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Patient information: Food allergy symptoms and diagnosis (Beyond the Basics)

FOOD ALLERGY OVERVIEW

Reactions to food are common, and can be divided into two categories, those caused by food allergy and all other reactions. It is important to know the difference between food allergies and other illnesses or symptoms caused by foods because the management of each is different.

  • Food allergies develop when the body's immune system has an abnormal reaction to one or more proteins in a food. Food allergies can lead to serious or even life-threatening allergic reactions.
  • Other food reactions are not caused by the immune system. These reactions cause unpleasant symptoms and are far more common than food allergies. Examples include lactose intolerance, heartburn (gastroesophageal reflux), bacterial food poisoning, and sensitivity to caffeine, just to name a few.

This topic discusses the signs and symptoms of food allergy and testing that may be recommended to diagnose food allergies. The avoidance of foods in people with food allergies is discussed separately. (See "Patient information: Food allergy treatment and avoidance (Beyond the Basics)".)

CLASSIC FOOD ALLERGIES

In people with "classic" food allergies, allergic antibodies, called IgE, develop in response to proteins in certain foods. When the person is exposed to that protein at a later time (eg, by eating peanuts), binding of the food protein to IgE triggers a release of chemicals, which cause the symptoms of an allergic reaction. This typically occurs quickly, within minutes to two hours after eating.

Sudden-onset symptoms — The symptoms of a food allergy can vary from mild to severe or even life threatening. It is not always possible to predict how severe symptoms will be based upon the symptoms experienced during a previous reaction. As an example, a person could have mild hives after eating peanuts on one occasion, and then have an anaphylactic reaction after eating peanuts another time (see "Clinical manifestations of food allergy: An overview"). However, reactions are not necessarily worse after each exposure.

The most common sudden-onset symptoms of food allergy include:

  • Skin: Itching, flushing, hives (urticaria, like mosquito bites), or swelling (angioedema)
  • Eyes: Itching, tearing, redness, or swelling of the skin around the eyes
  • Nose and mouth: Sneezing, runny nose, nasal congestion, swelling of the tongue, or a metallic taste
  • Lungs and throat: Difficulty getting air in or out, repeated coughing, chest tightness, wheezing or other sounds of labored breathing, increased mucus production, throat swelling or itching, hoarseness, change in voice, or a sensation of choking
  • Heart and circulation: Dizziness, weakness, fainting, rapid, slow, or irregular heart rate, or low blood pressure
  • Digestive system: Nausea, vomiting, abdominal cramps, or diarrhea
  • Nervous system: Anxiety, confusion, or a sense of impending doom

Associated conditions

Anaphylaxis — Generalized anaphylaxis is the most serious type of allergic reaction, and can cause life-threatening signs and symptoms, including difficulty breathing, swelling of the upper throat and/or tongue, a very rapid or irregular heartbeat, low blood pressure, or cardiac arrest (the heart stops beating). (See "Patient information: Anaphylaxis symptoms and diagnosis (Beyond the Basics)".)

Generalized anaphylaxis generally begins within 5 to 60 minutes of exposure to a trigger, although rarely, symptoms begin several hours after eating. A person who develops symptoms of anaphylaxis must be treated immediately with an injection of epinephrine. Treatment of anaphylaxis is discussed separately. (See "Patient information: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)".)

Many different foods can potentially trigger anaphylaxis. In adults, peanuts, tree nuts (eg, walnuts), fish, and shellfish cause most anaphylactic reactions. In children, peanuts and tree nuts are the most common causes of anaphylactic reactions.

Allergic rhinitis and conjunctivitis — Food allergies can trigger allergic symptoms in the nose, eyes, or throat. However, these symptoms usually occur along with whole-body symptoms, such as hives, difficulty breathing, diarrhea, etc. The most common nose, eye, and throat symptoms including a runny nose, congestion, sneezing, nasal itching, itchy or watery red eyes, or voices changes.

Oral allergy syndrome — Oral allergy syndrome, or pollen-food allergy syndrome, is seen in up to 50 percent of people with allergic rhinitis caused by pollen. In this condition, people who are allergic to pollen have an allergic reaction after eating certain raw (uncooked) fruits or vegetables. The reaction is immediate and can cause itching, irritation, and mild swelling of the lips, tongue, roof of the mouth, and throat. A list of pollens and foods that cross react is available here (figure 1 and figure 2). (See "Clinical manifestations and diagnosis of oral allergy syndrome (pollen-food allergy syndrome)" and "Management and prognosis of oral allergy syndrome (pollen-food allergy syndrome)".)

Symptoms of oral allergy syndrome may be more noticeable during the associated pollen season. Symptoms usually resolve within minutes after the person stops eating the food. Most people have only localized symptoms (eg, in the mouth).

