, as do various aspects of the paleo diet and the "golden age" thinking that implies.
 What's the harm in a fad diet?
A danger to genuine celiacs is that if gluten-free is perceived as a "lifestyle" choice then many food preparers may not educate themselves or treat the condition with the seriousness that it deserves, e.g. not bothering to prepare food separate from sources of contamination that won't harm people on some GF fad diet but can cause serious harm to a celiac sufferer. Because of this, gluten-free labeling is regulated in many parts of the world. In addition if the fad implodes as it did with the Atkins diet, then genuine sufferers are back to square one and left to deal with the misinformation fallout.
 The gluten-free lifestyle
The gluten-free diet is moderately tiresome to stick with and unless you are medically required to you probably shouldn't bother.
If you have a partner who is highly sensitive to cross-contamination, you may end up on a gluten-free diet yourself, depending on their sensitivity. Learn to love rice pasta, millet bread, and sorghum beer. Wine, cider, and distilled alcoholic beverages are safe too.
 Safe and unsafe foods
At its most basic, the diet means cutting out any source of gluten, primarily wheat, barley and rye. Kamut and spelt are "ancient" forms of wheat that still contain gluten.
The most obvious sources of gluten would be bread, biscuits and some cakes. But many foods have unexpected gluten. Wheat flour is often used as a thickener and starch, meaning that most restaurant soups and many sauces will contain gluten. Pseudo-grains such as millet, buckwheat, quinoa are fine, as are potatoes, rice and corn. Oats are frequently contaminated due to being processed on machinery also used to process wheat, but there are brands which are tested and certified free from contamination. Wheat gluten is often used in meat substitutes, and sausages may contain biscuit rusk. For those with celiac, cross-contamination in the manufacturing process can also be a source of hidden gluten so it's important to check packaging for shared equipment warnings.
Highly processed grain derived products such as maltodextrin, grain alcohol (yay) and malt vinegar are usually gluten-free. Some gluten-free products will use highly processed wheat starch which is also gluten-free.
In short it is vital to inspect the ingredients extremely carefully and if in any doubt, don't buy. It is common to find that one brand of a product (e.g. ketchup) is safe while another brand is not.
 Gluten-free alternatives
Many gluten-free substitute foods are available, such as bread and pasta made from rice & corn flour; some products also use balsa wood and styrofoam, or at least taste as if they do. The quality of bread varies widely based on the ingredients and preparation involved. The biggest problem is that bread tends to fall or part or squash because it is not elastic. Some gluten-free mixes use additives to preserve the shelf life of the product, with cellulose being a common ingredient.
Grocery shopping outlets such as QFC and Fred Meyer, and home-delivery services such as Amazon Fresh, have special sections for gluten-free foods. Many restaurants have a gluten-free alternate menu, or mark gluten-free items on the menu itself or in a nutrition guide.
Coeliac UK has a website with an online food database that includes many supermarket brands, and UK residents can also get gluten-free staples on prescription through the NHS. The additional cost of gluten-free foods is tax-deductible for US residents with a true coeliac diagnosis (as opposed to "thinking that gluten is bad for them").
 Food regulation
European food regulations require food advertised as gluten-free to possess 20 parts per million (ppm) or less of gluten. A second category of food possessing 21-100ppm may be advertised as very low gluten.  Some products may be "naturally gluten-free" which means there is nothing to imply gluten (e.g. a bag of carrots is naturally gluten-free) but no formal testing has been done to know for sure and in the case of some grains occasional contamination does occur.
A product must highlight allergens including gluten in its list of ingredients e.g. by putting them in bold. A separate allergens box is optional. Many products will state they "may" contain traces of gluten due to their manufacturing process.
There is no standard logo for gluten-free products however the UK Coeliac Society's "crossed grain" symbol has become the most commonly used throughout the EU and elsewhere . Other products might use a similar motif or just the words "gluten-free" somewhere on their packaging.
 United States
As of August 2014, the U.S. FDA requires all food labelled as "gluten-free" to contain 20 parts per million (ppm) or less of gluten, and to not be made with any unprocessed gluten-containing grain.
 Gluten-free makeup
No, we're not making this up. Since a lot of cosmetics use wheat starch as a cheap filler, several cosmetic companies now sell gluten-free product lines.
