Food allergies 101

My clinical practice focuses on patients with gastrointestinal motility disorders and functional gastrointestinal disorders (FGID). I am frequently asked to see patients who report gastrointestinal (GI) symptoms after eating. These symptoms are non-specific and run the gamut from abdominal pain or discomfort, to epigastric fullness, postprandial nausea, bloating and diarrhea. Although quite common in the general population, with approximately 20% of adults reporting adverse GI symptoms after eating, these symptoms are even more prevalent in patients with FGID. For example, nearly two-thirds of patients with irritable bowel syndrome (IBS) report GI adverse events after eating.1 The non-specific nature of these symptoms is highlighted by the fact that, in the general population, when patients with food related symptoms are re-challenged with the offending food, only two to three percent develop recurrent symptoms. As I evaluate these patients and ask about their concerns, a common refrain is a worry that their symptoms represent a food allergy. Many patients ask whether they should be tested for a food allergy. For these patients I follow a step-wise approach, which is outlined below. First, I discuss the prevalence of true food allergies and the difference between a food allergy and a food intolerance. I then inquire whether they have classic food allergy symptoms. Finally, I review the utility of food allergy testing in the context of their symptoms, and also discuss the myriad of tests available to measure food intolerances, most of which have little value.

To set the stage for a meaningful patient discussion, it is important to briefly review the basics of food allergies. Although many patients believe that they are allergic to food, true food allergies are not common and only occur in one to three percent of adults, in contrast to the large number of adults with reported food intolerances. Atopic individuals are more likely to have food allergies, but they are not more prevalent in IBS patients. The most prevalent food allergies in adults, based on IgE testing, are shellfish (2%), peanuts (0.6%), tree nuts (0.6%), fish (0.4%), wheat (0.4%), cow’s milk (0.3%), eggs (0.2%), and sesame (0.1%).2,3 Food allergies are immune-mediated events and are classified as IgE related, non-IgE related or a mixed (IgE and non-IgE) response.2 A true food allergy is characterized by symptoms that occur reproducibly on exposure to a given food, rapidly (often within minutes), and which are absent when avoiding that specific food. IgE-mediated food allergies must be accompanied by evidence of sensitization with the development of specific IgE antibodies to a food allergen. A peanut allergy is the classic example of an IgE-mediated food allergy. In contrast, non-IgE food allergies are mediated by T-cells, are usually confined to childhood, and include food protein induced enterocolitis syndrome and food protein induced enterocolitis. Mixed IgE- and non-IgE mediated food allergies include cow’s milk protein allergy, eosinophilic esophagitis and eosinophilic gastroenteritis.

Although many patients believe that they are allergic to food, true food allergies are not common and only occur in one to three percent of adults, in contrast to the large number of adults with reported food intolerances.

Next, I question patients about their symptoms that they believe represent a food allergy. Patients should report a reproducible reaction to a food (e.g., itching of the palate and lips, angioedema, rhinorrhea, periorbital edema, dysphagia, laryngospasm or bronchospasm, nausea, vomiting, abdominal pain, diarrhea, urticaria, hypotension, anaphylaxis). Identifying a true food allergy is important given the potential ramifications of a patient developing a severe allergic response to a food (e.g., anaphylaxis). If none of these symptoms are present, I reassure the patient and generally do not recommend testing. For patients with symptoms suggestive of a food allergy, I arrange a consultation with an allergist at which time appropriate testing can be performed. Two tests are commonly used: a skin prick test or serum-specific IgE levels. Neither test is perfect, however. The skin prick test may be positive only 50% of the time in patients with true food allergies; however, the negative predictive value is 90 percent.2 Serum IgE levels correlate with the likelihood of a clinically relevant reaction to food, although levels do not correlate with the intensity of the reaction.3 The sensitivity of serum IgE levels is low; up to 25%  of clinically significant reactions, including anaphylaxis, may be missed.3

I then discuss the concept of food intolerance. A food intolerance is simply defined as an undesirable reaction to a food that is not immune mediated. These reactions may develop for a variety of reasons, including pharmacologic effects of foods (e.g., salicylates, vasoactive amines, caffeine, glutamate, serotonin, tyramine, capsaicin), enzyme defects (e.g., lactose, fructose, sucrose-isomaltase), transport defects (e.g., glut-2, glut-5), functional disorders (e.g., dyspepsia) or psychological factors (e.g., anorexia, orthorexia). In contrast to a food allergy, food intolerances generally develop more slowly, do not worsen over time, and are often intermittent, unpredictable and poorly identified with re-challenge. Sensitivity to gluten is one of the most commonly reported reactions to food by IBS patients; it is a non-immunologically mediated event thought to represent an adverse reaction to fructan. I remind patients that food intolerances develop for multiple reasons and that there is no single test that can make this diagnosis. I caution patients about using tests sold online or by alternative health care providers. These tests have not been validated and have not been subjected to rigorous, blinded trials (e.g., cytotoxic assays, electrodermal tests, hair analysis, iridology, kinesiology, facial thermography). In addition, these tests can be quite costly and may cause patients to unnecessarily restrict their diet. Of note, there is interest in assessing IBS symptoms based on diet changes using a leukocyte activation test. Provocative pilot data will need to be confirmed in large, multicenter trials.

Key takeaways

  • Food allergies are not common, occurring in only 1-3% of the adult population.
  • Food intolerances are quite common, especially in patients with functional gastrointestinal disorders, such as IBS (approximately 2/3 of patients).
  • Food allergies occur rapidly, are reproducible with a food challenge and can usually be identified with either a skin prick test or serologic testing.
  • Food intolerances develop more slowly, are frequently not reproducible, and cannot reliably be identified with current technology.



1. Bohn L., Storsrud S., Tornblom H., Bengtsson U., Simren M. Self-reported food-related gastrointestinal symptoms in IBS are common and associated with more severe symptoms and reduced quality of life. Am J Gastroenterol. 2013;108: 634-641.

2. Sischerer S.H., Sampson H.A. Food allergy. J Allergy Clin Immunol. 2010;125: S116-S125.

3. Turnbull J.L., Adams H.N., Gorard D.A. Review article: the diagnosis and management of food allergy and food intolerances. Aliment Pharmacol Ther. 2015;41:3-25.

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