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In medicine, food allergy is hypersensitivity to dietary substances, leading to various types of gastrointestinal complaints. It occurs mainly, but not exclusively, in children. It is a common type of allergy, and is usually treated with an exclusion diet.
Signs and symptoms
Most patients present with diarrhea after ingesting certain foodstuffs, skin symptoms (rashes), bloating, vomiting and regurgitation. The digestive complaints usually develop within half an hour of ingesting the allergen.
Rarely, food allergy can lead to anaphylactic shock: hypotension (low blood pressure) and loss of consciousness. This is a medical emergency. An allergen associated with this type of reaction is peanut, although latex products can induce similar reactions. Initial treatment is with epinephrine (adrenalin), often carried by known patients in the form of an Epi-pen.
Food allergy is thought to develop more easily in patients with the atopic syndrome, a very common combination of diseases: allergic rhinitis and conjunctivitis, eczema and asthma. The syndrome has a strong inherited component; a family history of these diseases can be indicative of the atopic syndrome.
As meals tend to consist of different ingredients, it is not always easy to identify the allergen. Moreover, laboratory diagnosis is imprecise and expensive without a clinical indication which foods may cause the symptoms. Excluding very common allergens is therefore often attempted; in young children, this can be cow's milk (necessitating the use of soya products)eggs, or tree nuts and peanuts.
If an allergen cannot be identified, blood tests may help identify a cause. A full blood count is usually normal, but severe causes may reveal eosinophilia. Routine organ markers and electrolytes are usually normal (unless there has been longstanding, severe diarrhea). Total IgE can be elevated. RAST (radio-allergosorbent test) for specific antigens is generally tested in a panel (e.g. legumes, which can then be separately tested to identify the cause of the allergy). Anti-gliadin or Anti-tTG (tissue transglutaminase) is often measured to exclude coeliac disease (see below).
Important differential diagnoses are:
- Lactose intolerance; this generally develops later in life but can present in young patients in severe cases. This is due to an enzyme deficiency (lactase) and not allergy. It occurs in many non-Western people.
- Coeliac disease; this is a specific allergy against wheat protein (gliadin ).
- Irritable bowel syndrome (IBS); although many IBS cases might be due to food allergy, this is an important diagnosis in patients with diarrhea in whom no allergens can be identified.
- C1-esterase deficiency; this rare disease generally causes attacks of angioedema, but can present solely with abdominal pain and occasional diarrhea.
See also allergy for a general discussion of the background of allergy
Generally, introduction of allergens through the digestive tract is thought to induce immune tolerance . In individuals who are predisposed to developing allergies (atopic syndrome), the immune system produces IgE antibodies against protein epitopes on non-pathogenic substances, including dietary components. The IgE molecules are coated onto mast cells, which inhabit the mucosal lining of the digestive tract.
Upon ingesting an allergen, the IgE reacts with its protein epitopes and release (degranulate) a number of chemicals (including histamine), which lead to oedema of the intestinal wall, loss of fluid and altered motility. The product is diarrhea.
Some of the most common food allergies in North America and Europe are to eggs, milk, peanuts, soy, fish, shellfish, tree nuts, and wheat. The most serious, sometimes deadly, allergies are to peanuts, tree nuts, and shellfish.
Treatment options include an exclusion diet and hyposensibilisation. Exclusion diets prevent attacks by limiting exposure to the allergen. Some common allergens require a major overhaul of someone's choice of foods, and multiple allergies can make this approach a veritable nightmare. While young children generally comply due to their parents' involvement, teenagers often rebel or become impatient, requiring a dedicated approach by a nutritionist/dietician and a medical professional.
Hyposensibilisation is the creation of immune tolerance by introducing the allergen in very small amounts and gradually increasing the dose until the immune system is "used" to the allergen. This approach is difficult and often fails, but is the only effective treatment. There are no registered forms of medication that can adequately control symptoms of food allergy.
For reasons that are not understood, food allergies have become more common in Western nations in recent times. (This trend seems to apply to asthma as well.) In the United States, it is believe that about 4% of the population suffers from food allergies. In children, this number is believed to be significantly higher.
The most common food allergens include peanuts, milk, eggs, tree nuts, fish, shellfish, soy, and wheat - these foods account for about 90% of all allergic reactions.
Various medical specialties have a differing view on food allergies. What is called irritable bowel syndrome by many doctors (a stress-related functional disorder) is often interpreted by allergologists to be food allergy.
In addition, many practitioners of complementary and alternative medicine ascribe symptoms to food allergy where most clasically trained doctors would not find a causal relationship. Examples are headaches, tiredness and hyperactivity. Nevertheless, hypoallergenic diets can be of benefit in these conditions, indicating that the current medical views on food allergy may be too limited.
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