Merck Manual

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Food Allergy

By

Peter J. Delves

, PhD, University College London, London, UK

Last full review/revision Oct 2020| Content last modified Oct 2020
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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 additives can 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 suggests that regularly feeding infants foods that contain peanuts may help prevent them from developing a peanut allergy. More study of this approach 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 more common among children whose parents have food allergies, allergic rhinitis, or allergic asthma.

Symptoms

Symptoms of food allergies vary depend on which food causes the allergy and how old the person is.

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 start to react to inhaled allergens (such as pollen) and have asthma symptoms. They may wheeze, feel short of breath, or get a runny nose when they eat the food that triggers their allergy. By about age 10, children rarely have asthma symptoms after the food is eaten.

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, swelling (angioedema), and, occasionally, a runny nose and asthma. Food allergies sometimes cause symptoms such as light-headedness or fainting.

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

  • Skin prick tests or an allergen-specific immunoglobulin 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 irritable bowel 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) test may 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, such as one for peanuts. If binding occurs, the person has an allergy to that allergen.

If either test identifies a particular food, that food is eliminated from the diet. If eliminating the food relieves symptoms, the food is given to the person again to see whether symptoms develop after it is eaten. When possible, this step is done as part of an oral challenge test. The oral challenge test is 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.

Other ways to identify a food allergy include elimination diets:

  • A diet that eliminates only the food or foods suspected of causing the allergy

  • A diet that consists only of foods not likely to cause an allergic reaction

An elimination diet may be the only test used to diagnose a food allergy or may be used after a skin prick test or an allergen-specific serum IgE test.

For the first type of elimination diet, the person stops eating all foods that may be causing the symptoms for about 1 week.

The second type of elimination diet, which consists of food not likely to cause allergic reactions, can be tried instead of the first type of diet. The second type of diet involves the following:

  • Following a diet prescribed by the doctor

  • Eating only the foods and liquids specified in the diet and only using pure products (which excludes many commercially prepared foods)

There are several possible elimination diets, which vary in the foods that are eliminated and allowed. For example, one diet may eliminate beef and lamb and allow chicken. Another may eliminate lamb and poultry and allow beef.

Following an elimination 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 symptoms are not relieved after 1 week, doctors may recommend a different elimination diet.

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.

Prevention

For many years, doctors have advised against feeding young infants foods that commonly trigger an allergic reaction (such as peanuts) as a way to prevent food allergies. However, new evidence suggests that regularly feeding infants foods that contain peanuts may help prevent them from developing a peanut allergy. More study of this approach is needed.

Parents should talk to their pediatrician about the best way to prevent peanut allergy in their child.

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. For example, there is a new peanut preparation that helps make people with peanut allergy less sensitive to peanuts.

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 should 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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