A gluten-free diet (GFD) is a nutritional plan that strictly excludes gluten, which is a mixture of prolamin proteins found in wheat (and all of its species and hybrids, such as spelt, kamut, and triticale), as well as barley, rye, and oats. The inclusion of oats in a gluten-free diet remains controversial, and may depend on the oat cultivar and the frequent cross-contamination with other gluten-containing cereals.
Gluten may cause both gastrointestinal and systemic symptoms for those with gluten-related disorders, including coeliac disease (CD), non-coeliac gluten sensitivity (NCGS), and wheat allergy. In these people, the gluten-free diet is demonstrated as an effective treatment, but studies show that up to 79% of the people with coeliac disease have an incomplete recovery of the small bowel, despite a strict gluten-free diet. This is mainly caused by inadvertent ingestion of gluten. People with a poor understanding of a gluten-free diet often believe that they are strictly following the diet, but are making regular errors.
In addition, a gluten-free diet may, in at least some cases, improve gastrointestinal or systemic symptoms in diseases like irritable bowel syndrome, rheumatoid arthritis, or HIV enteropathy, among others. There is no good evidence that gluten-free diets are an alternative medical treatment for people with autism.
The grains that contain gluten are not essential in the human diet. However, an unbalanced selection of food and an incorrect choice of gluten-free replacement products may lead to nutritional deficiencies. Replacing flour from wheat or other gluten-containing cereals with gluten-free flours in commercial products may lead to a lower intake of important nutrients, such as iron and B vitamins. Some gluten-free commercial replacement products are not as enriched or fortified as their gluten-containing counterparts, and often have greater lipid/carbohydrate content. Children especially often over-consume these products, such as snacks and biscuits. Nutritional complications can be prevented by a correct dietary education.
A gluten-free diet may be based on gluten-free foods, such as meat, fish, eggs, milk and dairy products, legumes, nuts, fruits, vegetables, potatoes, rice, and corn. Gluten-free processed foods may be used. Pseudocereals (such as quinoa, amaranth, and buckwheat) and some minor cereals have been found to be suitable alternative choices that can provide adequate nutrition.
Rationale behind adoption of the diet
Coeliac disease
Coeliac disease (American English: celiac) (CD) is a chronic, immune-mediated, and mainly intestinal process, that appears in genetically predisposed people of all ages. It is caused by the ingestion of gluten, which is present in wheat, barley, rye and derivatives. Coeliac disease is not only a gastrointestinal disease, because it may affect several organs and cause an extensive variety of non-gastrointestinal symptoms, and most importantly, it may often be completely asymptomatic. Added difficulties for diagnosis are the fact that serological markers (anti-tissue transglutaminase [TG2]) are not always present, and many people with coeliac may have minor mucosal lesions, without atrophy of the intestinal villi.
Coeliac disease affects approximately 1–2% of the general population all over the world and is on the increase, but most cases remain unrecognized, undiagnosed and untreated, exposing patients to the risk of long-term complications. People may develop severe disease symptoms and be subjected to extensive investigations for many years before a proper diagnosis is achieved. Untreated coeliac disease may cause malabsorption, reduced quality of life, iron deficiency, osteoporosis, obstetric complications (stillbirth, intrauterine growth restriction, preterm birth, low birthweight, and small for gestational age), an increased risk of intestinal lymphomas and greater mortality. Coeliac disease is associated with some autoimmune diseases, such as diabetes mellitus type 1, thyroiditis, gluten ataxia, psoriasis, vitiligo, autoimmune hepatitis, dermatitis herpetiformis, primary sclerosing cholangitis, and more.
Coeliac disease with "classic symptoms", which include gastrointestinal manifestations such as chronic diarrhea and abdominal distention, malabsorption, loss of appetite, and impaired growth, is currently the least common presentation of the disease and affects predominantly small children generally younger than two years of age.
Coeliac disease with "non-classic symptoms" is the most common clinical type and occurs in older children (over two years old), adolescents and adults. It is characterized by milder or even absent gastrointestinal symptoms and a wide spectrum of non-intestinal manifestations that can involve any organ of the body, and very frequently may be completely asymptomatic both in children (at least in 43% of the cases) and adults.
Following a lifelong gluten-free diet is the only medically accepted treatment for people with coeliac disease.
Non-coeliac gluten sensitivity
Non-coeliac gluten sensitivity (NCGS) is described as a condition of multiple symptoms that improves when switching to a gluten-free diet, after coeliac disease and wheat allergy are excluded. People with NCGS may develop gastrointestinal symptoms, which resemble those of irritable bowel syndrome (IBS) or a variety of nongastrointestinal symptoms.
