An introduction to food allergies and intolerances: Adverse reactions to food

Food allergies and intolerances, better defined as adverse reactions to food, are on the rise in the Western world, as are allergies in general. European studies estimated a prevalence rate of around 7.5% in children and 2% in adults.

Despite a constant increase in the prevalence of these disorders, there is still a lot of uncertainty about the mechanisms that underlie them, as well as many uncertainties that exist in the diagnosis that is often performed with non-scientific methodologies (or with scientific methods used improperly, as in the case of IgG tests).

Classification of adverse reactions to food

Adverse reactions to foods are classified into:

• Toxic
• Non-toxic

The former can more precisely be called poisoning reactions and are caused by the presence of “toxins” in food; for example, the toxic and sometimes fatal reaction that occurs after the ingestion of inedible fungi, or the reaction to potato solanine, botulism, etc., are also known. This type of reaction can occur in any individual provided that he or she has ingested a dose of toxin that triggers the symptoms.

Non-toxic reactions, on the other hand, are what we are going to discuss here and they are commonly due to individual susceptibility. This type of adverse reactions can be divided into food allergies and food intolerances.

Food allergies are adverse reactions to foods mediated by the immune system. Immunological reactions to food are classified into several types.

IgE-mediated allergies are those that have been most clearly characterized. These types of reactions are well defined and distinguished into generalized reactions or anaphylaxis, and organ-specific reactions.

Non-IgE-mediated reactions are antibody-mediated reactions, involving classes of immunoglobulins other than the IgE, in this case, IgG, and especially cell-mediated reactions. Many clinically defined (e.g. celiac disease) and infant gastrointestinal diseases belong to this group.

Food intolerances are adverse reactions to food that can be traced back to a non-immunological mechanism. They can be:

• Enzymatic (that is, due to the lack or absence of a specific enzyme, e.g. lactose intolerance, favism);
• Pharmacological (i.e. reactions to vasoactive amines or to additives contained in certain foodstuffs).

3 Diagnosis of adverse reactions to food

The diagnosis of adverse reactions to foods is in general complex, and has to be carried out by specialists competent in this specific field; in fact, as explained above, there is no single responsible mechanism and the number of existing food allergens is high, many of which do not have validated diagnostic methods for a reliable diagnosis.

1. Anamnesis: in other words, the collection and interpretation of data resulting from the patient interview; this is the essential prerequisite for a correct diagnosis of an allergy. In fact, from the medical history, the patient can provide fundamental data and details, especially if the suspicious symptomatology occurs only a few minutes after taking the food. The symptoms described by the patient and the food to which the symptomatology is suspected can be identified, prompting the doctor to prescribe any tests to identify the allergen. In particular, the essential elements to be noted in the case history are:

• The latency time is the interval between ingestion of the suspected food and the onset of symptoms. If the symptoms occur immediately after ingestion of the food, and if the latency time is large, it will be less easy to identify the food;
• The type of symptoms, that is, when the food allergy manifests itself with a typical symptomatology of IgE-mediated allergic reactions (oral syndrome, hives, angioedema, eczema, rhinitis, asthma, acute gastroenteric symptoms);
• The duration of the symptoms, that is, whether the symptoms of the food allergy resolve after a few hours or not;
• The reproducibility, that is, when the symptoms related to the food allergy happen again every time the suspected food is ingested.

2. Diagnostic tests

Cutireation: This involves applying a drop of allergenic extract to the skin of the forearm, allowing it to penetrate into the superficial layers of the skin through the tip of a tiny sterile lancet. Reactions occur within 15-20 minutes of testing and positive reactions are identified by the swelling (similar to a mosquito bite) of the area where the allergen was injected. Cutireations cannot be performed if antihistamine therapy is being performed since the latter would inhibit the reaction. 

Total IgE Levels: The detection of high levels of total IgE (after excluding parasitic infections which also cause an increase) is an indication of an allergic state, but it cannot be used in the diagnosis of specific allergies. It should also be stressed that a finding of normal values does not exclude the diagnosis of an allergy.

Detection of specific IgE: Extracts of suspected foods are allowed to react with the patient’s plasma in a tube. If the subjects are allergic to a specific type of food, the food-specific antibodies will produce a reaction that can be measured with laboratory instruments.

The diagnostic accuracy of cutireations and the dosage of specific IgE can vary: the results should therefore always be interpreted considering the patient’s medical history and confirmed with the use of exclusion diets and triggering tests.

The presence of IgE in the blood is not always accompanied by the presence of symptoms of food allergy. The reason why the levels of IgE antibodies are not always in line with the symptomatology is not clear. However, if the immunological tests are all negative, an IgE-mediated food allergy can be excluded; if the tests are positive for a food that is clearly responsible for the symptoms, then an IgE-mediated allergy can be confirmed with both a triggering test or an exclusion diet specific for that food.

3. Exclusion diets

Exclusion diets may be helpful in patients with persistent symptoms when strongly suspected foods and/or diets that have been identified, for instance using the diagnostic tests mentioned before. These types of diets are conducted for three weeks and, in the case of an improvement, the diet period should be extended to achieve a clear result.

The use of the so-called oligoallergenic diets, which are based on very few foods considered to be poorly allergenic and which are usually prescribed for a period of three to five weeks, are no longer recommended. These diets are not based on scientific principles and they tend to produce some benefits only thanks to the placebo effect and when the foods that have been excluded are reintroduced into the diet, the symptoms reappear without a clear connection to any food.

4. Placebo-controlled oral provocation test

The only correct way to safely diagnose an adverse reaction to food is the so-called placebo-controlled double-blind oral provocation test. Translated into more understandable words, this means that a strictly-controlled triggering test is carried out using the suspected food and a placebo as the control. Neither the person carrying out the tests nor the patient should know what is being administered (either the allergenic food or the placebo). This test helps verify whether there is a genuine reaction to a specific food that does not occur when the placebo is administered.

It is used after the tests mentioned above have been carried out, in order to obtain a more certain diagnosis. During the test, the suspected food is administered in dry or freeze-dried form, in opaque capsules, or masked in an inert matrix. The placebo consists of equal-looking capsules containing dextrose or the inert matrix, which is made of liquid or solid foods that are known to be tolerated by the patient. This test needs to be done in an equipped health facility.