There has been tremendous interest in gluten-free diets in the last decade, which has led to the production of a multi-billion dollar gluten-free food market. Many people have attributed the success of the gluten-free market to fad, advertising, misleading information, and other factors with the truth being that there are very few scientific studies investigating gluten-free diets in people who are not afflicted with celiac disease (gluten intolerance) or wheat allergy. In fact, non-celiac gluten sensitivity (NCGS) was only defined as a clinical condition in 2010 (Sapone et al., 2010) and the details surrounding its prevalence and pathology remain poorly defined (Catassi et al., 2013). This article describes the three gluten-related disorders as well as the current state of the scientific field regarding the prevalence, risk, symptoms, diagnosis, and treatment of these disorders. Parts 2 and 3 of this series cover considerations for implementing a gluten-free diet in the absence of diagnosed gluten sensitivity, and practical advice on how to implement a gluten-free diet, respectively.

Which foods contain gluten?
Gluten is protein that is found in wheat, rye, and barley (Fasano, 2014). Although oats do not inherently contain gluten, oat-containing products are often contaminated due to harvesting and processing in facilities with gluten-containing grains (Ellis & Ciclitira, 2008; Thompson, 2004). In addition, some people with celiac disease have a sensitivity to oat-specific peptides (Ellis & Ciclitira, 2008)).

The Codex Alimentarius Commission of the World Health Organization of the United Nations provides international gluten-free food standards for manufacturers. If a product contains less than 20 ppm gluten it is considered gluten-free. In August 2013 the United States adopted this limit so that any product with the "gluten-free" label has 20 ppm gluten or less (strictly enforced starting August 2014) (Food and Drug Administration, 2013).

The immune system
In order to understand the differences between the different gluten spectrum disorders, it is helpful to understand how the immune system is activated in the presence of invaders. There are two branches of the immune response: innate immunity and adaptive immunity (Fisher, 2008).

The innate immune system provides non-specific defenses against the outside world. Examples of components of the innate immune system are physical barriers such as skin and the intestinal wall and phagocytes, which are white blood cells that ingest foreign bacteria, particles, and dead cells. Due to its non-specific action, the innate immune system is active upon infection and up to 12 hours after infection.

The adaptive immune system provides more specific action against invading microbes. It is composed of white blood cells called B-cells and T-cells. B-cells produce antibodies (also known as immunoglobulins, Ig) against specific invaders. Antibodies are able to recognize many different chemical structures that may be associated with an invader (e.g. peptides, foreign sugar molecules). These chemical structures are called antigens. B- and T-cells, once activated by an antigen, will remain in one's system indefinitely and will be able to respond faster if the person is exposed a second time. Because it takes time to synthesize B-cells and T-cells, the adaptive immune system is most active 1-6 days after infection.

An inflammatory response is initiated when an invader is identified. Cells of the immune system secrete inflammatory cytokines, which act as signals to other cells that the body is under attack. In addition, histamine (most commonly known for its cause of runny nose and watery eyes during an allergic reaction) and tryptase are released which can increase secretion of mucous and, in the gastrointestinal tract, lead to cramping and diarrhea (Barbara et al., 2004). In the case of a food allergy/intolerance, continuous activation of this system can compound the symptoms and lead to discomfort and declining intestinal health.

Both the innate and the adaptive immune systems are optimized to protect against bacterial and viral invaders. These invaders possess unique amino acid sequences that the immune system uses to identify them. Unfortunately, in the case of allergy or intolerance, the body misidentifies an innocuous peptide as a dangerous peptide and reacts.

Gluten-related disorders
Three disorders have been characterized related to immune reactions to gluten: wheat allergy, celiac disease, and NCGS. The similarities and differences of these disorders will be described here. For completeness, irritable bowel syndrome (IBS) and Crohn's disease are also disorders of the gut and sometimes overlap with gluten sensitivity, so these two conditions are also briefly discussed.

Wheat allergy is an IgE-mediated hypersensitivity to particular proteins in wheat (Inomata, 2009). The most common reaction is to ω-5 gliadin, a protein in gluten, but other components of wheat can serve as allergens (Tatham & Shewry, 2008). As with all allergies, wheat allergy primarily involves the adaptive immune system, though the innate immune system is also responsible for some aspects of the extreme response. (The details of the allergic response are beyond the scope of this article. For more information, see (Inomata, 2009) and (Fasano, 2014).)

