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* This is the Consumer Version. *

Food Allergy

By Peter J. Delves, PhD, University College London, London, UK

A food allergy is an allergic reaction to a particular food.

  • Food allergies are commonly triggered by certain nuts, peanuts, shellfish, fish, milk, eggs, wheat, and soybeans.

  • Symptoms vary by age and may include rashes, wheezing, a runny nose, and, occasionally in adults, more serious symptoms.

  • Skin prick tests, blood tests, and an elimination diet may help doctors identify the food triggering the allergy.

  • The only effective treatment is to eliminate the food from the diet.

Many different foods can cause allergic reactions. Allergic reactions to foods may be severe and sometimes include an anaphylactic reaction.

Food allergies may start during infancy. Children may outgrow a food allergy. Thus, food allergies are less common among adults. But if adults have food allergies, the allergies tend to persist throughout life.

Food allergies are sometimes blamed for such disorders as hyperactivity in children, chronic fatigue, arthritis, and depression, as well as poor athletic performance. However, these associations have not been substantiated.

Other reactions to food

Some reactions to food are not an allergic reaction.

Food intolerance differs from a food allergy because it does not involve the immune system. Instead, it involves a reaction in the digestive tract that results in digestive upset. For example, some people lack an enzyme necessary for digesting the sugar in milk (called lactose intolerance).

Other reactions to a food may result from contamination or deterioration of the food.

In some people, food additivescan cause a reaction that resembles but is not an allergic reaction. For example, some preservatives (such as metabisulfite) and dyes (such as tartrazine, which is a yellow dye used in candies, soft drinks, and other foods) can cause symptoms such as asthma and hives. Similarly, eating certain foods, such as cheese, wine, and chocolate, triggers migraine headaches in some people.

Causes

Almost any food or food additive can cause an allergic reaction. The most common triggers vary by age group.

Infants and young children with food allergies tend to be allergic to the most common allergy triggers (allergens), such as those in the following:

  • Eggs

  • Milk

  • Wheat

  • Peanuts

  • Soybeans

To prevent such allergies from developing, many parents avoid exposing their young children to these foods. However, new evidence calls this approach into question, and more study is needed.

For older children and adults, the most common triggers are allergens in

  • Nuts

  • Seafood

Being exposed to other allergens that are similar to those in foods (such as pollen) may trigger the production of antibodies to substances in food, resulting in a food allergy. This process is called sensitization. For example, children with peanut allergy may have been sensitized to peanuts when topical creams containing peanut oil were used to treat rashes. Also, many people who are allergic to latex are also allergic to bananas, kiwis, avocados, or a combination. Latex and these fruits contain similar allergens.

Food allergies are most common among children whose parents have food allergies, allergic rhinitis, or allergic asthma.

Symptoms

In infants, the first symptom of a food allergy may be a rash such as eczema ( atopic dermatitis) or a rash that resembles hives. The rash may be accompanied by nausea, vomiting, and diarrhea. By about age 1 year, the rash tends to develop less often, but children may wheeze, feel short of breath, or get a runny nose when they eat the food that triggers their allergy. By about age 10, food allergies—most commonly to milk and less commonly to eggs and peanuts—tend to subside. Allergies to airborne substances, such as allergic asthma and hay fever, may develop as food allergies subside.

When food allergies persist in older children and adults, reactions tend to be more severe. In adults, food allergies cause itching in the mouth, hives, eczema, and, occasionally, a runny nose and asthma. For some adults with a food allergy, eating a tiny amount of the food may trigger a sudden, severe reaction. A rash may cover the entire body, the throat may swell, and the airways may narrow, making breathing difficult—an anaphylactic reaction, which can be life threatening.

For some people, allergic reactions to food (especially wheat or shrimp) occur only if they exercise immediately after eating the food (called exercise-induced allergic reactions).

Some allergic reactions to food take hours to develop and cause symptoms such as abdominal pain, nausea, cramping, and diarrhea.

Diagnosis

  • A doctor's evaluation

  • Sometimes skin prick tests or an allergen-specific immunoglobulin test

  • An oral challenge test

  • An elimination diet

Doctors suspect a food allergy based primarily on the person’s history. Usually in adults, the allergy is obvious. But diagnosing a food allergy in children may be difficult. Some food allergies may be difficult to distinguish from many other digestive problems, such as celiac disease.

Skin prick tests with extracts from various foods may be done if a food allergy is suspected. A drop of each extract is placed on the person’s skin, which is then pricked with a needle. A skin reaction to a food tested does not necessarily mean that a person is allergic to that food, but no skin reaction means that an allergy to that food is unlikely.

Alternatively, an allergen-specific immunoglobulin (IgE) testmay be done. The immune system produces a different type of IgE in response to each allergen. For example, the IgE that is produced after pollen is inhaled differs from the IgE that is produced when nuts are eaten. For the test, doctors withdraw a sample of blood and determine whether IgE in the person's blood binds to a specific allergen used for the test. If binding occurs, the person has an allergy to that allergen.

If either test identifies a particular food, an oral challenge test may be done to confirm the diagnosis.

In an oral challenge test, the person is given another food (such as milk or applesauce) in two batches: one with the suspected food in it and one without the suspected food in it. Then the doctor observes as the person eats the food:

  • If no symptoms develop after the suspected food is eaten, the person is not allergic to the food.

  • If symptoms develop after the suspected food is eaten and not after the other food is eaten, the person is probably allergic to the suspected food.

An elimination diet is another way to identify a food allergy. Doctors may use only this diet to diagnose a food allergy or may use it after a skin prick test or an allergen-specific serum IgE test. An elimination diet involves the following:

  • The person stops eating all foods that may be causing the symptoms for about 1 week.

  • The doctor provides the diet the person is to follow.

  • Only the foods or fluids specified in the diet may be eaten, and only pure products should be used.

Following such a diet is not easy because many food products have ingredients that are not obvious or expected. For example, many rye breads contain some wheat flour. Eating in restaurants is not advisable because the person and the doctor need to know the ingredients of every meal eaten. If no symptoms occur, foods are added back one at a time. Each added food is given for more than 24 hours or until symptoms appear, and thus the allergen is identified. Or the doctor may ask the person to eat a small amount of a food in the office. The doctor then observes the person’s reaction to the food.

Did You Know...

  • People with severe food allergies should always carry antihistamines and an epinephrine syringe in case they have a severe reaction.

Treatment

  • An elimination diet

  • Drugs to relieve symptoms

People with food allergies must eliminate the foods that trigger their allergies from their diet.

Desensitization by first eliminating the food, then eating small amounts of the food, or by placing drops of food extracts under the tongue is being studied.

Antihistamines are useful only for relieving hives and swelling. Cromolyn, taken by mouth, can also relieve symptoms. This form of cromolyn is available only by prescription.

People with severe food allergies often carry antihistamines to take immediately if a reaction starts. They should also carry a self-injecting syringe of epinephrine to use when needed for severe reactions.

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* This is the Consumer Version. *