While eating is necessary to sustain life, most people find eating to be an enjoyable experience given the variety and abundance of food available to them. However, for individuals with food allergies and related illnesses, consuming certain foods can be a debilitating, and possibly even lifethreatening, experience.

Consequently, the joy of eating is diminished by the ever-present fear of consuming a food or food ingredient that will cause an adverse reaction. Food-allergic consumers must assiduously avoid the offending foods and/or food ingredients because strict avoidance diets are the only available preventive strategy.

For such consumers, food selection often becomes a tedious task requiring meticulous reading of ingredient lists on labels, dependence on food manufacturers to maintain accurate labels, and a continual search for more knowledge about food composition.

For these individuals, food preparation requires careful attention to detail, cooking ‘from scratch’, and seeking alternative recipes for many dishes.

Because very small amounts of the offending food can elicit allergic reactions in some affected individuals, these consumers live in constant fear that, despite their caution, trace amounts of the offending food, sufficient to elicit an adverse reaction, might still exist in the foods that they consume.

They are concerned about ingredients derived from the offending food because such ingredients might contain residual allergenic proteins from the source food. This fear is compounded by the fact that declaration of the source of ingredients used in foods is not always required on food labels.

Definition and classification

Food allergies can be defined as adverse, immune-mediated reactions to foods that occur in certain individuals. Often, the public and even some within the medical community categorize all individualistic reactions to foods as food allergies.

However, true food allergies should be restricted to those individualistic reactions to foods that are mediated by the immune system. The term ‘food sensitivity’ can be used to refer to all types of individualistic adverse reactions to foods. These food-related illnesses are individualistic because they affect only a few people in the population.

Food intolerances are individualistic adverse reactions to foods that occur through nonimmunological mechanisms. Knowing the difference between immunological food allergies and non-immunological food intolerances is critical to proper management of these illnesses.

Food intolerances are often controlled by limiting the amount of food eaten; with food allergies, much more strict avoidance of the offending food is usually necessary. Food allergy is an abnormal immunological response to a food or food component; food allergens are almost always proteins. Examples include allergic reactions to common foods such as peanuts and milk.

Within this category are immediate hypersensitivity reactions where symptoms ensue within minutes to an hour after ingestion of the offending food and delayed hypersensitivity reactions where the onset of symptoms occurs 6–24 or more hours after ingestion of the offending food.

Immediate hypersensitivity reactions are mediated by immunoglobulin E (IgE) antibodies. Exercise-induced food allergies are a subset of food allergies involving immediate reactions that occur only when the specific food is ingested just before or after exercise, although many cases of exerciseinduced allergies are not associated with foods.

Delayed hypersensitivity reactions are cell-mediated, normally involving sensitized immune cells in the small intestine, usually lymphocytes, that are sensitized to the specific substance that triggers the reaction.

The ultimate result is tissue inflammation often restricted to certain sites in the body with symptoms appearing on a more delayed basis, as much as 24 or more hours after consumption of the offending food. Food intolerances, in contrast to true food allergies, do not involve abnormal responses of the immune system.

Anaphylactoid reactions involve the release of the chemical mediators (mostly histamine) of allergic reactions into the body without the intervention of IgE antibodies. Foods such as strawberries and chocolate are thought to allegedly induce such reactions, but definitive proof for this type of food intolerance does not exist.

Metabolic food disorders are genetically determined metabolic deficiencies that result in adverse reactions to a food component. Lactose intolerance serves as a good example of a metabolic food disorder.

In lactose intolerance, the affected individual has a deficiency of the intestinal enzyme, β-galactosidase, which is essential for the metabolism of the lactose in milk. Consequently, lactose cannot be absorbed from the intestinal lumen leading to bacterial fermentation of the lactose in the colon with resultant flatulence and frothy diarrhea.

Food idiosyncrasies are adverse reactions to foods or a food component that occur through unknown mechanisms. Examples include sulfite-induced asthma and tartrazine-induced asthma.

In many cases, the cause-and-effect relationship between the food or food component and the particular adverse reaction remains unproven; this would be the situation with tartrazine-induced asthma. Psychosomatic illnesses are included in this category.

Allergy-like intoxications are worth some mention here because these illnesses can be confused diagnostically with food allergies. Unlike food allergies, everyone in the population is probably susceptible. This reaction occurs as a result of the ingestion of histamine, one of the primary mediators of allergic disease.

Histamine is released from cells within the body in true food allergies and anaphylactoid reactions but is ingested in the case of allergy-like intoxications.

Histamine poisoning (also known as scombroid fish poisoning) is commonly associated with the ingestion of spoiled tuna, mackerel, mahi-mahi, and other fish and also occasionally with cheese. The symptoms mimic some of the most common symptoms encountered in true food allergies.