Food Allergy: Latest Guidelines From the NIH

Patients often attribute an adverse reaction to food contaminants or additives to “food allergy.” However, true food allergy—hypersensitivity to a specific food triggered by the immune system—is uncommon; about 4% of adults and 5% of children in the United States are affected. Nonetheless, the prevalence of food allergy may be increasing.¹ 
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Because such allergies can cause devastating and even fatal anaphylactic reactions, diagnosis and treatment of these reactions are critical. The National Institute of Allergy and Infectious Diseases (NIAID) recently developed clinical guidelines for the diagnosis and management of food allergy. Highlights of those guidelines are presented here.

COMMON FOOD ALLERGIES

In adults, foods often associated with allergy include shellfish (eg, crab, crayfish, lobster, shrimp), peanuts, fish (Figure), and eggs. In children, the most common food allergens are eggs, milk, and peanuts. Patients allergic to a specific food, such as peanuts or crayfish, may also experience cross-reactions if they eat other types of nuts or shellfish.

DIFFERENTIAL DIAGNOSIS

The differential diagnosis of food allergy is wide-ranging. Conditions that can produce symptoms that resemble those of food hypersensitivity reactions include:

•Contamination. Infections caused by food-borne bacterial pathogens, particularly in contaminated meat, can cause symptoms similar to those reactions commonly associated with food hypersensitivity.

•Histamine toxicity. High levels of histamine may occur naturally in cheese and some wines. Histamine may also be found in tuna, mackerel, and other fish, possibly because of bacterial contamination.

•Lactase deficiency. At least 10% of the US population are unable to digest most milk products because of insufficient amounts of this enzyme. Persons with this deficiency may experience bloating, abdominal pain, and diarrhea after ingesting milk products.

•Additives. Products that are added to foods can cause adverse reactions similar to hypersensitivity reactions. Monosodium glutamate, for example, may cause flushing, headache, chest pain, and other symptoms. Yellow dye number 5 occasionally causes hives. Sulfites, which occur naturally in food but may also be added to prevent mold growth, may provoke severe bronchospasm in patients with asthma.

•Gastrointestinal (GI) conditions. Both ulcers and cancers of the GI tract may cause diarrhea, vomiting, or abdominal pain. These conditions are particularly easy to confuse with food hypersensitivity reactions because the symptoms are exacerbated by eating.

•Psychological conditions. Occasionally, a seemingly allergic reaction to a particular food is triggered by a psychological association between eating that food and an unpleasant event (that often occurred years earlier).

DIAGNOSIS

An important tool for diagnosing possible food allergy is a detailed history. However, the history alone is not sufficient to make the diagnosis. The “gold standard” for diagnosis is the double-blind placebo-controlled food challenge.¹

History. Ascertain as precisely as possible the timing and circumstances of your patient’s allergic reaction, including the following:

•Specificity. Is the reaction consistently related to ingestion of a specific food? Was your patient the only one who became sick after eating this food? If others also fell ill, this would raise suspicion of food contamination or a similar cause and decrease the likelihood of a hypersensitivity reaction.

•Timing. Allergic reactions occur quickly, usually within 1 hour of ingestion. If the adverse reaction took longer, find out what other foods were eaten at the same time; some foods can delay digestion and consequently any allergic reaction.

•Quantity. The amount of suspect food eaten can affect the severity of the reaction.

•Preparation. If the food was undercooked, your patient may have reacted to contaminants that were not destroyed by complete cooking.

•Reaction to treatment. If antiallergy treatments relieve your patient’s symptoms, this raises suspicion of allergy, although it may not rule out other causes.

Dietary testing. You might also ask your patient to record the contents of each meal and any adverse reaction in a diet diary. This will help to narrow down the possible culprits for a food allergy.

An elimination diet involves deleting the suspect food from the diet. If this provides symptomatic relief, you can make a provisional diagnosis. The return of symptoms when the patient resumes eating this food further supports the diagnosis. However, this strategy will not be effective if the patient’s symptoms occur infrequently, and it is contraindicated if those symptoms are severe.

Skin tests. Prick or scratch tests are quick, easy, and safe. However, they are not conclusive; your patient may have a positive reaction to a specific food allergen but not be allergic to that food. A conclusive diagnosis includes a positive test result and a history of allergic reaction to that food.

Skin testing is inappropriate for patients with severe eczema or anaphylactic reactions.

Laboratory tests. If a skin test is contraindicated, you may order a radioallergosorbent test (RAST) or enzyme-linked immunosorbent assay (ELISA). Again, a positive RAST or ELISA result is not in itself conclusive.

Food challenges. The procedure involves placing various foods, some of which are possible causes of allergic reactions, in opaque capsules. The patient swallows a capsule; if no allergic reaction develops, he or she swallows the next capsule, and continues until all the capsules have been consumed. In a double-blind procedure, another clinician prepares the capsules so that neither you nor the patient knows what foods are in which capsules.

Although this test is the gold standard for allergy testing, it is expensive, time-consuming, and inappropriate for patients with a history of severe allergic reactions or multiple food allergies.

TREATMENT

Avoidance. The only effective treatment for food allergy is to remove the offending food from the patient’s diet. Unfortunately, some of the foods most often associated with allergic reactions are used as ingredients in an astonishing variety of products. Even a tiny amount of an allergy-producing food (eg, 1/44,000 of a peanut) can produce a reaction in highly allergic patients.

Patients can avoid this danger by reading the list of ingredients that appears on the label or packaging of prepared foods. Make sure that your patient knows how to read these labels carefully. Also advise patients to avoid eating in restaurants as much as possible, since the chef may use recipes that include the food in question.

Inadvertent exposure. Even the most compliant patient may ignore or misread a label, and few will avoid restaurant meals altogether. Patients with a history of anaphylactic reactions to a food should wear medical alert bracelets warning that they are subject to severe food allergy reactions. Provide such patients with a syringe of epinephrine (eg, EpiPen) and instruct them on how to self-administer it in the event of a hypersensitivity reaction. Advise them to go to an emergency department or seek medical assistance immediately.

Antihistamines may provide relief for less severe allergic reactions, such as GI symptoms, respiratory symptoms, or hives. Bronchodilators can relieve asthma symptoms.

Children. Treating allergies in infants and children requires special measures. Elemental formulas that consist mainly of sugars and amino acids and contain few allergens can be used if an infant has allergic reactions to cow’s milk or soy formula. Exclusive breast-feeding can delay the onset of allergies by eliminating exposure to allergy-producing foods and is recommended for the first 4 to 6 months of life.

Parents of children with food allergies need instruction in treating inadvertent exposures, including the administration of epinephrine. Also, confirm that any institutions that provide care for the child, such as schools or day-care centers, are prepared to supervise the child’s diet and to respond to all levels of allergic reactions.