The Differences Between Food Allergies and Food Intolerance

Peanut butter sandwich with a brown paper bag and bunch of grapes
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Adverse reactions to foods have become mainstream and almost “normal” in our society. People talk about their “food allergies” as openly as they discuss the weather and current events. In fact, with so many celebrities talking about their food allergies and intolerances, it’s in vogue and fashionable to have some type of adverse reaction to food.

But what about the science behind these reactions? What is an allergy and what is not? We often tell patients that a symptom caused by eating a certain food isn’t necessarily a food allergy. When an allergist refers to an allergy, this means that there is an IgE-mediated process causing the release of allergic chemicals leading to symptoms consistent with a food allergy. There are also a number of reactions to foods that are not allergic, but rather food intolerances. Some of these intolerances can be caused by the immune system, while others are not.

IgE Food Allergy

There are a number of reactions to foods that can be considered a true food allergy, meaning that IgE is involved in causing the reaction. The symptoms of the most severe type of food allergy typically includes some form of skin symptom (urticaria and angioedema, itching or flushing) and may include other symptoms such as gastrointestinal (nausea, vomiting, abdominal cramping, diarrhea), respiratory (coughing, shortness of breath, wheezing), cardiovascular (dizziness, low blood pressure), ear-nose-throat (sneezing, throat clearing, itchy nose and eyes) and other symptoms (sense of impending doom).

There are also a number of IgE-mediated food allergies that cause less severe symptoms. These include worsening of atopic dermatitis, oral allergy syndrome, and eosinophilic esophagitis/eosinophilic gastrointestinal diseases (although there are non-IgE food allergy causes of these conditions as well).

IgE-mediated food allergies are usually diagnosed quite easily with the use of allergy skin testing and/or blood testing.

Immunologic Food Intolerances

There are a couple of reactions to foods that are not due to IgE but are still caused by the immune system. These reactions affect the gastrointestinal tract for the most part.

Celiac disease, or gluten-sensitive enteropathy, is caused by an antibody made by the immune system against the lining of the small intestinal as a result of eating gluten. Gluten is found in many cereal grains such as wheat, rye, and barley. Symptoms of celiac disease include bloating, cramping, diarrhea, weight loss and malabsorption of nutrients (leading to other complications such as anemia) – but constipation can also occur. Some people with celiac disease can experience a skin rash can dermatitis herpetiformis, which is characterized by blisters that are extremely itchy and tend to occur on the elbows and knees as well as the lower back and scalp. The diagnosis of celiac disease is best made with a biopsy of the lining of the small intestine but can also be made with a blood test for antibodies (tissue transglutaminase and endomysial). Some people will experience symptoms of celiac disease but with normal tests – this is often termed “gluten intolerance”.

Another type of immune-mediated food intolerance is FPIES (food protein induced enterocolitis syndrome). In this condition, which usually affects young children but can affect older children and adults, symptoms can include severe projectile vomiting, severe diarrhea, and lethargy. Children can actually appear “septic” and are often admitted to the hospital for possible severe infection. There are no skin rashes or respiratory symptoms with this condition. FPIES is difficult to diagnose because allergy testing does not show signs of a food allergy. The diagnosis is often made by clinical suspicion, although specialized medical centers can perform oral food challenges in a hospital setting and follow serial white blood cell counts to help make a diagnosis. A mild variant of FPIES is food protein induced proctitis, which is most commonly seen in infants who are started on milk or soy-based formulas and have blood in their stools as a result. This version is quite mild and the infant often has no other symptoms; the condition typically resolves when the child is switched to hypoallergenic baby formulas.

Non-Immune Mediated Intolerances

Reactions to foods not caused by allergy or other immune reactions are difficult to test for, hard to define, and are probably what most people are experiencing when they describe a food intolerance.

The most common of these reactions is lactose intolerance, is which characterized by bloating, cramping and diarrhea within hours of eating lactose-containing foods (dairy products). This condition is caused by a deficiency of lactase, the enzyme that breaks down lactose, leading to water entering the small intestine from the body as a result of the large lactose molecule present in the intestine. Allergy testing will not show signs of milk allergy. While a lactose tolerance test is available, the diagnosis of lactose intolerance is usually made on a clinical basis.

There are many other foods that can cause a non-IgE, non-immune mediated food intolerance. Again, most of these conditions result in some type of gastrointestinal upset. There are generally no tests available for these types of reactions.

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  • The American Academy of Allergy, Asthma and Immunology, and Food Allergy Practice Parameters. Ann Allergy Asthma Immunol. 2006; 96:S1-68.