This is divided into food aversion, where a person simply avoids a food they dislike; true food intolerance, where taking the food causes symptoms; and food allergy, where the symptoms can be shown to be caused by an immunological reaction. Some cases of food intolerance are due to idiosyncrasy – that is, a genetic defect in the patient, such as alactasia, where the intestine lacks the enzyme that digests milk sugar, with the result that individuals so affected develop diarrhoea when they drink milk. Intolerance to specific foods, as distinct from allergy, is probably quite common and may be an important factor in the AETIOLOGY of IRRITABLE BOWEL SYNDROME (IBS).

For the diagnosis of true food allergy, it is necessary to demonstrate that there is a reproducible intolerance to a specific food given blindly, that is with neither doctor nor patient knowing whether it is the food in question or a PLACEBO substitute and that there is clinically observed or laboratory evidence of an abnormal immunological reaction having occurred. Occasionally the allergic response may not be to the food itself but to contaminants such as penicillin, or additives such as tartrazine. There may also be non-immunological reactions to foods which have pharmacological (‘medicine-like’) effects, such as caffeine in coffee or histamine in fermented cheese. Such reactions may also be due to the irritant effect on the intestinal mucosa (especially if it is already diseased) by, say, highly spiced curries.

Testing blood and skin for food allergy is beloved of some alternative practitioners but, in practice, the results of such tests do not always agree with what happens when the food is taken. Therefore, a careful history is often as useful in making a diagnosis as is any test.