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Many people ascribe their various symptoms to food, and many such sufferers are seen and started on exclusion diets. The scientific evidence that food does harm in most instances is weak, although adverse reactions to food certainly exist. These can be divided into those that involve immune mechanisms (food allergy) and those that do not (food intolerance).

Food allergy

Food allergy, which is estimated to affect up to about 5% of young children and about 1-2% of adults, may be IgE mediated or non-IgE mediated (T-cell mediated). The IgE-mediated reactions tend to occur early after a food challenge (within minutes to an hour). Adults tend to be allergic to fish, shellfish and peanuts, while children tend to be allergic to cow's milk, egg white, wheat, and soy. Peanuts are very allergenic and peanut allergy persists throughout life. The following conditions can result from food allergy:
Acute hypersensitivity. An example is urticaria, vomiting or diarrhoea after eating nuts, strawberries or shellfish. These IgE-mediated reactions do not usually produce clinical problems as the patients have already learned to avoid the suspected food. Inadvertent ingestion of the incriminating food can sometimes occur, leading to angioneurotic oedema.

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  Eczema and asthma. These tend to affect young children and are often due to egg and are IgE mediated.
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  Rhinitis and asthma. These have been produced by foods such as milk and chocolate, mainly in atopic subjects.
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  Chronic urticaria. This has been treated successfully by an exclusion diet.
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  Food-sensitive enteropathy. This may manifest itself as coeliac disease (gluten (wheat) sensitive enteropathy), and cow's milk enteropathy (in infants) and is T-cell mediated.
  
  Food intolerance

Migraine. This sometimes follows the intake of foods such as chocolate, cheese and alcohol, which are rich in certain amines, such as tyramine. Patients on monoamine oxidase inhibitors, which are involved in the metabolism of these amines, are particularly vulnerable.

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  Irritable bowel syndrome. In some patients this seems to be related to ingestion of certain food items, such as wheat, but the mechanisms are not clearly defined.
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  Chinese restaurant syndrome. Monosodium glutamate, a flavour enhancer used in cooking Chinese food, may produce dizziness, faintness, nausea, sweating and chest pains.
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  Lactose intolerance. Patients develop abdominal bloating and diarrhoea following ingestion of lactose, which is present in milk. This is probably the commonest form of food intolerance world-wide, and may be genetic in origin.
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  Phenylketonuria. This can also be classified as a form of food intolerance, and is due to lack of phenylalanine hydroxylase, which is necessary for the metabolism of phenylalanine present in dietary protein.

A number of other inborn errors of metabolism can also be regarded as forms of food intolerance.

Food intolerance may be due to a constituent of food (e.g. the histamine in mackerel or canned food, or the tyramine in cheeses), chemical mediators released by food (e.g. histamine may be released by tomatoes or strawberries), or toxic chemicals found in food (e.g. the food additive tartrazine).

Many other additives and compounds with certain E numbers have been implicated as causing reactions, but here the evidence for this is poor.

There is little or no evidence to suggest that diseases such as arthritis, behaviour and affective disorders and Crohn's disease are due to ingestion of a particular food.

Multiple vague symptoms such as tiredness or malaise are also not due to food allergy. Most of the patients in this group are suffering from a psychiatric disorder.

Management

A careful history may help to delineate the causative agent, particularly when the effects are immediate.

Skin-prick testing with allergen and measurement in the serum of antigen or antibodies have not correlated with symptoms and are usually misleading. 'Fringe' techniques such as hair analysis, although widely advertised, are of limited value.

Diagnostic exclusion diets are sometimes used, but they are time-consuming. They can occasionally be of value in identifying a particular food causing problems.

Dietary challenge consists of the food and the test being given sublingually or by inhalation in an attempt to reproduce the symptoms. Again this may be helpful in a few cases.

Most people who have acute reactions to food realize it and stop the food, and do not require medical attention. In the remainder of patients, a small minority seem to be helped by modifying their diet, but there is no good scientific evidence to support these exclusion diets.