Food allergy

A food allergy is an adverse immune response to a food protein.^[1]^[2] Food allergy is distinct from other adverse responses to food, such as food intolerance, pharmacologic reactions, and toxin-mediated reactions.

The food protein triggering the allergic response is termed a food allergen. It is estimated that up to 12 million Americans have food allergies,^[3] and the prevalence is rising.^[4] Six to eight percent of children under the age of three have food allergies and nearly four percent of adults have them.^[5] Food allergy causes roughly 30,000 emergency room visits and 100 to 200 deaths per year in the United States. ^[6] The most common food allergies in adults are shellfish, peanuts, tree nuts, fish, and eggs^[5], and the most common food allergies in children are milk, eggs, peanuts, and tree nuts.^[5]

Treatment consists of avoidance diets, in which the allergic person avoids all forms of the food to which they are allergic. For people who are extremely sensitive, this may involve the total avoidance of any exposure with the allergen, including touching or inhaling the problematic food as well as touching any surfaces that may have come into contact with it. Areas of research include anti-IgE antibody (omalizumab, or Xolair) and specific oral tolerance induction (SOTI), which have shown some promise for treatment of certain food allergies. Persons diagnosed with a food allergy may carry an autoinjector of epinephrine such as an EpiPen or Twinject, wear some form of medical alert jewelry, or develop an emergency action plan, in accordance with their doctor.

Signs and symptoms

Classic immunoglobulin-E (IgE)-mediated food allergies are classified as type-I immediate hypersensitivity reactions. These allergic reactions have an acute onset (from seconds to one hour) and may include:^[7]

The reaction may progress to anaphylactic shock: A systemic reaction involving several different bodily systems including hypotension (low blood pressure),loss of consciousness, and possibly death. Allergens most frequently associated with this type of reaction are peanuts, nuts, milk, egg, and seafood, though many food allergens have been reported as triggers for anaphylaxis.

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.^[8] The syndrome has a strong inherited component; a family history of allergic diseases can be indicative of the atopic syndrome.

Conditions caused by food allergies are classified into 3 groups according to the mechanism of the allergic response:

The Big Eight

These are often referred to as "the big eight."^[10] They account for over 90% of the food allergies in the United States.^[11]

The top allergens vary somewhat from country to country but milk, eggs, peanuts, treenuts, fish, shellfish, soy, wheat and sesame tend to be in the top 10 in many countries.^{citation needed}

Rarer Food Allergies

Likelihood of allergy can increase with exposure^{citation needed}. For example, rice allergy is more common in East Asia where rice forms a large part of the diet.^[12]

In Central Europe, celery allergy is more common. In Japan, allergy to buckwheat flour, used for Soba noodles, is more common.

Diagnosis

The best method for diagnosing food allergy is to be assessed by an allergist. The allergist will review the patient's history and the symptoms or reactions that have been noted after food ingestion. If the allergist feels the symptoms or reactions are consistent with food allergy, he/she will perform allergy tests.

Pathophysiology

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.^{citation needed} 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.^{citation needed}

Causes

The immune system's eosinophils, once activated in a histamine reaction, will register any foreign proteins they see. One theory regarding the causes of food allergies focuses on proteins presented in the blood along with vaccines, which are designed to provoke an immune response. Influenza vaccines and the Yellow Fever vaccine are still egg-based, but the Measles-Mumps-Rubella vaccine stopped using eggs in 1994.^[19] There is resistance to this theory, especially as it applies to autoimmune disease.^[20]

Another theory focuses on whether an infant's immune system is ready for complex proteins in a new food when it is first introduced.^[21]

The most popular theory at this time is the Hygiene hypothesis. Researches speculate^{citation needed} that in the modern, industrialized nations, such as the United States, food allergy is more common due to the lack of early exposure to dirt and germs, in part due to the over use of antibiotics and antibiotic cleansers. This theory is based partly on studies showing less allergy in third world countries.^{citation needed} Research has found^{citation needed} that the body, with less dirt and germs to fight off, turns on itself and attacks food proteins as if they were foreign invaders.

Prevention

According to a report issued by the American Academy of Pediatrics, "There is evidence that breastfeeding for at least 4 months, compared with feeding infants formula made with intact cow milk protein, prevents or delays the occurrence of atopic dermatitis, cow milk allergy, and wheezing in early childhood."^[22]

Treatment

The mainstay of treatment for food allergy is avoidance of the foods that have been identified as allergens.

If the food is accidentally ingested and a systemic reaction occurs, then epinephrine (best delivered with an autoinjector of epinephrine such as an Epipen or Twinject) should be used. It is possible that a second dose of epinephrine may be required for severe reactions.^{citation needed} The patient should also seek medical care immediately.

