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Gluten sensitivity (also gluten intolerance) is a spectrum of disorders, including celiac disease and wheat allergy, in which gluten has an adverse effect on the body. Symptoms include bloating, abdominal discomfort or pain, diarrhea, muscular disturbances and bone or joint pain. It can also cause other unexpected symptoms such as headaches and vertigo, among many others. 
Gluten gives elasticity to dough helping it to rise and to keep its shape. It is found in many staple foods in the Western diet. It is a protein composite found in wheat and other grains, including barley and rye and processed foods thereof. Gluten is composed of a gliadin fraction (alcohol soluble) and a glutenin fraction (only soluble in dilute acids or alkali). Gluten is not naturally occurring in corn, rice, or oats, but may be cross-contaminated by facilities that also process wheat, barley, or rye products.
Symptoms of gluten sensitivity include bloating, abdominal discomfort or pain, constipation and diarrhea, and might present extraintestinal symptoms including muscular disturbances and bone or joint pain.
In a recent clinical paper, gluten sensitivity was defined as one or more of a variety of immunological, morphological or symptomatic manifestations that may also be shared by celiac disease and irritable bowel syndrome (IBS). In cases where there is reactivity to gluten, yet celiac disease and wheat allergy are eliminated as possibilities, gluten sensitivity may be considered. Whilst the general clinical picture for gluten sensitivity is similar to celiac disease in particular, it is usually less severe and neither anti-tissue transglutaminase antibodies nor autoimmune comorbidities are found.
It is believed that approximately 40-50% of gluten sensitivity patients may have IgG or IgA anti-gliadin antibodies (AGA). There is also a study identifying approximately 50% of gluten sensitivity patients, few more than the general population, carry either HLA DQ 2 or 8. On closer inspection, it has also been found that such gluten-sensitive subjects lack histological lesions; fall below Marsh 2 celiac disease score and indicating that there may be some infiltration of intraepithelial lymphocytes. In addition, it has been found that they have normal intestinal permeability and an increased expression of Toll Like receptors 2 (TLR2) but no change in the cytokines involved in adaptive immune responses Th1 and Th17 such as IL-6, IL-17 A, IL 21, which are increased only in patients with celiac disease. The types of responses indicate an innate immune system involvement, but not autoimmune disease.
Gluten sensitivity should have a defined cause, although not apparent always with first examination, affected individuals should eventually fall into gluten-sensitive enteropathy (GSE) or wheat allergy. Only rarely should gluten sensitivity be idiopathic. Idiopathic gluten sensitivity (IGS) arises spontaneously or from an obscure or unknown cause and may involve neuropathy, myopathy, dermal, or intestinal abnormalities. Anti-gliadin antibodies are the primary link between gluten and idiopathic sensitivity in instances "in which enteropathy or allergy are not clearly involved".
When enteropathy develops in early childhood symptomatic disease is more rapidly evident. A survey of geriatrics with celiac disease in Finland revealed that the incidence of disease was much higher than the general population. Allergic disease may rise or fall with age; certain evidence points to the increased or daily use of non-steroidal anti-inflammatory factors (aspirin, ibuprofen) as an increased risk factor for urticaria or anaphylaxis, and the sensitizing dose may include low-dose aspirin therapy used in the treatment of heart disease. Idiopathic disease appears largely late onset.
Researchers reported extreme fatigue and pain in patients without celiac disease, with gliadin antibodies. They called this a "non-celiac gluten intolerance" for which there is no explanation as to the mechanisms involved.
The fruiting bodies of plants contain genes as well as reserves of nutrients that allow seedlings to grow. The enrichment of nutrients is an attractant to herbivores and omnivores. For annual grasses that release seeds during a brief period each year there is a need to protect seeds during maturation from insects or animals, which might stock seeds for year round usage. For wheat, alpha-gliadins are seed-storage proteins, but also act as inhibitors of alpha-amylase activity in some animals, particularly in insects. It is also known that wheat gliadins create intestinal disease when fed to very young rodents.
One study examined the effect of ω-5 gliadin, the primary cause of wheat dependent exercise/aspirin induced anaphylaxis, and found increased permeability of intestinal cells caused by this gliadin and another wheat albumin.
Another line of research shows gliadin binds a chemoattractant receptor and causes increases of a factor that destroys tight junctions. These junctions prevent leakage around the cells that line the small intestine, resulting in the leaking of food proteins into the body.
Triticeae glutens are important factors in several inflammatory diseases. The immunochemistry can be subdivided into innate responses (direct stimulation of immune system), class II mediated presentation (HLA DQ), class I mediated stimulation of killer cells, and antibody recognition. The responses to gluten proteins and polypeptide regions differs according to the type of gluten sensitivity. The response is also dependent on the genetic makeup of the human leukocyte antigen genes. In enteropathy, there are at least 3 types of recognition, innate immunity (a form of cellular immunity priming), HLA-DQ and antibody recognition of gliadin and transglutaminase.
The three dominant sequences responsible for the antibody reaction have been identified. With idiopathic disease only antibody recognition to gliadin has been resolved. In wheat allergy, there appears to be an innate component and the response pathways are mediated through IgE against gliadin and other wheat proteins.
People suspected of having celiac disease may be tested for anti-transglutaminase antibodies followed by duodenal biopsy; this test will confirm or refute active celiac disease. The study that recommends this has some ATA positive/biopsy-negative individuals, this could result from patchy villous atrophy or subclinical pathology.