Less than 10 percent of people have systemic symptoms to fruits and vegetables (eg, vomiting or diarrhea) and 1 to 2 percent of people develop generalized anaphylaxis (see 'Anaphylaxis' above). People with a history of systemic symptoms should carry epinephrine autoinjectors.

The reaction does not usually occur if the fruits or vegetables are cooked. Tree nuts and peanuts are an exception to this, meaning that anyone with a history of an oral allergy to nuts should avoid them in all forms (raw, roasted, cooked).

Food dependent exercise-induced anaphylaxis — There are some people who develop an anaphylactic reaction after eating a certain food and then exercising up to four hours later. A reaction can occasionally occur after exercising first and then eating. The particular food does not cause anaphylaxis if the person does not exercise. This is called food dependent exercise-induced anaphylaxis. (See "Exercise-induced anaphylaxis: Clinical manifestations, epidemiology, pathogenesis, and diagnosis" and "Exercise-induced anaphylaxis: Management and prognosis".)

The most common foods associated with this condition include wheat, celery, and seafood, although some people react after eating any food and then exercising. Not eating for several hours before exercise can usually prevent this type of reaction.

CONDITIONS THAT MAY BE RELATED TO FOOD ALLERGIES

There are several conditions that may be food related, such as eosinophilic gastrointestinal disorders (eosinophilic esophagitis) and atopic dermatitis (eczema). These are discussed in detail separately. (See "Clinical manifestations and diagnosis of eosinophilic esophagitis" and "Patient information: Atopic dermatitis (eczema) (Beyond the Basics)".)

NON IgE FOOD ALLERGIES

Food allergies can occur without involving IgE. The symptoms of this type of food allergy are usually slower to develop and longer lasting than those of classic food allergies.

The three main types of non IgE food allergies are:

Most of these conditions cause symptoms of the digestive system, such as vomiting, diarrhea, abdominal pain, and/or blood in the stool. Food protein-induced enterocolitis and proctitis/proctocolitis are more commonly seen in infants. (See "Food protein-induced proctitis/colitis and enteropathy of infancy".)

FOOD ALLERGY DIAGNOSIS

Anyone who has signs or symptoms of a food allergy should see their healthcare provider. Between 20 and 30 percent of people report food allergy in themselves or their children. However, only 6 to 8 percent of children under the age of five and 3 to 4 percent of adults have a true food allergy.

Laboratory testing and/or skin testing is often used to confirm the food allergy and determine if avoidance of a particular food is necessary. (See "Diagnostic evaluation of food allergy".)

Medical history — During a medical history, the healthcare provider will ask questions about the person's past allergic reactions:

  • What symptoms of food allergy did you have?
  • What particular food do you think provoked the reaction? Have you eaten this food before? Have you reacted before?
  • How much of this food did you eat?
  • What other foods did you eat at that time? Do you know all the ingredients of the food you ate? Include all foods: appetizer, main dish, sauces, dressings, breads, beverages, and side dishes.
  • How was the food prepared? As an example, could the food have been fried in oil used to prepare other foods?
  • Were any of following eaten: peanuts, tree nuts, shellfish, fish, milk, eggs, wheat, or soy?
  • How much time passed between eating the food and the first symptoms?
  • Did you exercise or exert yourself after eating?
  • Did you take any medications, herbs, vitamins, nonprescription medications, or drink any alcohol before or after eating?
  • How was the reaction treated? Did it resolve without treatment or did you take any medications? How long were the medications continued and were there any later symptoms?

Depending upon the answers to these questions and the physical examination, the healthcare provider may decide to order laboratory testing. In other cases, the provider will refer the person to a specialist (allergist or gastroenterologist) for further evaluation.

Allergy testing — Testing for food allergies often includes skin testing and/or blood tests. Depending upon the situation, tests may be done to determine if a person is allergic to pollens, insects, latex, and other allergens. However, testing is only recommended if the person is suspected to have an allergy. As an example, if a person had a reaction after eating peanuts, but has never reacted to wheat or eggs and eats them regularly, it is not necessary to test for allergy to wheat or eggs.

Skin testing — Skin testing involves pricking/scratching the skin with a tiny probe that is coated with food extract or fresh food. The pricks are usually done on the forearm or upper back after the skin is cleaned with alcohol. The skin prick is not usually painful.

Adults and children of any age may have skin testing. The test may result in small hives (like a mosquito bite) at the site of the prick/scratch. Only a trained healthcare provider, usually an allergy specialist, should do skin testing. This specialist will evaluate the size of the test results to assist in making a diagnosis.

Blood tests — Blood tests are available to assist doctors in making a diagnosis. Neither the skin test nor the blood test can be depended upon to make a diagnosis without a clinician considering the medical history and other supporting information. Blood tests are widely available and do not require an allergy specialist to perform the test. However, consultation with an allergy specialist may be recommended to interpret the results of the test.

Elimination diets — An elimination diet is a specially designed diet that eliminates one or more foods or groups of food from a person's diet for a period of time. The food is then added back to determine if signs or symptoms of a food allergy develop.

