Food intolerance and allergy--a review.
Specific food intolerance needs to be distinguished from obsessional states in which those who are affected have an aversion to numerous foods. Even in cases where specific food intolerance can be demonstrated, the diagnosis of food allergy depends on additional evidence that the patient's reaction is based on an abnormal immunological response. In food allergy, skin and laboratory tests may detect the presence of an IgE-mediated reaction, particularly in patients with asthma or eczema and especially where the foods involved are highly allergenic--such as egg, fish, nuts and milk. However, many patients with proven food intolerance have negative tests, suggesting that other immunological or non-immunological mechanisms are responsible. Laboratory tests for non-IgE reactions are unreliable. Where it is difficult to show a connection between individual foods and an allergic response--as in patients with urticaria provoked by food additives--one of the reasons for diagnostic difficulty is that the offending substances may be present in a wide range of common foods. If the diagnosis is to be firmly established in such cases, it is necessary to show that symptoms remit on an elimination diet and recur after a placebo-controlled challenge.