At this time, there is no cure for food allergies.^[23] There are no allergy desensitization or allergy "shots" available for food allergies.^{citation needed} Some doctors feel they do not work in food allergies because even minute amounts of the food in question or even food extracts (as in the case of allergy shots) can cause an allergic response in many sufferers.

Statistics

For reasons that are not entirely understood, the diagnosis of food allergies has apparently become more common in Western nations in recent times.^[4] In the United States food allergy affects as many as 5% of infants less than three years of age^[25] and 3% to 4% of adults.^[26] There is a similar prevalence in Canada.^[27]

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.^{citation needed}

Differing views

Various medical practitioners have a differing views on food allergies. Irritable Bowel Syndrome (IBS) patients have been studied with regards to food allergies. Some studies have reported on the role of food allergy in IBS; only one epidemiological study on functional dyspepsia and food allergy has been published. However, since 2005 several studies have demonstrated strong correlation between IgG and/or IgE food allergy and IBS symptoms ^[28]^[29] ^[30] The mechanisms by which food activates mucosal immune system are incompletely understood, but food specific IgE and IgG4 appeared to mediate the hypersensitivity reaction in a subgroup of IBS patients. Specific chemicals and receptors have been demonstrated to be critical in food allergy development in murine models.^[31] Exclusion diets based on skin prick test, RAST for IgE or IgG4, hypoallergic diet and clinical trials with oral disodium cromoglycate have been conducted, and some success has been reported in a subset of IBS patients.^[32]

Studies comparing skin prick testing and ELISA blood testing have found that the results of skin prick testing correlate poorly with symptoms of irritable bowel syndrome that correlate with food allergies demonstrated through ELISA testing and dietary challenge. ^[33]

Extensive clinical experience has demonstrated significant improvement of patients with IBS whose ELISA-based food allergy testing is positive and where treatment includes a careful exclusion diet. ^[34]

In addition, many practitioners of alternative medicine ascribe symptoms to food allergy where other doctors do not. The causal relationships between some of these conditions and food allergies have not been studied extensively enough to provide sufficient evidence to become authoritative. The interaction of histamine with the nervous system receptors has been demonstrated, but more study is needed.^[35] Other immune response effects are commonly known (swelling, irritation, etc.), but their relationships to some conditions has not been extensively studied. Examples are arthritis, fatigue, headaches, and hyperactivity. Nevertheless, hypoallergenic diets reportedly can be of benefit in these conditions, indicating that the current medical views on food allergy may be too narrow. Holford and Brady (2005) suggest three levels of response; classical immediate-onset allergy (IgE), delayed-onset allergy (giving a positive response on an ELISA IgG test but rarely on an IgE skin prick test), and food intolerance (non-allergic), and claim the last two to be more common.^[36] It is important to note that IgG is present in the body and is known to respond to foods. So some medical practitioners, especially allergists, claim that there is no predictive value to these types of tests, despite the studies cited above.

In children

Milk and soy allergies in children can often go undiagnosed for many months, causing much worry for parents and health risks for infants and children. Many infants with milk and soy allergies can show signs of colic, blood in the stool, mucous in the stool, reflux, rashes and other harmful medical conditions.^{citation needed} These conditions are often misdiagnosed as viruses or colic.

Some children who are allergic to cow's milk protein also show a cross sensitivity to soy-based products. ^[37] There are infant formulas in which the milk and soy proteins are degraded so when taken by an infant, their immune system does not recognize the allergen and they can safely consume the product. Hypoallergenic infant formulas can be based on hydrolyzed proteins, which are proteins partially predigested in a less antigenic form. Other formulas, based on free amino acids, are the least antigenic and provide complete nutrition support in severe forms of milk allergy.

About 50% of children with allergies to milk, egg, soy, and wheat will outgrow their allergy by the age of 6. Those that don't, and those that are still allergic by the age of 12 or so, have less than an 8% chance of outgrowing the allergy ^[38].

Peanut and tree nut allergies are less likely to be outgrown, although evidence now shows ^[39] that about 20% of those with peanut allergies and 9% of those with tree nut allergies ^[40] will outgrow their allergies. In such a case, they need to consume nuts in some regular fashion to maintain the non-allergic status.^{citation needed} This should be discussed with a doctor.

Labeling laws

In response to the risk that certain foods pose to those with food allergies, countries have responded by instituting labeling laws that require food products to clearly inform consumers if their products contain major allergens or by-products of major allergens.

United States law

Under the Food Allergen Labeling and Consumer Protection Act of 2004 (Public Law 108-282), companies are required to disclose on the label whether the product contains a major food allergen in clear, plain language. The allergens have to clearly be called out in the ingredient statement. Most companies are responsible enough to list allergens in a statement separate from the ingredient statement.^[41]

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