Food allergy symptoms are most common in babies and children, but they can appear at any age. You can even develop an allergy to foods you have eaten for years with no problems.

Symptoms

The body’s immune system keeps you healthy by fighting off infections and other dangers to good health. A food allergy reaction occurs when your immune system overreacts to a food or a substance in a food, identifying it as a danger and triggering a protective response.

While allergies tend to run in families, it is impossible to predict whether a child will inherit a parent’s food allergy or whether siblings will have a similar condition. Some research does suggest that the younger siblings of a child with a peanut allergy will also be allergic to peanuts.

Symptoms of a food allergy can range from mild to severe. Just because an initial reaction causes few problems doesn’t mean that all reactions will be similar; a food that triggered only mild symptoms on one occasion may cause more severe symptoms at another time.

The most severe allergic reaction is anaphylaxis — a life-threatening whole-body allergic reaction that can impair your breathing, cause a dramatic drop in your blood pressure and affect your heart rate. Anaphylaxis can come on within minutes of exposure to the trigger food. It can be fatal and must be treated promptly with an injection of epinephrine (adrenaline).

While any food can cause an adverse reaction, eight types of food account for about 90 percent of all reactions:

  • Eggs
  • Milk
  • Peanuts
  • Tree nuts
  • Fish
  • Shellfish
  • Wheat
  • Soy

Certain seeds, including sesame and mustard seeds (the main ingredient in the condiment mustard), also are common food allergy triggers and considered a major allergen in some countries.

Symptoms of an allergic reaction may involve the skin, the gastrointestinal tract, the cardiovascular system and the respiratory tract. They can surface in one or more of the following ways:

  • Vomiting and/or stomach cramps
  • Hives
  • Shortness of breath
  • Wheezing
  • Repetitive cough
  • Shock or circulatory collapse
  • Tight, hoarse throat; trouble swallowing
  • Swelling of the tongue, affecting the ability to talk or breathe
  • Weak pulse
  • Pale or blue coloring of skin
  • Dizziness or feeling faint
  • Anaphylaxis, a potentially life-threatening reaction that can impair breathing and send the body into shock; reactions may simultaneously affect different parts of the body (for example, a stomachache accompanied by a rash)

Most food-related symptoms occur within two hours of ingestion; often they start within minutes. In some very rare cases, the reaction may be delayed by four to six hours or even longer. Delayed reactions are most typically seen in children who develop eczema as a symptom of food allergy and in people with a rare allergy to red meat caused by the bite of a lone star tick.

Another type of delayed food allergy reaction stems from food protein-induced enterocolitis syndrome (FPIES), a severe gastrointestinal reaction that generally occurs two to six hours after consuming milk, soy, certain grains and some other solid foods. It mostly occurs in young infants who are being exposed to these foods for the first time or who are being weaned. FPIES often involves repetitive vomiting and can lead to dehydration. In some instances, babies will develop bloody diarrhea. Because the symptoms resemble those of a viral illness or bacterial infection, diagnosis of FPIES may be delayed. FPIES is a medical emergency that should be treated with IV rehydration.

Not everyone who experiences symptoms after eating certain foods has a food allergy or needs to avoid that food entirely; for instance, some people experience an itchy mouth and throat after eating a raw or uncooked fruit or vegetable. This may indicate oral allergy syndrome – a reaction to pollen, not to the food itself. The immune system recognizes the pollen and similar proteins in the food and directs an allergic response to it. The allergen is destroyed by heating the food, which can then be consumed with no problem.

Triggers

Once a food allergy is diagnosed, the most effective treatment is to avoid the food. The foods most associated with food allergy in children are:

  • Milk
  • Eggs
  • Peanuts

Children may outgrow their allergic reactions to milk and to eggs. Peanut and tree nut allergies are likely to persist.

The most common food allergens in adults are:

  • Fruit and vegetable pollen (oral allergy syndrome)
  • Peanuts and tree nuts
  • Fish and shellfish

People allergic to a specific food may also potentially have a reaction to related foods. A person allergic to one tree nut may be cross-reactive to others. Those allergic to shrimp may react to crab and lobster. Someone allergic to peanuts – which actually are legumes (beans), not nuts – may have problems with tree nuts, such as pecans, walnuts, almonds and cashews; in very rare circumstances they may have problems with other legumes (excluding soy).

Learning about patterns of cross-reactivity and what must be avoided is one of the reasons why people with food allergies should receive care from a board-certified allergist. Determining if you are cross-reactive is not straightforward. Allergy testing to many items in the same “family” may not be specific enough – many times, these tests are positive, given how similar two food items in a “family” may look to the test. If you have tolerated it well in the past, a food that is theoretically cross-reactive may not have to be avoided at all.

Negative tests may be very useful in ruling out an allergy. In the case of positive tests to foods that you have never eaten but that are related to items to which you’ve had an allergic reaction, an oral food challenge is the best way to determine whether the food poses a danger.

Diagnosing Food Allergies

A food allergy will usually cause some sort of reaction every time the trigger food is eaten. Symptoms can vary from person to person, and you may not always experience the same symptoms during every reaction. Allergic reactions to food can affect the skin, respiratory tract, gastrointestinal tract and cardiovascular system. It is impossible to predict how severe the next reaction might be, and all patients with food allergies should be carefully counseled about the risk of anaphylaxis, a potentially fatal reaction that is treated with epinephrine (adrenaline).

While food allergies may develop at any age, most appear in early childhood. If you suspect a food allergy, see an allergist, who will take your family and medical history, decide which tests to perform (if any) and use this information to determine if a food allergy exists.

To make a diagnosis, allergists ask detailed questions about your medical history and your symptoms. Be prepared to answer questions about:

  • What and how much you ate
  • How long it took for symptoms to develop
  • What symptoms you experienced and how long they lasted.

After taking your history, your allergist may order skin tests and/or blood tests, which indicate whether food-specific immunoglobulin E (IgE) antibodies are present in your body:

  • Skin-prick tests provide results in about 20 minutes. A liquid containing a tiny amount of the food allergen is placed on the skin of your arm or back. Your skin is pricked with a small, sterile probe, allowing the liquid to seep under the skin. The test, which isn’t painful but can be uncomfortable, is considered positive if a wheal (resembling the bump from a mosquito bite) develops at the site where the suspected allergen was placed. As a control, you’ll also get a skin prick with a liquid that doesn’t contain the allergen; this should not provoke a reaction, allowing comparison between the two test sites.
  • Blood tests, which are a bit less exact than skin tests, measure the amount of IgE antibody to the specific food(s) being tested. Results are typically available in about a week and are reported as a numerical value.

Your allergist will use the results of these tests in making a diagnosis. A positive result does not necessarily indicate that there is an allergy, though a negative result is useful in ruling one out.

In some cases, an allergist may wish to conduct an oral food challenge, which is considered the most accurate way to make a food allergy diagnosis. During an oral food challenge, which is conducted under strict medical supervision, the patient is fed tiny amounts of the suspected trigger food in increasing doses over a period of time, followed by a few hours of observation to see if a reaction occurs. This test is helpful when the patient history is unclear or if the skin or blood tests are inconclusive. It also can be used to determine if an allergy has been outgrown.

Because of the possibility of a severe reaction, an oral food challenge should be conducted only by experienced allergists in a doctor’s office or at a food challenge center, with emergency medication and equipment on hand.