Wheat can be an allergen in food-dependent, exercise-induced anaphylaxis. In this case the condition is known as wheat-dependent, exercise-induced anaphylaxis (WDEIA). Wheat dosage and exercise intensity can affect the severity of symptoms, and aspirin can exacerbate symptoms due to facilitated absorption of the allergens (Inomata, 2009).

Celiac disease is an autoimmune disorder that has strong genetic and environmental (i.e. exposure to gluten) components. With regular gluten ingestion, patients' small intestines becomes "leaky", thereby compromising the intestinal barrier and allowing passage of microbes and other offensive agents that trigger inflammation. This inflammation also precipitates an autoimmune response against the villi of the small intestine, which are the tiny projections in the intestine that absorb nutrients. Celiac disease is unique in that it can lead to poor absorption of nutrients and consequent deficiencies. Celiac disease is a response of both the adaptive immune system involving B-cells and T-cells and the innate immune system. (For more information see (Guandalini, Discepolo, Newland, & Kupfer, 2014)).

Non-celiac gluten sensitivity (NCGS) was characterized as an ailment separate from wheat allergy and celiac disease in 2010 (Sapone et al., 2010) and understanding its pathology is still in its infancy. NCGS has been, and still is, considered a fad by some people who believe that the benefits reported from a gluten-free diet are caused by the placebo effect or an effect of eating healthier in general (e.g. avoidance of cookies, cake, and battered-and-fried foods; purchasing salads instead of pizza or cheeseburgers; etc.). However, people with NCGS experience improvements in symptoms above those of the placebo effect in blinded clinical trials (Biesiekierski et al., 2011). Individuals with NCGS who follow a strict gluten-free diet may experience relief from gas, bloating, and other gastrointestinal dysfunction, alleviation of headache or other unexplained ailments, and/or elevation in mood, energy, and focus (Catassi et al., 2013).

The pathology of NCGS is largely unknown, though is distinctly different than that of celiac disease or wheat allergy. In contrast to wheat allergy and celiac disease, patients with NCGS do not present IgE antibody responses to wheat or signs of autoimmune dysfunction. However, gluten appears to activate the innate immune system via toll-like receptor 2 (TLR2) and, to a lesser extent, TLR1. These TLRs are elevated in many NCGS patients (Fasano, 2014). In addition, patients with NCGS showed levels of CD3+ intestinal intraepithelial lymphocytes (a specific type of white blood cell) that fell between celiac patients and healthy controls, indicating moderate inflammation in the intestinal mucosa due to an innate immune response (Catassi & Sapone, 2014).

There is inconsistency as to whether gluten per se (Biesiekierski et al., 2011) or another component of wheat (Biesiekierski et al., 2013) is responsible for the gastrointestinal symptoms. Experts are aware that NCGS may not be an accurate term and the condition could be more accurately described as "non-celiac wheat sensitivity". At this time, it is unknown whether or not other gluten-containing foods such as barley and rye are offensive to NCGS patients (Catassi et al., 2013). In addition, amylase-trypsin inhibitors (ATIs) are wheat proteins that are mostly resistant to digestion; these proteins trigger an immune reaction in celiac patients and are allergens in baker's asthma (Junker et al., 2012; Tatham & Shewry, 2008). ATIs have not yet been studied in patients with NCGS. One study identified "fermentable, oligo-, di- and monosaccharides and polyols" (FODMAPs) as alternative compounds in wheat products and other foods that could trigger bowel dysfunction (Biesiekierski et al., 2013). FODMAPs are not digested by human enzymes and are fermented in the intestine by gut bacteria. Examples of FODMAPs include fructans, lactose, fructose, xylitol, maltitol, and other sugar-related compounds. In the study by Biesiekierski et al., 37 individuals with NCGS were recruited. After consuming a gluten- and FODMAP-free diet for 2 weeks, the participants on average experienced a decrease in symptom severity (10 out of 37 participants reported severity scores of 20/100 or greater before the treatment; all of these participants experienced relief from symptoms). In addition, there was no greater increase in pain, bloating, tiredness, or other symptoms when gluten was added to the FODMAP-free diet compared to whey (Biesiekierski et al., 2013).