It is possible, depending on where the makeup in question is applied, that a little of it could be ingested. This could trigger a reaction in a celiac sufferer. But there's no evidence that gluten can be absorbed through your skin. So you probably shouldn't worry about gluten in your eye shadow, unless your celiac bedmate likes to nibble on your eyeballs.
 See also
- Gluten-Free Certification Organization
- Celiac disease
- Diagnosis of gluten sensitivity
- Gluten-free children's play sand -- no, really!
- Against All Grain, founded by a woman with an autoimmune disorder who decided not to stop at just excluding gluten
- The Reality Behind Gluten-Free Diets Advice from the University of Wisconsin-Madison about gluten-free diets, healthy people are advised to eat gluten.
- Dear America: Quit Flipping Out About Gluten, Foodspin, 27-Oct-2014
- ↑ Pedestrian Question, What is Gluten?
- ↑ WebMD: The Truth About Gluten
- ↑ The Extant works of Aretaeus, the Cappadocian.
- ↑ Coeliac prevalence and screening, Coeliac UK
- ↑ WebMD: Celiac Disease — Topic Overview
- ↑ Pioneer in the gluten-free diet: Willem-Karel Dicke 1905-1962, over 50 years of gluten-free diet
- ↑ Nexvax2® – Lead therapeutic
- ↑ … or is it? Does gluten sensitivity in the absence of coeliac disease exist?, Imran Aziz et al., British Medical Journal, 30-Nov-2012
- ↑ What if your gluten intolerance is all in your head?, New Scientist, 11-July-2013
- ↑ Non-Celiac Gluten Sensitivity May Not Exist, RealClearScience blog, 14-May-2014
- ↑ Biesiekierski et al., No effects of gluten in patients with self-reported non-celiac gluten sensitivity after dietary reduction of fermentable, poorly absorbed, short-chain carbohydrates, Gastroenterology, vol. 145 no. 2 (August 2013), pp. 320-328
- ↑ Fermentable Oligo-, Di-, and Mono-saccharides And Polyols. See wp:FODMAP.
- ↑ the one that low-FODMAP dieters are encouraged to follow for 2-6 weeks before easing off a bit to see what they can cope with
- ↑ Risk of thyroid disease in individuals with celiac disease, J. Clin. Endocrinol. Metab. 2008 Oct
- ↑ e.g. Chris Kresser (a licensed acupuncturist and practitioner of integrative medicine), The Gluten-Thyroid Connection
- ↑ The GFCF (Gluten-Free, Casein-Free) Diet for Autism Spectrum Disorders
- ↑ GlutenFreeHelp.info: Louder Than Words by Jenny McCarthy
- ↑ WebMD: Gluten-Free/Casein-Free Diets for Autism
- ↑ ABC News: A Gluten-Free, Casein-Free Diet No Remedy for Autism
- ↑ Evaluation, Diagnosis, and Treatment of Gastrointestinal Disorders in Individuals With ASDs: A Consensus Report
- ↑ INSAR: Persistence of Complementary and Alternative Medicine (CAM) Use In ASD
- ↑ example
- ↑ SparkPeople: Celiac Disease and Gluten-Free Group
- ↑ Weight Watchers: Gluten-Free Main Dishes
- ↑ http://www.ncbi.nlm.nih.gov/pubmed/24901744
- ↑ http://well.blogs.nytimes.com/2016/01/20/when-athletes-go-gluten-free/
- ↑ http://www.ncbi.nlm.nih.gov/pubmed/25970665
- ↑ If people want to add a few gluten-free products to a normal diet including, for example regular bread with gluten this is a best a harmless, expensive placebo.
- ↑ Kill me now. — PintofStout
- ↑ WebMD: Hidden Gluten — Topic Overview
- ↑ Celiac.com's list of safe ingredients
- ↑ Udi's Gluten Free Bread
- ↑ Archive copy at the Wayback Machine
- ↑ Amazon: Learn How to Shop for Gluten Free
- ↑ concerning the composition and labelling of foodstuffs suitable for people intolerant to gluten
- ↑ 
- ↑ Gluten-Free Labeling of Foods, FDA, last retrieved 8-Oct-2015
- ↑ Are Gluten-Free Cosmetics Necessary?, U.S. News and World Report, 11-September-2012