FOOD AND DRINK
ADULT FOOD ALLERGY Dr Isabel Skypala Consultant Allergy Dietitian, Royal Brompton & Harefield NHS Foundation Trust
Food allergy used to be a disease of childhood, but is now on the increase in adults. In 1994, a study reported that the prevalence of adult food allergy in the UK was 1.4%; 20 years later, 5.8% of UK adults in another study reported reactions consistent with an IgE-mediated food allergy.1,2 This is similar to Europe.
A dietitian for 38 years who established and leads the adult food allergy service, Isabel is also honorary Senior Clinical Lecturer at Imperial College and actively engaged in food allergy research.
A study of eight European countries reported an overall prevalence rate of 4.4% for diagnosed adult food allergy, ranging from 0.5% in Lithuania to 8.4% in Spain.3 Most recently, Turner et al reported an increase of 105% in food induced anaphylaxis in the 15 to 59 age group, with the mean age of fatal food induced anaphylaxis being 25 years.4 DIAGNOSIS
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Taking a detailed history of foods involved, symptom type and speed of onset is the key to determining whether these reactions are due to an IgE-mediated food allergy.5 The foods suspected should be confirmed as culprits by undertaking skin prick or specific IgE tests. But it is important to only test for foods implicated in the reaction because many adults who already have an allergic disease, such as eczema or allergic rhinitis, will often have specific IgE antibodies against many foods, so food sensitisation is not predictive of the development of a food allergy in adults.6 This is mainly due to pollen sensitisation (trees, grasses and weeds) causing positive tests to foods, such as tree nuts and peanuts, due to similarities between the allergen epitopes (binding sites) of pollens and food proteins.7 Another food which might be positive without relevance is milk, which rarely presents as an allergy in adult life, but many who have severe eczema often have specific IgE antibodies to milk.8 It is important that individuals with specific IgE antibodies against a food consumed
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regularly and without symptoms, continue to eat the food, so that allergen recognition will continue. Complete elimination of the food can result in non-recognition by the IgE antibodies; one study found that 19% of patients developed new immediate food reactions after initiation of an elimination diet, 30% of which were severe and usually milk and egg were the culprits.9 The risk of this occurring was unrelated to the level of specific IgE antibody and the authors recommended that strict elimination diets need to be thoughtfully prescribed, as they may lead to decreased oral tolerance. Where there is doubt about the diagnosis, e.g. if tests are negative in the face of a strong history, or the history is weak, but tests are positive, then an oral food challenge should be performed.10 CHILDHOOD ALLERGY TO MILK AND EGG
Allergies to milk, egg, wheat and soy usually resolve by the age of 18 years, with those having high levels of specific IgE antibodies being least likely to experience remission.11 Such patients will transition to the adult clinic, but should continue to be monitored as resolution may still occur. Molecular allergy testing with individual milk allergens Bos d 4 (alpha lactoglobulin), Bos d 5 (beta lactalbumin) and Bos d 8 (casein) can be useful; a level of 10IU/ml for casein had a 95% positive predictive value (PPV) for a positive oral food challenge to baked milk in one study, although other studies argue that a positive skin prick test to fresh milk
gives similar results.12,13 For eggs, acquisition of tolerance is associated with a decrease in specific IgE antibody to egg white and also to Gal d 1 (ovomucoid), a low level of which can be predictive of a resolving egg allergy.14 DOES PEANUT ALLERGY RESOLVE?