Irritable bowel syndrome (IBS) is not a condition related to gluten sensitivity, specifically, but gluten can elicit symptoms in some patients (Catassi et al., 2013). IBS is characterized by abdominal pain, bloating, flatulence, and diarrhea or constipation. There are many causes and just as many treatment strategies.

Crohn's disease is also a condition of the bowel that is characterized by gastrointestinal discomfort. Like celiac disease, severity of symptoms is related to both genetic and environmental factors. In fact, there may be a genetic link between Crohn's disease and celiac disease (Festen et al., 2011) suggesting that a large percentage of Crohn's patients may benefit from a gluten-free diet (Tursi, Giorgetti, Brandimarte, & Elisei, 2005).

Prevalence
About 0.1% of the population has a wheat allergy (Pietzak, 2012). Wheat allergy is most prevalent in children between 3-5 years old who have other food allergies. Often, the children outgrow it.

Approximately 1% of people in Europe and North America have been diagnosed with celiac disease (Guandalini et al., 2014). (See Part 2 of this series for more details.)

The prevalence of NCGS is difficult to determine because many patients are self-diagnosed, but it appears may be more prevalent than celiac disease (Catassi et al., 2013). In addition, about two-thirds of people following self-initiated gluten-free diets have not clinically excluded celiac disease (Biesiekierski, Newnham, Shepherd, Muir, & Gibson, 2014). With that said, it has been estimated from epidemiological and clinical studies that 2-6% of the population has gluten sensitivity (Catassi et al., 2013). Prevalence appears to be higher among females (Catassi & Sapone, 2014).

Risk
Gluten-related disorders have a genetic component. If a person has a first degree relative with celiac disease, their chance of developing celiac disease is up to 20% greater than someone without a first degree relative with the disease (Guandalini et al., 2014). The risk is also increased in people with autoimmune thyroid disease, type 1 diabetes, and Down's syndrome (Silvester & Duerksen, 2013). The disease can develop at any age (Silvester & Duerksen, 2013), and females tend to be afflicted at a greater rate than males (Catassi & Sapone, 2014).

Symptoms
Wheat allergy, celiac disease, and NCGS all share the same gastrointestinal symptoms: cramps, bloating, nausea, flatulence, diarrhea or constipation, and abdominal pain, especially after exposure to wheat or gluten. After gluten ingestion, symptoms can present between several hours and several days later (Catassi & Sapone, 2014). Wheat allergy causes symptoms that extend beyond the gastrointestinal tract: swelling, itching, and irritation in the mouth, nose, eyes, and throat; a rash or hives; and/or difficulty breathing (Pietzak, 2012). Severe exposures can cause life-threatening anaphylactic shock. However, wheat allergy does not cause permanent damage to the intestine the way celiac disease does. In addition to gastrointestinal discomfort, celiac patients may present unexplained iron-deficient anemia or osteoporosis, fatigue, weight loss, or joint/bone symptoms due to poor absorption of nutrients (Silvester & Duerksen, 2013). Dermatitis herpetiformis is a rash that develops in celiac patients, often unaccompanied by gastrointestinal symptoms, and is characteristic of celiac disease specifically (Pietzak, 2012). Celiac patients as well as NCGS patients can also experience systemic manifestations such as "foggy mind", headache, fatigue, joint and muscle pain, leg or arm numbness, depression, and/or anemia (Catassi et al., 2013). Indeed, patients with gluten sensitivity can experience serious neurological and cognitive dysfunction, even when gastrointestinal symptoms are not present (for an excellent discussion on gluten sensitivity and the brain, see (Hadjivassiliou et al., 2010)).

Diagnosis
Wheat allergy
Wheat allergies can be diagnosed with a radioallergosorbent test, which tests for IgE antibodies to wheat, and a skin prick test, which introduces wheat allergens into the body in a controlled fashion to observe the response. However, sometimes the IgE test and the skin prick test are not accurate because wheat contains many potential allergenic peptides; in the case of a negative response, a person may subsequently be subjected to an oral food challenge (Inomata, 2009).