An elimination diet may be recommended as part of the process of determining if a person has food allergies. An allergist or dietitian must be involved in designing an elimination diet because avoiding entire groups of foods (eg, milk) could potentially lead to malnutrition, especially in infants and children. An elimination diet, by itself, does not often lead to the diagnosis of food allergy.

During an elimination diet, it is important to read food labels carefully. In the United States, the Food Allergen Labeling and Consumer Protection Act mandates that nutritional labels on food packages plainly identify eight specified food allergen sources (milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, wheat, and soy), although other foods may still appear under multiple names. (See "Patient information: Food allergy treatment and avoidance (Beyond the Basics)".)

In addition, patients must understand that "substitute" foods, which reduce or eliminate fat or other components of a food, still contain the allergenic proteins. As an example, some egg substitutes (which are lower in cholesterol) still contain egg white proteins.

Food diary — A healthcare provider may request that a person keep a complete record of everything they eat over a period of time, including all foods, drinks, condiments, and candies. A table to record this information is available here (form 1).

Food challenges — If it is not clear if a person has a food allergy, based upon their medical history and allergy testing, he or she may be offered a medically supervised gradual feeding or food challenge. A food challenge may also be recommended if there is reason to believe that the food allergy has resolved. A food challenge is done by giving the person a tiny amount of the potentially allergenic food to eat. (See "Oral food challenges for diagnosis and management of food allergies".)

After the person is given the first sample of food, he/she is observed for 10 to 15 minutes. If there is no reaction, a slightly larger amount of the food is given. This is continued for approximately 90 minutes or more. If the person develops signs or symptoms of an allergic reaction, the food challenge is immediately stopped.

Food challenges should only be performed in a setting where the personnel and equipment needed to treat anaphylaxis are available; this can be an office or hospital setting.

  • Preparing for the food challenge — It is important to prepare for a food challenge by not eating or drinking for two hours before the test, and certain medications may need to be stopped days or weeks before. The person should bring their epinephrine autoinjector to the food challenge in case they develop a delayed allergic reaction on the way home.
  • If there is no reaction during the food challenge — If the person does not have any signs of an allergic reaction during the food challenge, he/she probably does not have an allergy to the tested food. However, the person may have allergies to other foods, so it is important to understand when/if foods should continue to be avoided.

WHEN TO SEEK HELP

It is sometimes difficult to know if a reaction is caused by a true food allergy or a food intolerance. Anyone who has one or more of the following symptoms after eating should seek medical care:

  • Nausea or vomiting
  • Cramping, abdominal pain, or diarrhea, especially if there is blood or mucus in the stool
  • Itching or raised red welts on the skin
  • Flushed (reddened, warm) skin
  • Swelling of the lips, mouth, face, or throat
  • Wheezing, coughing, or difficulty breathing
  • Lightheadedness or passing out

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem.

This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient level information — UpToDate offers two types of patient education materials.

The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.

Patient information: Food allergy (The Basics)
Patient information: Starting solid foods with babies (The Basics)
Patient information: Lactose intolerance (The Basics)
Patient information: Angioedema (The Basics)
Patient information: Eosinophilic esophagitis (The Basics)
Patient information: Allergy skin testing (The Basics)
Patient information: Peanut allergy (The Basics)

Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.

Patient information: Food allergy treatment and avoidance (Beyond the Basics)
Patient information: Anaphylaxis symptoms and diagnosis (Beyond the Basics)
Patient information: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)
Patient information: Atopic dermatitis (eczema) (Beyond the Basics)
Patient information: Celiac disease in adults (Beyond the Basics)

Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.

Allergic and asthmatic reactions to food additives
Clinical manifestations of food allergy: An overview
Clinical manifestations and diagnosis of oral allergy syndrome (pollen-food allergy syndrome)
Management and prognosis of oral allergy syndrome (pollen-food allergy syndrome)
Diagnostic evaluation of food allergy
Management of food allergy: Avoidance
Food allergy in schools and camps
Food-induced anaphylaxis
Future therapies for food allergy
Oral food challenges for diagnosis and management of food allergies
Primary prevention of allergic disease: Maternal avoidance diets in pregnancy and lactation
Respiratory manifestations of food allergy
Peanut, tree nut, and seed allergy: Clinical features
Seafood allergies: Fish and shellfish
Milk allergy: Clinical features and diagnosis
Egg allergy: Clinical features and diagnosis
The impact of breastfeeding on the development of allergic disease
Food allergy in children: Prevalence, natural history, and monitoring for resolution
Anaphylaxis in infants
Clinical manifestations and diagnosis of eosinophilic esophagitis
Food protein-induced proctitis/colitis and enteropathy of infancy

The following organizations also provide reliable health information.

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Literature review current through: Aug 2014. | This topic last updated: May 24, 2013.
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