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The elimination diet is a diagnostic tool or method used to determine whether patient’s symptoms are food related. The term is sometimes used incorrectly to describe a diet which eliminates certain foods for a patient; this type of diet is more correctly called a treatment diet.

Adverse reactions to food can be due to several mechanisms. Correct identification of the type of reaction in an individual is important, as different approaches to management are required. The area of adverse reactions to food has been controversial and the subject of ongoing research. It has been characterised in the past by lack of universal acceptance of definitions, diagnosis and treatment. ‘food allergy’ is a widely misused term and has been used incorrectly to refer to all kinds of food reactions both immunological and non-immunological, similarly for the term ‘food intolerance’.

Definitions

Food allergy is defined as an immunological hypersensitivity which occurs most commonly to food proteins such as egg, milk, seafood, shellfish, tree nuts, soya, wheat and peanuts, usually by production of IgE (immunoglobulin E) antibodies.

Food intolerance is defined as a non-immunological abnormal physiological response. While true allergies are associated with fast-acting IgE responses, it can be difficult to determine the offending food causing an intolerance reaction because if the immune system is involved, the response is likely to be IgG mediated and takes place over a prolonged period of time. Thus the causative agent and the response are separated in time, and may not be obviously related. Food intolerance reactions may include pharmacologic, metabolic, toxic, or psychological responses to foods or food components.

• Pharmacological reactions are generally to low molecular weight chemicals which occur either as natural compounds such as salicylates, amines, glutamates and MSG or to artificially added substances such as preservatives, colouring, emulsifiers and taste enhancers. These chemicals are capable of causing drug like (biochemical) side effects in susceptible individuals.
• Metabolic food reactions are due to an inborn or acquired errors of metabolism of nutrients such as in diabetes, lactase deficiency, phenylketonuria and favism. Toxic food reactions are caused by the direct action of a food or additive without immune involvement.
• Toxins may either be present naturally in food or released by bacteria or from contamination of food products.
• Psychological reactions involve manifestation of clinical symptoms caused not by the food but by emotions associated with the food. The symptoms do not occur when the food is given in an unrecognisable form.

Elimination diets are useful to assist in the diagnosis of food allergy and pharmacological food intolerance. Metabolic, toxic and psychological reactions can be diagnosed by other means.

Diagnosis

Food allergy is principally diagnosed by careful history and examination. When reactions occur immediately after certain food ingestion then diagnosis is straight forward and can be documented by using carefully performed tests such as SPT skin prick test and RAST sensitive tests to detect specific IgE antibodies to specific food (proteins) and aero-allergens. However false positive results to SPT do occur, when diagnosis is doubtful it can be confirmed by exclusion of the suspected food or allergen from the patient followed by appropriately timed challenge under careful medical supervision. If there is no change of symptoms after 2 to 4 weeks of avoidance of the protein then food allergy is unlikely to be the cause and other causes such as food intolerance should be investigated.

Food intolerance due to pharmacological reaction is more common than food allergy and has been estimated to occur in 10% of the population and unlike food allergy can occur in non-atopic individuals. It is also more difficult to diagnose. Individual food chemicals are widespread and can occur across a range of foods, such that eliminating one at a time is unhelpful. Natural chemicals such as benzoates and salicylates found in food are identical to artificial additives in food processing and can provoke the same response. As the specific component is not readily known and as the reactions are often delayed up to 48 hours after ingestion, it can be difficult to identify suspect foods. Added to that is that these chemicals often exhibit dose-response relationships and so the food may not trigger the same response each time. There is currently no skin or blood test available to identify the offending chemical(s) consequently elimination diets aimed at identifying food intolerances need to be carefully designed and exactingly applied. All patients with suspected food intolerance should consult a physician first to eliminate other possible causes.

Elimination diet

The elimination diet must be comprehensive containing only those foods unlikely to provoke a reaction, but be able to provide complete nutrition and energy, for the weeks it will be applied, professional nutritional advice is needed. Thorough education about the elimination diet is essential to ensure patients and the parents of children with suspected food intolerance understand the importance of complete adherence to the diet. As inadvertent consumption of an offending chemical can prevent resolution of symptoms and render challenge results useless. Whilst on the elimination diet records are kept of all foods eaten, medications taken and severity of any symptoms. Patients are advised that withdrawal symptoms can occur in the first weeks on the elimination diet, some of the patients symptoms can flare or worsen initially before settling. Whilst on the diet some patients become sensitive to fumes and odours, which may cause symptoms. They are advised to avoid such exposures as this can complicate the elimination and challenge procedures. Particularly to petroleum products, paints, cleaning agents, perfumes, smoke and pressure pack sprays. Once the procedure is complete this sensitivity becomes less of a problem.

Clinical improvement usually occurs over a 2 to 4 week period, if there is no change after a strict adherence to the elimination diet and precipitating factors, then food intolerance is unlikely to be the cause. A normal diet can then be resumed by gradually introducing suspected and eliminated foods or chemical group of foods one at a time. Gradually increasing the amount up to high doses over 3 to 7 days to see if exacerbated reactions are provoked before permanently reintroducing that food to the diet. A strict elimination diet is not usually recommended during pregnancy, although a reduction in suspected foods that reduce symptoms can be helpful.

Challenge testing

Challenge testing is not carried out until all symptoms have cleared or improved significantly for five days after a minium period on the elimination diet of two weeks. The elimination diet is continued throughout the challenge period. Open food challenges on wheat and milk can be carried out first. Followed by challenge with natural food chemicals then additives. Challenges can take the form of purified food chemicals or with foods grouped according to food chemical. Purified food chemicals are used in double blind placebo controlled testing, and food challenges involve foods containing only one suspect food chemical eaten several times a day over 3 to 7 days. If a reaction occurs patients must wait until all symptoms subside completely and then wait a further 3 days (to overcome a refractory period) before recommencing challenges. Patients with a history of asthma, laryngeal oedema or anaphylaxis may be hospitalised as inpatients or attended in specialist clinics where resuscitation facilities are available for the testing.

If any results are doubtful the testing is repeated, only when all tests are completed is a treatment diet determined for the patient. The diet restricts only those compounds to which the patient has reacted and over time liberalisation is attempted. In some patients food allergy and food intolerance can coexist, with symptoms such as asthma, eczema and rhinitis. In such cases the elimination diet for food intolerance is used for dietary investigation. Any foods identified by SPT or RAST as suspect should not be included in the elimination diet.

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Further excellent information is available at this site, under the Food Intolerance tab; http://www.cs.nsw.gov.au/rpa/allergy/

Another good paper on the scientific basis of food intolerance is in Chapter 57 of the Nightingale book on CFS; " The role of Food Intolerance in CFS" by Robert Loblay and Anne Swain.