One study recommended biopsy samples running distally from the duodenum to avoid the risk of false negatives. Eliminating the possibility of celiac disease can generally be done by adding HLA-DQ typing, in which DQ2 and DQ8 are found in enteropathy 98% of the time in Caucasians, DQ7.5 the remaining 1.6% and 0.4% not found with either of these 3. Without ATA or HLA-DQ2/8 positivity, celiac disease is not likely the cause of the sensitivity. In either case, other avenues of diagnostics, such as allergy testing are available.
Rarely gluten sensitivity may be idiopathic, a potential that wheat proteins play a role in other disease, in these instances DQ1 may be associated with sensitivity. There is research showing that in certain patients with gluten ataxia early diagnosis and treatment with a GFD can improve ataxia and prevent its progression.
Depending on testing somewhere between 3 and 15% of the normal population have anti-gliadin antibodies (AGA). Studies using anti-gliadin antibodies (AGA) reveal that in undiagnosed or untreated individuals with AGA, have an increasing risk for lymphoid cancers and decreased risk for other conditions associated with affluence.
Antibodies to α-gliadin have been significantly increased in non-celiacs individuals with oral ulceration. Anti-α-gliadin antibodies are frequently found in celiac disease (CD), to a lesser degree subclinical CD, but are also found in a subset who do not have the disease. The 1991 reference comes from a period when testing for subclinical CD was undeveloped. Of people with pseudo-exfoliation syndrome, 25% showed increased levels of anti-gliadin IgA. One fourth of people with Sjögren's syndrome had responses to gluten, of 5 that had positive response to gluten, only one could be confirmed as CD and another was potentially GSE, the remaining 3 appear to be gluten-sensitive. All were HLA-DQ2 and/or DQ8-positive.
Treatment to produce remission of Crohns disease (CrD) symptoms on elimination diet indicated the most important foods provoking symptoms were wheat and dairy. A later paper showed little IgE mediated response except to the dairy, while another paper showed no significant anti-food IgE association.
From the perspective of gluten sensitivity there is no single definition of gluten that concisely defines all potentially pathogenic glutens. With wheat allergies, there can be a wide spectrum of species that may trigger allergies with similar proteins, the omega-gliadin proteins have similar proteins found in oats at high frequency, but omega-gliadin allergy is not a predictor of oat allergy or intolerance. A person can have an allergy to wheat, but not rye.
Glutelins have not been characterized over broad taxa. With idiopathic gluten sensitivity, the antibodies that correlate with disease are anti-gliadin antibodies. Whether these antibodies are pathogenic or are indicators of circulating gliadin is unknown. For gluten-sensitive enteropathy, gliadin and homologous proteins from rye and barley cause disease. T-cell epitopes implicated in disease have been found in glutinous protein genes in all species sequenced within the tribe Triticeae.
Oats are a species within the grass tribe Aveneae, which is in the Pooideae subfamily along with Triticeae (contains wheat, rye, barley and many other genera). Oats are the most closely related cereal species to Triticeae cereals. Some, but not all, cultivars of oat contain the pathogenic proteins that provoke a response in gluten sensitive individuals and those with celiac disease.
After World War II, wheat was suspected as the cause of celiac disease, and the gluten from wheat was identified as a cause soon after. At the time, duodenal biopsy—the current "gold standard" of diagnosis—had not yet been developed; indirect measures of disease were used. In two studies, three children were fed 75 to 150 grams of oats per day and developed symptoms. In three concurrent studies, 10 children and two adults were allowed to eat 28 to 60 grams of oats and developed no symptoms. Since wheat, barley and sometimes rye are common contaminants in oats, until this was investigated, oats were considered to be toxic to celiacs.
A study published in February 2011 uncovered differing levels of toxicity amongst different varieties of oat, indicating that cross-contamination is not the only reason why some oats provoke reactions in some people with a gluten intolerance. A study published in June 2008 found that of 109 sources of oats screened, 85 had unacceptable levels of gluten from wheat, barley or rye.
In summary of recent developments, oats can be tolerated in a gluten-free diet, but oat products should be limited in contamination from Triticeae derived gluten to 20 PPM (20 mg per kg). US states are free to deny the GF-label standard for oat products, if warranted.
As of February 2011, G12, the newest monoclonal antibody (moAb) available, was the only one proven to detect both cross-contamination in oats and also the inherent gluten / avenin that is found in some varieties of oat.
A barley-sensitive ELISA called the R5 sandwich assay does not detect gluten in any of 25 pure oat varieties, but it does detect barley, wheat and rye. Disease-sensitive farming practices, antibody testing and species specific genetic testing are capable of producing pure oats. In the United States, 3 domestic GF-brands are available and one brand imported from Ireland 'reckons' to be 99.95% pure oats. Two brands in the United States use the R5 antibody test and claim to be below 20 PPM in defined gluten.
Gluten-free oats can provide a valuable source of fiber, vitamin B, iron, zinc and complex carbohydrates. Recent studies show that gluten-sensitive individuals on a gluten-free diet often get too much simple carbohydrate, too little fiber and vitamin B. Currently most guidelines do not include oats in a gluten-free diet. While this is likely to change, oats are not recommended within a year of diagnosis because of the oat-sensitive enteropathy (ASE) risk, the desire to establish a clinical baseline and complexity of the contamination issue.
Consuming oats when anti-gliadin antibodies or gliadin are present increases anti-avenin antibodies, and may promote ASE. Duodenal biopsy may be recommended after oat consumption is initiated. The DQ phenotype of all 3 ASE individuals studied so far indicated DQ2 homozygotes are at risk for ASE. Preferably, newly diagnosed celiacs seek the help of a dietician. Guidelines are also available for the introduction of pure, uncontaminated oats into the gluten-free diet.