Gastrointestinal symptoms may include any of the following: abdominal pain, bloating, bowel habit abnormalities (either diarrhea or constipation), nausea, aerophagia, gastroesophageal reflux disease, and aphthous stomatitis. A range of extra-intestinal symptoms, said to be the only manifestation of NCGS in the absence of gastrointestinal symptoms, have been suggested, but remain controversial. These include: headache, migraine, "brain fog", fatigue, fibromyalgia, joint and muscle pain, leg or arm numbness, tingling of the extremities, dermatitis (eczema or skin rash), atopic disorders such as asthma, rhinitis, other allergies, depression, anxiety, iron-deficiency anemia, folate deficiency or autoimmune diseases. NCGS has also been controversially implicated in some neuropsychiatric disorders, including schizophrenia, eating disorders, autism, peripheral neuropathy, ataxia and attention deficit hyperactivity disorder (ADHD). Above 20% of people with NCGS have IgE-mediated allergy to one or more inhalants, foods or metals, among which most common are mites, graminaceae, parietaria, cat or dog hair, shellfish and nickel. Approximately, 35% of people with NCGS have other food intolerances, mainly lactose intolerance.
The pathogenesis of NCGS is not yet well understood. For this reason, it is a controversial syndrome and some authors still question it. There is evidence that not only gliadin (the main cytotoxic antigen of gluten), but also other proteins named ATIs which are present in gluten-containing cereals (wheat, rye, barley, and their derivatives) may have a role in the development of symptoms. ATIs are potent activators of the innate immune system. FODMAPs, especially fructans, are present in small amounts in gluten-containing grains and have been identified as a possible cause of some gastrointestinal symptoms in persons with NCGS. As of 2019, reviews have concluded that although FODMAPs may play a role in NCGS, they only explain certain gastrointestinal symptoms, such as bloating, but not the extra-digestive symptoms that people with NCGS may develop, such as neurological disorders, fibromyalgia, psychological disturbances, and dermatitis.
After exclusion of coeliac disease and wheat allergy, the subsequent step for diagnosis and treatment of NCGS is to start a strict gluten-free diet to assess if symptoms improve or resolve completely. This may occur within days to weeks of starting a GFD, but improvement may also be due to a non-specific, placebo response. Recommendations may resemble those for coeliac disease, for the diet to be strict and maintained, with no transgression. The degree of gluten cross contamination tolerated by people with NCGS is not clear but there is some evidence that they can present with symptoms even after consumption of small amounts. It is not yet known whether NCGS is a permanent or a transient condition. A trial of gluten reintroduction to observe any reaction after one–two years of strict gluten-free diet might be performed.
A subgroup of people with NCGS may not improve by eating commercially available gluten-free products, which are usually rich of preservatives and additives, because chemical additives (such as sulphites, glutamates, nitrates and benzoates) might have a role in evoking functional gastrointestinal symptoms of NCGS. These people may benefit from a diet with a low content of preservatives and additives.
NCGS, which is possibly immune-mediated, now appears to be more common than coeliac disease, with prevalence rates between 0.5 and 13% in the general population.
Wheat allergy
People can also experience adverse effects of wheat as result of a wheat allergy. Gastrointestinal symptoms of wheat allergy are similar to those of coeliac disease and non-coeliac gluten sensitivity, but there is a different interval between exposure to wheat and onset of symptoms. Other symptoms such as dermal reactions like as rashes or hyperpigmentation may also occur in some people. Wheat allergy has a fast onset (from minutes to hours) after the consumption of food containing wheat and could be anaphylaxis.
The management of wheat allergy consists of complete withdrawal of any food containing wheat and other gluten-containing cereals. Nevertheless, some people with wheat allergy can tolerate barley, rye or oats.
Gluten ataxia
Gluten ataxia is an autoimmune disease triggered by the ingestion of gluten. With gluten ataxia, damage takes place in the cerebellum, the balance center of the brain that controls coordination and complex movements like walking, speaking and swallowing, with loss of Purkinje cells. People with gluten ataxia usually present gait abnormality or incoordination and tremor of the upper limbs. Gaze-evoked nystagmus and other ocular signs of cerebellar dysfunction are common. Myoclonus, palatal tremor, and opsoclonus-myoclonus may also appear.
Early diagnosis and treatment with a gluten-free diet can improve ataxia and prevent its progression. The effectiveness of the treatment depends on the elapsed time from the onset of the ataxia until diagnosis, because the death of neurons in the cerebellum as a result of gluten exposure is irreversible.
Gluten ataxia accounts for 40% of ataxias of unknown origin and 15% of all ataxias. Less than 10% of people with gluten ataxia present any gastrointestinal symptom, yet about 40% have intestinal damage.
As a non-medical diet
Since the beginning of the 21st century, the gluten-free diet has become a fad diet.
The gluten-free diet has been advocated and followed by celebrities to lose weight and some professional athletes. There is no published experimental evidence to support that the gluten-free diet contributes to weight loss.
There is a consensus in the medical community that people should consult a physician before going on a gluten-free diet, so that a medical professional can accurately test for coeliac disease or any other gluten-related disorders.
Research In a 2013 double-blind, placebo-controlled challenge (DBPC) by Biesiekierski et al. in a few people with irritable bowel syndrome, the authors found no difference between gluten or placebo groups and the concept of non-celiac gluten sensitivity as a syndrome was questioned. Nevertheless, this study had design errors and an incorrect selection of participants, and probably the reintroduction of both gluten and whey protein had a nocebo effect similar in all people, and this could have masked the true effect of gluten/wheat reintroduction.