Peanut allergy resolution is more likely to occur in early childhood; in one study, 225 of children with peanut allergy experienced resolution by the age of four years15 and another study with a similar resolution rate found that most children outgrew their peanut allergy by the age of eight years.16 Adults reporting peanut allergy diagnosed in childhood should always be re-tested, as many were not robustly diagnosed as children and, in keeping with the advice at the time, were usually told to also avoid all tree nuts. However, only onethird of peanut allergic individuals will be allergic to one or more tree nuts17 and so, the need to avoid tree nuts should be individually assessed, especially as severe reactions are more frequent in teenagers and adults.18,19 Sensitisation to the primary peanut allergens Ara h (Arachis hypogaea) 1, Ara h 2 and Ara h 3 is usually acquired in childhood, so it is unlikely that adults will develop new-onset peanut allergy.20 One allergen, Ara h 2, has been shown to be the best predictor of the presence or absence of a peanut allergy; one study reported that an Ara h 2 level of 14.4kU/l had a 90% probability for a positive peanut challenge.21 However, other studies in different populations have shown different results; Eller et al reported that a level of 1.63kU/l for Ara h 2 had 100% specificity and 70% sensitivity.22 NEW-ONSET FOOD ALLERGIES IN ADULT LIFE
A retrospective study on 2.7 million adults identified 3.6% had one or more food allergies and intolerances, with seafood and fruits and vegetables being the most common trigger foods.23 Of the seafood, shellfish is more likely to cause new-onset IgE-mediated food allergy presenting in adult life, whereas allergy to fish is more likely to occur in childhood.24,25 Reported reactions to shellfish are most often to crustaceans rather than molluscs.26 It has been estimated that 60% of patients who have a crustacean allergy are sensitised to the pan allergen tropomyosin, but another allergen, sarcoplasmic calcium-binding
protein, has also been associated with clinical reactivity to prawns..27 Sensitisation may be to individual allergens, which may vary depending on the species of prawn, so it is recommended that diagnosis should be made using skin prick tests to the specific species of prawn involved in the reaction, using the fresh material.28,29 New-onset symptoms to tree nuts and occasionally to peanuts presenting in adult life are usually due to pollen antibodies recognising and reacting to cross-reacting proteins. This condition, known as pollen-food syndrome or oral allergy syndrome, affects 1-4% of UK adults, depending on geographical location.2 Testing with individual peanut and tree nut allergens is very useful in determining whether symptoms to nuts are due to PFS or a primary nut allergy. The key allergens provoking PFS are those which are homologous to the birch pollen allergen Bet v 1, and these include Ara h 8 (peanut) and Cor a 1 (hazelnut).30 Raw fruits and, to a lesser extent, vegetables, are also common causes of PFS, often provoking mild symptoms, although concentrated amounts of allergen, such as freshly made fruit juices, smoothies, or soy milk, which contains the birch cross-reacting allergen Gly m 4, can cause severe reactions.31,32 Severe or anaphylactic reactions to fruits and vegetables are more likely to occur due to lipid transfer proteins (LTP), especially the peach LTP allergen, Pru p 3. This condition was originally described only in South Mediterranean countries, but more recently has been recognised in some northern European countries.33,34 These allergens are found in both raw and cooked fruits, vegetables, nuts and cereals and are often linked to sensitisation to pollens such as plane tree and mugwort.35 Apart from peaches, tomatoes are also a common food trigger, but LTP allergens have been sequenced in many foods including lettuce and so-called ‘super foods’, such as goji berries and linseeds.36-39 WHEN IT IS NOT IGE-MEDIATED FOOD ALLERGY
Non-IgE-mediated food allergy does exist in adults. Adult-onset eosinophilic oesophagitis presents with classic symptoms, such as dysphagia to solid food, reflux and food impaction, but also rings, strictures and narrowing of the oesophagus.40 Symptom management often includes highdose topical corticosteroids, but also elimination www.NHDmag.com August/September 2017 - Issue 127
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FOOD & DRINK diets. Both 6-food (milk, egg, wheat, seafood, peanuts/tree nuts and soy) and 4-food (milk, wheat, egg, soy) elimination have been found to improve symptoms, reduce eosinophilic infiltrate in oesophageal mucosa and improve endoscopic markers of inflammation.41 Although Food Protein Induced Enterocolitis Syndrome (FPIES) is considered to be a disease of childhood, shellfish is thought to cause a similar type of delayed non-IgE-mediated food allergy in adults.42 However, many adults presenting in the allergy clinic will report nonspecific reactions linked to foods, which are most frequently due to functional gut disorders, spontaneous urticaria/angio-oedema or, more rarely, intolerance to sulphites, benzoates, or naturally occurring vaso-active amines.43 Avoidance of gluten-containing foods dominates; one systematic review found selfreported wheat allergy was 3.6%, compared to food-challenge-defined wheat allergy which was 0.1%.44 In 2009, 3% of UK adults reported wheat to be a trigger food and more recently, 7.3% of Australian adults and 6.25% of adults in the Netherlands reported symptoms to wheat.45,46
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This is of concern, not least because Elli and colleagues have demonstrated that only 14% of those who suspect they are gluten intolerant are likely to be so.47 Also, gluten-free products have consistently lower levels of protein than their gluten-containing counterparts and gluten-free diets may be low in calcium, iron, zinc, folate and fibre, affecting not only nutritional status, but also the intestinal microbiota.48,49 CONCLUSION
Adult food allergy is increasing in prevalence. The key to diagnostic accuracy is to take a thorough history to determine the likelihood of an IgEmediated food allergy. Specific IgE tests should only be undertaken to suspected foods with oral food challenge performed should there be any mismatch between history and test results. Shellfish, fruits, and nuts are common triggers of IgE-mediated food allergy, but wheat is the food most often excluded by the patient, often leading to sub-optimal nutritional intakes. An excellent knowledge of food ingredients is essential when diagnosing and managing adult food allergy, making the dietitian an essential part of the allergy team.