Chapter 10 FOOD ALLERGIES AND INTOLERANCES

KEY POINTS

• Food allergies may affect up to 6% of the population.

• Food allergies are adverse reactions to foods mediated by the immune system. Food allergies may be mediated by antibodies e.g. IgE (immediate hypersensitivity) or they may be cell mediated (delayed hypersensitivity).

• Intolerances are adverse reactions to foods that are not mediated by the immune system.

• A thorough history is the most important aspect in the evaluation of a patient with a suspected food adverse reaction.

• Allergy tests may aid in the diagnosis of a food allergy but should never be exclusively relied upon in the absence of a supportive history.

• Exclusion diets and re-challenge remain the ‘gold standard’ for the diagnosis of many clinical food allergy-related syndromes.

• Appropriate education regarding both the risk of a severe reaction and food avoidance is the most important aspect in the management of patients with food adverse reactions.

DEFINITIONS

Food adverse reaction

Any abnormal reaction that is related to the ingestion of a food.

Food allergy

An adverse reaction to food resulting from an immune-mediated response to a food protein. Food allergic reactions are usually either IgE mediated (immediate hypersensitivity e.g. urticaria) or non-IgE (cell) mediated (delayed hypersensitivity e.g. food protein enteropathy). Some allergic diseases are associated with demonstrable IgE to foods and other allergens but a causative link has not been firmly established (e.g. eosinophilic gastroenteropathy). Isolated gastrointestinal food allergies are usually either delayed or IgE-associated.

Food intolerance

Any non-immune-mediated adverse reaction to food. Intolerances are usually caused by an inherent characteristic of the patient e.g. a disaccharidase deficiency causing lactose intolerance. Intolerances may require a much larger amount of the food to cause clinical symptoms and are usually delayed in onset.

Toxic reactions

Adverse reactions to foods mediated by a toxin contained in the food e.g. food poisoning or histamine in scombroid fish. These reactions occur in all people who ingest the food and are not dependent on either immune- or non-immune-mediated susceptibility.

EPIDEMIOLOGY

Food allergy of all types may affect up to 6% of children and 4% of adults. Estimates from the USA suggest that around 4% of the general population have IgE-mediated food allergy while IgE-mediated allergy to specific foods varies from between 0.5% to around 2% of the general population, especially in children. The incidence of IgE-mediated food allergy is much higher in individuals with other atopic diseases e.g. up to one-third of children with atopic dermatitis have been reported to have concomitant food allergy. IgE-mediated food allergy is also found more frequently among asthmatic patients, but is rarely the cause of wheeze in the absence of other symptoms.

CLINICAL SYNDROMES

IgE-mediated immediate hypersensitivity

A relatively small number of foods, including cow’s milk, hen’s egg, wheat, peanut and shellfish cause the majority of IgE-mediated immediate hypersensitivity reactions. Symptoms usually occur within minutes of ingestion of the food and very rarely occur more than 2 hours following the ingestion. Symptoms involving the gastrointestinal system include abdominal pain, nausea, vomiting and diarrhoea. Immediate hypersensitivity reactions rarely involve the gastrointestinal system in the absence of other systems. The most severe form of IgE-mediated allergy is called anaphylaxis and is defined as the involvement of any two organ systems and the respiratory or cardiovascular system. Anaphylaxis is a medical emergency and can be fatal in some cases.

Food protein-induced proctocolitis

Cell-mediated (eosinophilic) hypersensitivity, usually to cow’s milk or soy, is characterised by macroscopic blood and mucus in the stool of an otherwise healthy, usually breastfed, infant. Onset is at around 2 months on average. Usually no special investigations are needed to make the diagnosis. Treatment is avoidance of the causative protein by breastfeeding mothers or, if symptoms persist or the infant is not breastfed, a hydrolysed formula may be used. This usually resolves between 1 and 2 years of age.

Food protein enteropathy

Cell-mediated (T cell) hypersensitivity, usually to cow’s milk but occasionally also grains, egg and shellfish, is characterised by chronic vomiting, diarrhoea and malabsorption. The onset is usually in infancy in formula-fed infants and failure to thrive is common. Protein loss may lead to oedema and iron deficiency may follow. Diagnosis is made through a combination of endoscopy (biopsy), allergen avoidance and re-challenge. This condition generally resolves between 1 and 2 years of age.

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