Celiac disease
Because of overlap between positive test results and various conditions, patients undergo several assessments for celiac diagnosis including evaluation for antibodies in the serum, genetic testing, assessment of the pathology of the small intestine, and/or evaluation of symptoms on a gluten-containing vs. a gluten-free diet. The most widely used test is a serologic (blood) screen for transglutamine IgA, which is an autoimmune antibody produced against tissue transglutaminase (tTG) and is highly elevated in the blood in patients with celiac disease (Catassi & Sapone, 2014). The transglutamine IgA test has a 90-96% sensitivity (i.e. 4-10% of people with celiac disease are not identified with the test, often due to low total levels of IgA) and a 95% specificity (i.e. there is a 5% false positive rate) (Silvester & Duerksen, 2013). The technology of diagnostic tests is rapidly developing, so other antibodies may be included in the screen as well (e.g. endomysial antibodies (EMA-IgA)). Genetic tests are used primarily for excluding celiac disease, because celiac patients express the human leukocyte antigen (HLA) class II haplotypes DQ2 and/or DQ8. People with celiac disease almost always possess HLA-DQ2 and/or HLA-DQ8 genes, though not all individuals who harbor these genes will be afflicted (Guandalini et al., 2014). Next, in order to confirm the serologic tests, an intestinal biopsy will reveal whether there are collapsed villi, markers of inflammation, or other physical markers of celiac disease. Most importantly, implementation of a strict gluten-free diet will alleviate symptoms in a patient with celiac disease.

NCGS
There is not currently a way to definitively diagnose NCGS (Catassi et al., 2013), so diagnosis of NCGS is generally due to exclusion criteria (i.e. exclusion of wheat allergy, celiac disease, irritable bowel syndrome, and Crohn's disease) (Fasano, 2014). Many NCGS patients (56.4%) present with an elevated prevalence of first-generation IgG anti-gliadin antibodies (AGA). However, only 7.7% of patients with NCGS show high levels of IgA ADA (Catassi et al., 2013). (IgG and IgA are both elevated in patients with celiac disease.) NCGS can be diagnosed by commencing a gluten-free diet and then, after at least 3 weeks, undergoing a "gluten challenge" to see if symptoms are alleviated during the gluten-free diet and then return during the gluten challenge. In order to rule out placebo effects, a "double-placebo" gluten challenge is optimal for diagnosis: The patient consumes two diets provided by the clinician. One diet has gluten, the other does not, and the patient does not know which is which (Catassi & Sapone, 2014).

Due to the seemingly high but unknown prevalence of NCGS, it is useful for a person to see a doctor to be evaluated for wheat allergy, celiac disease, and other food-related disorders before self-starting a gluten-free diet. This will allow the patient and the doctor to design an optimal diet that will relieve symptoms (Silvester & Duerksen, 2013).

Treatment
For any person diagnosed with a gluten-related disorder, it is important for them to strictly adhere to a gluten-free diet in order to prevent gastrointestinal disturbances, optimize nutrient absorption (thereby reducing risk of anemia and osteoporosis), and, in the case of celiac disease, reduce the risk of intestinal lymphoma (cancer related to elevated inflammation) (Silvester & Duerksen, 2013). Based on the diagnosis and individual factors, the diet will not be equivalent for every patient. For example, people with wheat allergy do not need to avoid rye and barley (Pietzak, 2012). Also, some, but not all, people with celiac disease experience a relief of irritable bowel symptoms when they strictly follow a gluten-free diet and other offensive foods should be explored (Catassi et al., 2013). For practical advice on how to follow a gluten-free diet, see Part 3 of this series.

Conclusions
Both prevalence and awareness of wheat/gluten-related conditions are increasing. People who experience chronic gastrointestinal distress may benefit from following a gluten-free diet, though it is important to discuss this with a physician before making any drastic dietary changes. Although the pathology of wheat allergy and celiac disease are relatively well characterized, the understanding of gluten sensitivity (or is it wheat protein sensitivity? or FODMAP sensitivity?) is sorely lacking. There are many open questions in this field, such as: Why do people experience gluten sensitivity? Can sensitivity change over the lifespan? Could a gluten-free diet aid in non-gastrointestinal-related ailments? Can our food be altered to prevent these problems? And many others. Fortunately, the amount of research dedicated to gluten-related disorders has skyrocketed in the last 10 years (Catassi et al., 2013), and hopefully the answers to these questions will come sooner rather than later.

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