Patient education: 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 a true food allergy and all other reactions. It is important to know the difference between food allergies and other types of reactions 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.

This article discusses the signs and symptoms of food allergy and tests that may be recommended to diagnose food allergies. The management of food allergy generally involves avoiding that food; this is discussed separately. (See "Patient education: Food allergen avoidance (Beyond the Basics)".)

CLASSIC (IgE-MEDIATED) FOOD ALLERGIES — In people with "classic" food allergies, the body reacts to proteins in certain foods as foreign or harmful and produces antibodies in response. These antibodies are called immunoglobulin E (IgE) antibodies. Then, when the person is exposed to that food protein again (through consuming the food), the food protein binds to the IgE antibodies, triggering a release of chemicals. This causes the symptoms of an allergic reaction. This typically occurs quickly, within minutes to two hours after eating. A person with a food allergy may also have a "local reaction" if the food touches their skin (hives or a rash at the site of contact), even if they don't actually eat it.

You may hear classic food allergy referred to as "IgE-mediated" food allergy.

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 a serious anaphylactic reaction after eating peanuts another time (see 'Anaphylaxis' below). However, reactions do not necessarily get worse after each exposure.

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

●Skin – Itching, flushing, hives (itchy bumps, also called "urticaria"), 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, changes in heart rate (fast, slow, or irregular), or low blood pressure

●Digestive system – Nausea, vomiting, abdominal cramps, or diarrhea

●Nervous system – Anxiety, confusion, or a sense of impending doom

Specific presentations

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 education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)".)

Generalized anaphylaxis generally begins within 5 to 60 minutes of exposure to a trigger (allergen), although, in rare cases, symptoms don't start until several hours after eating. A person who develops symptoms of anaphylaxis must be treated immediately with an injection of epinephrine. For this reason, if you or your child have been diagnosed with a severe (or potentially severe) food allergy, your clinician will prescribe an epinephrine autoinjector device to keep with you at all times (as well as extras if needed; for example, to keep at school or with a caregiver). The treatment of anaphylaxis is discussed separately. (See "Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)" and "Patient education: Using an epinephrine autoinjector (Beyond the Basics)".)

Allergic rhinitis and conjunctivitis — Food allergies can trigger allergic symptoms in the nose, eyes, or throat. The most common nose, eye, and throat symptoms include a runny nose; congestion; sneezing; nasal itching; itchy or watery red eyes; an itchy mouth, tongue, or throat; or voice changes. These can happen along with or before other whole-body symptoms such as hives, difficulty breathing, diarrhea, etc but rarely occur as the only sign of an allergic reaction to a food. One exception is that an itchy throat or mouth without any other symptoms typically represents oral allergy syndrome. (See 'Oral allergy syndrome' below.)

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 (also known as seasonal allergies). 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 in the figure (figure 1).

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

Less than 10 percent of people develop body-wide symptoms from 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 may be an exception to this as they are associated with a higher risk of severe reactions. If you have a history of an oral allergy to nuts, your allergist may recommend avoiding them in all forms (raw, roasted, or cooked) and/or limiting the amount you eat at once.

Food-dependent exercise-induced anaphylaxis — Some people develop an anaphylactic reaction after eating a certain food and then exercising afterwards (within about four hours of consuming the food). This is called "food-dependent exercise-induced anaphylaxis." A reaction can occasionally occur after exercising first and then eating the food. In this situation, the food does not cause anaphylaxis if the person does not exercise.

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.

MIXED IgE- AND NON-IgE-MEDIATED 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 (EoE)" and "Patient education: Eczema (atopic dermatitis) (Beyond the Basics)".)

NON-IgE FOOD ALLERGIES — It is possible to have a food allergy that does not involve IgE antibodies. With this type of food allergy, symptoms are usually slower to develop and last longer than those of classic (IgE-mediated) food allergies.

The three main types of non-IgE food allergies are:

●Food protein-induced enterocolitis syndrome (FPIES) – This is a serious type of allergy that mostly affects infants; it causes severe gastrointestinal symptoms like vomiting and diarrhea and can also cause low blood pressure. FPIES is most often triggered by cow's milk or soy protein, which are found in many infant formulas. It often resolves by the age of three to five years.

●Food protein-induced allergic proctocolitis (FPIAP) – This condition affects the lower part of the colon. It can cause rectal bleeding and diarrhea in infants.

●Celiac disease and dermatitis herpetiformis – Celiac disease is a condition in which the immune system responds abnormally to a protein called gluten, which then leads to damage to the lining of the small intestine. Gluten is found in wheat, rye, barley, and many prepared foods. Dermatitis herpetiformis is a skin condition that is also related to gluten sensitivity. (See "Patient education: Celiac disease in adults (Beyond the Basics)".)

FOOD ALLERGY DIAGNOSIS — If you suspect that you or your child may have a food allergy, it's important to see a clinician for evaluation. They will learn about your history (including past exposures to the food and what symptoms you have experienced) and do tests to determine whether you have a true food allergy and need to avoid a particular food. Taken together, all of this information can help a clinician diagnose a food allergy.

