DEFINITION

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.

EPIDEMIOLOGY

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 younger than 3 years have a true food allergy.¹

As seen in Box 1, prevalence of a specific food allergy depends on the patient’s age. In children, the most common foods are milk, soy, eggs, wheat, and peanuts. Peanuts are a particular concern from a public health standpoint, because the prevalence of peanut allergy in American children has doubled in the past decade.¹ Most children lose their allergies to egg, soy, milk, and wheat by school age, but they usually retain their allergies to peanuts, tree nuts (walnut, pecan, brazil nut, cashew, hazelnut, pistachio), and seafood throughout their lives. Recent studies of peanut allergy show that although about 20% of children lose this 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 of these adults have retained their allergy since childhood. Some with tree nut allergy have reactions to all tree nuts; others react to only one or two nuts and consume other nuts without problems.

PATHOPHYSIOLOGY

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.

SIGNS AND SYMPTOMS

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

Diagnosis of food allergy relies on a history consistent with IgE-mediated reaction to a particular food or foods. Important details include the patient’s age, route of exposure, amount of food needed to cause symptoms, timing between the exposure to the food 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 (e.g., fruits or vegetables) can require use of the actual food for skin testing: 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 based on 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 done 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, not eliminating the food or foods completely, not allowing enough time to achieve improvement, and selecting the wrong foods will give false results.

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.

Summary

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