Diseases & Conditions
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 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 (see Anaphylactoid Versus Anaphylactic ).
Food allergies may start during infancy. They are most common among children whose parents have food allergies, allergic rhinitis, or allergic asthma. Infants and young children with food allergies tend to be allergic to the most common allergic triggers (allergens), such as those in eggs, milk, wheat, peanuts, and 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. Older children and adults tend to be allergic to nuts and seafood. Children may outgrow a food allergy. Thus, food allergies are less common among adults. However, 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, poor athletic performance, and depression. However, these associations have not been substantiated.
Some reactions to food are not an allergic reaction. For example, 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 (lactose—see Malabsorption: 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, monosodium glutamate (MSG) (see Gastroenteritis: Chinese Restaurant Syndrome ), some preservatives (such as metabisulfite), and dyes (such as tartrazine, 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.
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.
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 severe reaction. A rash may cover the entire body, the throat may swell, and the airways may narrow, making breathing difficult. Occasionally, this reaction is severe and becomes life threatening?an anaphylactic reaction.
For some people, allergic reactions to food (especially wheat or celery) occur only if they exercise immediately after eating the food. In a few people, eating certain foods triggers or worsens a migraine headache.
Doctors suspect a food allergy based primarily on the person's history. If a food allergy is suspected, skin prick tests with extracts from various foods may be done. 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, a radioallergosorbent test (RAST) may be done. A sample of blood is withdrawn, and doctors measure how much immunoglobulin E (IgE), a type of antibody, binds to various allergens. If the level of IgE against any of these allergens is abnormally high, that allergen is probably triggering the allergy.
If either test identifies a particular food, an oral challenge test may be done to confirm the diagnosis. In this 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.
Another way to identify the food allergy is an elimination diet. 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 several days 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 Some Trade Names ADRENALIN syringe in case they have a severe reaction.
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 placing drops of food extracts under the tongue is not effective.
Antihistamines are useful only for relieving hives and swelling. Cromolyn Some Trade Names CROLOM INTAL NASALCROM , taken by mouth, can also relieve symptoms. This form of cromolyn Some Trade Names CROLOM INTAL NASALCROM 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 Some Trade Names ADRENALIN to use when needed for severe reactions.
Last full review/revision September 2008 by Peter J. Delves, PhD
Source: The Merck Manual Home Edition