Medical history — During a medical history, the clinician will ask questions about your past reactions to food, such as:

●What symptoms did you have?

●What particular food do you think caused the reaction? Had you eaten this food before? If yes, how often were you eating it, when did you last eat it, and had you eaten the food without having a reaction?

●How much of the 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 the following eaten: peanuts, tree nuts, sesame, shellfish, fish, milk, eggs, wheat, or soy?

●How much time passed between eating the food and the first symptoms?

●Did you exercise or do other physical activity 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 clinician may decide to order blood tests (see 'Blood tests' below). In other cases, they will refer you to a specialist (such as an 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. Neither the skin test nor the blood test alone is sufficient to diagnose food allergy; the clinician must also consider the person's medical history and other supporting information.

Skin testing — Skin testing involves pricking/scratching the skin with a tiny needle that is coated with food extract or fresh food. It is done by a trained clinician (usually an allergy specialist). The pricks are usually done on the forearm or upper back after the skin is cleaned with alcohol. This should not be very painful, and any mild discomfort should resolve quickly.

Skin testing can be done on adults and children. If the person is allergic to a food used in the test, an itchy bump (hive) will form where the skin was pricked. After a period of time, the specialist will examine the skin to see whether hives have developed and, if so, measure the size. This can help inform a diagnosis, although the clinician will use other information as well.

Blood tests — Blood tests can check to see if a person has IgE antibodies in their system. 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 removes 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 see if signs or symptoms of an 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. It is especially important to remember that 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 nine specified food allergen sources (milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, sesame, wheat, and soy), although other foods may still appear under multiple names. (See "Patient education: Food allergen avoidance (Beyond the Basics)".)

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

Food diary — Your clinician may suggest keeping a complete record of everything you eat over a period of time, including all foods, drinks, condiments, and candies. A table to record this information is available here (form 1). As is true with an elimination diet, it is important to remember that a food diary by itself does not often lead to the diagnosis of food allergy.

Food challenges — If it is not clear if a person has a food allergy based upon their medical history and the results of allergy testing, a clinician might suggest a medically supervised "food challenge." This may also be recommended if there is reason to believe that the food allergy is improving or 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 in a hospital or allergy clinic; it involves giving the person a tiny amount of the potentially allergenic food to eat. After eating the first sample of food, the person 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, and treatment is given if necessary.

Food challenges should only be performed in a setting where the personnel and equipment needed to treat anaphylaxis are available.

●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. If you have an epinephrine autoinjector, you should bring it with you to the food challenge in case you develop a delayed allergic reaction on the way home.

●If there is no reaction during the food challenge – If you did the food challenge to find out if you have an allergy, and you do not have any signs of an allergic reaction during the food challenge, you are most likely not allergic to that food. However, you could still have allergies to other foods, so be sure you understand whether and when foods should continue to be avoided.

If you already have a known allergy to a food (such as egg or milk) but pass a challenge for that food in extensively heated form, you will still need 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 your 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

Having a food allergy can be challenging. But it is possible to have a full life and enjoy cooking and eating, as long as you are prepared to recognize and treat symptoms of an allergic reaction.

WHERE TO GET MORE INFORMATION — Your health care 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 website (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for health care 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 education: Food allergy (The Basics)
Patient education: Starting solid foods with babies (The Basics)
Patient education: Lactose intolerance (The Basics)
Patient education: Angioedema (The Basics)
Patient education: Eosinophilic esophagitis (The Basics)
Patient education: Allergy skin testing (The Basics)
Patient education: Peanut, tree nut, and seed 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 education: Food allergen avoidance (Beyond the Basics)
Patient education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)
Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)
Patient education: Eczema (atopic dermatitis) (Beyond the Basics)
Patient education: 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 IgE-mediated food allergy
Management of food allergy: Avoidance
Food allergy in schools and camps
Food-induced anaphylaxis
Experimental therapies for food allergy: Immunotherapy and nonspecific therapies
Oral food challenges for diagnosis and management of food allergies
Primary prevention of allergic disease: Maternal diet 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 (EoE)
Food protein-induced allergic proctocolitis of infancy

The following organizations also provide reliable health information.

●Medline Plus (medlineplus.gov/foodallergy.html; available in Spanish)

●American Academy of Allergy, Asthma & Immunology (AAAAI) (www.aaaai.org/conditions-and-treatments)

●Food Allergy Research & Education (FARE) (www.foodallergy.org/)

●American College of Allergy, Asthma & Immunology (ACAAI) (acaai.org/)

●National Institute of Allergy and Infectious Diseases (NIAID) (www.niaid.nih.gov/)

●Asthma and Allergy Foundation of America (AAFA) (www.aafa.org/)

●US Food and Drug Administration (FDA) (www.fda.gov/)

●US Department of Health and Human Services (healthfinder.gov/FindServices/)

●US Centers for Disease Control and Prevention (CDC) (www.cdc.gov/healthyschools/foodallergies/index.htm)