The term food allergy is often misapplied, leading many people to believe that they are allergic to certain foods. A more accurate term would be food intolerance for many of these people. Food allergy symptoms can be caused either by a true allergic reaction to food or by food intolerance to specific components of a food.
Food allergy is an allergic reaction that involves the immune system and is not related to the amount of food eaten. Food intolerance, on the other hand, is an adverse reaction to a food component that does not involve the immune system and may vary with the amount of food eaten.
Symptoms may range from an upset stomach to life-threatening anaphylaxis. Identification and avoidance of any foods that trigger a reaction is the only cure available for food allergy or food intolerance.
What Is a Food Allergy?
A food allergy refers to a specific allergic reaction to food that involves the immune system. It is triggered by a particular food and has reproducible effects. In other words, the same symptoms—for example, wheezing or rash—must occur each time the food is eaten. It can also be called a hypersensitivity reaction.
What Causes a Food Hypersensitivity Reaction?
Food hypersensitivity is triggered by allergens—food components (usually proteins) that react with the immune system. Many different allergens can be present in the same food. Cow's milk, eggs, shellfish (shrimp, crab, or lobster), peanuts, tree nuts (walnuts, cashews, Brazil nuts), fish, soy, wheat, and even sesame seeds are the allergens that often trigger food hypersensitivity reactions.
Food hypersensitivity, especially to cow's milk, is seen more often in children than in adults. Many children outgrow food hypersensitivities in later years, with sensitivity to milk, egg, and soy the most likely to wane over time. People who are sensitive to seafood or nuts however, will probably have to avoid those foods forever.
Why Do Reactions Occur?
Like other types of allergies, such as hayfever, people who are allergic to foods are more reactive to the allergens found in these foods. Allergic reactions to food result from the actions of a specific group of proteins called antibodies. Antibodies, an important part of the body's defense system, are activated when the offending food is eaten. Their role is to recognize foreign invaders (antigens)—in this case, allergens—and get rid of them. It is not known why the body will suddenly react to these food antigens as foreign invaders, but it appears to be genetic.
Certain types of antibodies, known as IgE, are more highly reactive, and usually more abundant, in people with food hypersensitivity.
When IgE antibodies encounter a food allergen to which they are sensitized, they attach themselves onto the food proteins. This attachment causes the immune system to release mediators—chemical messengers that travel through the bloodstream to alert other organs to the presence of an unwelcome protein guest.
Release of these mediators causes the uncomfortable—or occasionally dangerous—symptoms of an allergic reaction. The extent of the reaction depends on the quantity of food eaten, age and health status, and the route taken by the mediators. The 3 most common reaction sites include:
Symptoms of allergic reaction in the skin include rash, hives, and swelling of the skin and tissues of the face, lips, mouth, and throat, larynx, extremities, and genitalia. Itching and eczema (an itchy, scaly skin rash) are also common but usually occur several hours to days after the offending food is ingested. Hives and swelling are usually immediate reactions, typically occurring within minutes (sometimes seconds) of food ingestion. They sometimes require immediate treatment.
Mediators that travel to the GI tract may cause symptoms along its entire length: swelling of the lips, itchy mouth or throat, nausea and vomiting, cramps and bloating, abdominal distention or intense abdominal pain, and diarrhea.
Anaphylactic shock is an extremely severe and life-threatening type of allergic reaction. The symptoms occur in rapid succession, progressing from itching or throat swelling to difficulty breathing, and loss of consciousness or even death, if emergency treatment is not started.
Food intolerance is an adverse reaction to a food component that does not involve the immune system. Reactions to the suspected food may vary in severity, sometimes depending on the amount of food with that component eaten. Therefore, food intolerance symptoms may be inconsistent. Distinguishing food intolerance from food hypersensitivity is often difficult, since the symptoms may be quite similar. Food intolerances can be attributed to a number of different causes, including:
- Enzyme deficiency, such as lactose intolerance (the inability to produce the enzyme lactase, which digests lactose sugar in milk)
- Gluten sensitivity
- Celiac disease
- Chemical additives, such as aspartame, dyes, nitrites, MSG, and sulfites
- Psychological disorder, such as food aversion
Sometimes the cause of food intolerance is unknown.
Diagnosing Food Hypersensitivities
Diagnostic tests must be done to determine if a true food allergy is present. These might include:
- Skin prick test—Extracts of suspected food antigens are dropped on the skin and the area is pricked or scratched through the drop. A positive result (raised bump) proves that immune cells in the skin are reacting to the antigen.
- Blood tests—These test indicate the presence of IgE, the antibody most active in food hypersensitivity reactions. The test can help identify which foods to avoid by matching up specific food antigens and IgE antibodies.
- Challenge test—The patient is fed suspected foods under carefully controlled conditions, and any reactions are noted. This is can be dangerous, however, if there is a risk of anaphylactic shock. The challenge test is only performed in certain situations.
- Food diary—If the diagnosis remains in question, the patient may be asked to keep a food diary for 1-2 weeks, writing down all foods eaten and noting any reactions. Any suspected foods are then eliminated to see if the symptoms clear up.
- Patch testing—While this type of test is not commonly recommended, it may be used in some cases. The food is placed under a dressing on the skin for 48 hours and the area is watched for any signs of reaction.
What Are the Treatment Options?
Dietary modification is the usual method of treating food hypersensitivity with elimination of the allergy-causing food. An elimination diet can become very limiting, so you may be referred to a dietitian who can explain the diet and make sure your nutrition requirements are being met.
Although allergy injections and antihistamine medications work for pollen or inhaled allergens, these remedies are not used for food hypersensitivities or food intolerances. Strict avoidance of the offending foods is one treatment. This is a serious challenge—all restaurant dishes and food labels must be examined.
Sublingual immunotherapy may be used. This type of treatment involves putting the allergic substances under the tongue to decrease or eliminate your response to a particular allergen.
When food hypersensitivity occurs, treatment may be needed right away. In the face of anaphylaxis, such action may be life-saving. You may need to have an epinephrine injection to stop the reaction. Further treatment in an emergency room or doctor’s office is generally required. This may include antihistamines, steroids, or both.
While an initial anaphylactic reaction may not be severe, the second may be fatal because it occurs after the body has had time to build up antibodies specific to the offending food allergen. An allergist can help you determine if you are at risk for this reaction. Most people who have had significant anaphylactic reactions should always carry an automatic epinephrine injector device. It allows them to rapidly treat symptoms until they can receive emergency treatment.
Allergy & Asthma Network
American Academy of Allergy, Asthma & Immunology
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Last reviewed October 2016 by Michael Woods, MD Last Updated: 10/25/2016