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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 review 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.
CLASSIC FOOD ALLERGIES — In people with “classic” food allergies, allergic antibodies, called immunoglobulin E (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.
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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. 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
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).
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.
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.
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 diarrhoea), and 1 to 2 percent of people develop generalised anaphylaxis. 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.
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).
NON-IgE FOOD ALLERGIES — Food allergies can occur without involving immunoglobulin E (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:
- Food protein-induced enterocolitis
- Food protein-induced proctocolitis
- Celiac disease and dermatitis herpetiformis
Most of these conditions cause symptoms of the digestive system, such as vomiting, diarrhoea, abdominal pain, and/or blood in the stool. Food protein-induced enterocolitis and proctocolitis are more commonly seen in infants.
FOOD ALLERGY DIAGNOSIS — Anyone who has signs or symptoms of a food allergy should see his or her 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.
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 Labelling 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.
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.
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. In addition, some foods such and egg and milk become less allergenic when they are extensively heated (eg, baked in bread or muffins), and a challenge may be performed to find out if the person is able to eat the food in this form. A food challenge is done by giving the person a tiny amount of the potentially allergenic food to eat.
After the person is given the first sample of food, he/she is observed for 10 to 30 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 – The doctor or nurse will provide specific instructions before the food challenge. It is important to prepare 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 did the food challenge to find out if he or she has an allergy and 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.
If the person already has a known allergy to a food (such as egg or milk) but passes a challenge for that food in extensively heated form, he or she still needs to be careful to avoid the food in raw or less cooked form. For example, a person with a milk allergy might be able to eat bread or processed foods that contain milk but still need to avoid drinking milk and eating dairy products like cheese or yogurt. A doctor or nurse will discuss the results of the food challenge and give recommendations on what to do moving forward.
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