Published: October 2012
An allergic reaction is but one kind of adverse reaction to food. Other reactions include intolerance to compounds such as lactose, reactions to toxins in cases of food poisoning, and nonallergic immune reactions such as celiac disease. As is the case for any allergy, true food allergy is a reaction mediated by immunoglobulin E (IgE) (allergic) antibodies. Such antibodies are directed at protein allergens in food items. Food allergy is much less common than other kinds of adverse food reactions.
Food allergy is not as prevalent as commonly believed, given the sizeable proportion of the population who react adversely to various kinds of foods. As much as 25% of the population report adverse reactions to foods. Many of these people, and even some of their physicians, call these reactions an “allergy,” but the overwhelming majority are not. Only 4% of the general population and 6% to 7% of children aged < 3 years have a true food allergy.1
As seen in Table 1, the prevalence of specific food allergies depends on the patient’s age. In children, the most common allergies are to milk, soy, eggs, wheat, and peanuts. The prevalence of peanut allergy in American children has doubled in the past decade,1 which has created a particular public health concern. While most children outgrow allergies to other foods by school age, allergies to peanuts, tree nuts (walnut, pecan, Brazil nut, cashew, hazelnut, pistachio), and seafood are usually lifelong. Recent studies of peanut allergy show that although about 20% of children lose the condition with avoidance,2-4 some reacquire it.5 Currently, no validated method has been established to predict either outcome.
In adults, the most common food allergies are those to peanuts, tree nuts, and seafood. Many allergic adults have retained the allergy from childhood. Some with tree nut allergy have reactions to all tree nuts; others react to only 1 or 2 nuts and can consume other nuts without problems.
IgE antibodies are generated against food allergens after exposure through the gastrointestinal tract, respiratory tract, or nonintact skin. The clinical manifestations depend upon the characteristics of the offending proteins, the genetic susceptibility of the person, and the route of sensitization. Food allergy is more common in patients who have other allergic conditions, such as atopic dermatitis and allergic rhinitis, and who have a family history of atopy.
IgE-mediated (allergic) reactions occur promptly, in many cases within minutes of ingesting the offending agent. Typical symptoms include itching or burning of the mouth or lips, swelling of the mouth or face, hives, itching, flushing, vomiting, diarrhea, lightheadedness, loss of consciousness, anxiety, and dyspnea. The allergic person might experience one or several of these symptoms. Sometimes the food allergy manifests as a worsening of eczema instead of with systemic or anaphylactic symptoms.
Diagnosis of food allergy relies on a history consistent with IgE-mediated reaction to a particular food or foods. Important details include the patientâ€™s age, route of exposure, amount of food exposure required to cause symptoms, time interval between the exposure and the onset of symptoms, clinical manifestations of the reaction, duration of symptoms, treatment of the symptoms and response to the treatment, and whether the reaction occurs consistently with exposure to the suspected food. Physical examination is also important because it can reveal other conditions associated with food allergy, such as atopic dermatitis or allergic rhinitis.
Allergists detect food-specific IgE antibodies by percutaneous skin tests and serum assays. Skin tests commonly use commercial extracts. However, the labile nature of some food proteins (eg, those in fruits or vegetables) can require the use of the actual food for skin testing. In these tests, the food is pricked and then the skin is pricked with the same instrument. Intracutaneous skin testing for foods is not recommended because it has been associated with greater risk for systemic reactions; moreover, this method is overly sensitive and can lead to false-positive results.
Various serum assays exist for measuring IgE antibodies to specific foods. Prick skin tests and serum detection of IgE antibodies are highly sensitive; however, skin testing is preferred because of its more favorable negative predictive value. A negative skin test indicates a 95% (or higher) probability that food allergy is not present. However, a positive skin test result is clinically significant only 50% of the time.6 For this reason, skin and serum tests for IgE antibodies to specific foods should be performed only after a proper history has been taken and the clinician is able to generate a pretest probability of food allergy.
Trial elimination diets and oral food challenges are also used to diagnose food allergy. Elimination diets can be used to determine whether foods are contributing to chronic conditions such as gastrointestinal disorders or atopic dermatitis. However, many factors can cloud the results of an elimination diet. For instance, false results might be observed if the patient does not eliminate the food or foods completely from his or her diet or does not allow enough time to achieve improvement, or if the wrong foods have been identified for elimination.
The gold standard for confirming food allergy is a double-blind, placebo-controlled challenge procedure. However, because this is time and labor intensive, an open challenge is often carried out. This challenge occurs under the direct supervision and observation of the physician. The patient consumes graded doses of the suspected food over time and is observed carefully for signs of reaction. An oral food challenge is the definitive way to assess or rule out allergy to a food, but it carries the risk of inducing anaphylactic reaction. For this reason, such challenges should only be performed under the care of a board-certified allergist.
Oral allergy syndrome is a condition in which patients with a pollen allergy experience oropharyngeal symptoms within minutes of eating raw fruits or vegetables. Shortly after the foods come in contact with the oral mucosa, the patient typically experiences itching or burning, and occasionally swelling, of the lips, palate, tongue, and throat. Affected people are usually able to tolerate the cooked versions of these foods, due to the labile nature of the proteins that are responsible for the symptoms. Oral allergy syndrome is caused by shared proteins between pollens and foods (Table 2).
|Apples||Hazel nuts, walnuts||Plums|
|Birch (tree) pollen||Melons||Ragweed pollen|
|Cantaloupe||Mugwort weed pollen||Soy|
Monosodium glutamate (MSG) is a flavor enhancer commonly added to certain food items, such as processed and preserved foods, and traditionally to American Chinese food as an ingredient in sauces and spice mixes. Some people consider MSG to be responsible for a constellation of symptoms that can occur shortly after ingestion including, headache, flushing, sweating, palpitations, chest pain, nausea, and weakness. However, there is no definitive evidence showing that ingestion of MSG in foods causes these symptoms. The symptoms are generally mild and resolve without treatment in a relatively short time. MSG is not associated with IgE-mediated reactions.
|Bottled lemon, lime, grape, and other juices||Maraschino cherries|
|Dried fruits||Potato chips|
|Fresh or frozen shrimp||Wine|
Sulfites are preservatives added to some foods, beverages, and medications (Table 3). They are used in wine making. Some asthmatics are sensitive to sulfites and will experience asthma symptoms, including life-threatening asthmatic reactions, following sulfite ingestion. There are rare reports of sulfite allergy causing generalized anaphylaxis. Patients with proven sulfite sensitivity or allergy should read labels to avoid ingestion of these preservatives (Table 4).
Patients with food allergy should be counseled to avoid the offending food completely. These patients should have access to self-injectable epinephrine at all times in case of accidental ingestion and subsequent reaction. There are no medications that will reliably prevent an allergic reaction to the food.
Immunotherapy is under investigation as a potential treatment for food allergy. Possible routes of administration include sublingual and subcutaneous. However, immunotherapy to prevent or alleviate food allergy is considered neither safe nor effective at this time.
Omalizumab is a humanized monoclonal anti-IgE antibody that is FDA-approved for management of moderate to severe allergic asthma refractory to combination controller therapy (see "Asthma" elsewhere in this section). Omalizumab may prove beneficial for patients with food allergy for preventing anaphylaxis during accidental exposure to an allergen or for decreasing the severity of the reaction. Omalizumab is thought to be of potential value for patients with food allergy, but it has not been validated.
It is hoped that allergists will have more than avoidance to offer to patients with food allergy in the future.