Adult Food Allergy—Public Perception and Reality

Although the prevalence of IgE-mediated food allergy (IgE-FA) in adults varies, it is now considered to be equal to that seen in children and also increasingly affecting older adults [1]. However, the foods involved are often at odds with the public perception of common food triggers, and the study on wheat allergy by Neyer and colleagues in this issue demonstrates this clearly. Extrapolation of their data showed that the prevalence of self-reported wheat sensitivity was 13.1% of the German population, but the confirmed prevalence of wheat allergy in adults was 0.25% [95% CI 0.08–0.9]. Although wheat is predominantly a carbohydrate, 10%–15% of the mass is made up of a complex collection of proteins, 28 of which have been identified as allergens (Figure 1). Wheat proteins are known to be associated with several diseases, including coeliac disease (Figure 2), which affects ~1% of the UK population, and bakers' asthma, which affects ~5% of bakers. Wheat allergy affects ~0.5% of children, often starting in infancy (and frequently associated with allergies to milk and egg) as a systemic IgE-mediated allergic reaction, and usually outgrown by adolescence (> 80%) [2].

The study from Neyer and colleagues measured the prevalence of allergy to wheat in adults (and adolescents). Of note, only three subjects showed positive serology to whole wheat extract (and only one to wheat allergen components Tri a 19 and Tri a 14). The authors do not comment on whether these were adult-onset cases, or whether they were co-factor dependent. That is, ~1 in 50 of those with symptoms had confirmed food allergy to wheat. However, the design of the study, where sensitisation was only assessed in those with a history of reproducible symptoms on ingestion of wheat, risks missing cases. Wheat allergy can present with a variety of manifestations in adults and is not always obvious from the history as reactions can be infrequent and may only occur in the presence of cofactors. One study reported that 80% of patients with wheat anaphylaxis only reacted in conjunction with exercise [3]. Other co-factors include non-steroidal anti-inflammatory drugs and alcohol. Adults presenting with moderate or severe reactions, where no discernible food trigger seems to be implicated due to perhaps having eaten more than 2 h before or after the symptoms, should always be investigated for co-factor involvement. Wheat dependent exercise-induced anaphylaxis is notoriously difficult to diagnose although sensitisation to ω-5 gliadin has been shown to be a good diagnostic marker. Symptoms may not be reproducible on challenge as a large dose of gluten protein is frequently required to elicit symptoms. This was reflected in the challenge protocol employed in this study, where gluten doses roughly equating to 20 slices of ordinary bread were given [4]. The prevalence figures found in this study are similar to those observed by others usually using similar study designs, showing IgE-mediated wheat allergy is rare, with a lower prevalence than celiac disease.

Ensuring the correct diagnosis is paramount, especially since not all those with WDEIA need to exclude wheat completely. Also, a wheat/gluten-free diet can be nutritionally disadvantageous and associated with health risks. Therefore, whatever the eventual diagnosis, the exclusion of wheat must be undertaken with dietary support to ensure appropriate selection of alternative foods to mitigate the potential loss of the benefits of wholegrain cereals. Population-based surveys have identified a recent increase in the adoption of gluten-free diets for no medical reason (from 0.52% to 1.69% of the population [5]), in the belief it is healthier.

Although wheat is a food most often avoided by adults due to concerns about an allergy, there is a similar picture with milk; Lyons and colleagues found that out of eight common allergenic foods, milk was the most likely to be reported to trigger symptoms by adults, whereas hazelnuts were the most frequent cause of confirmed food allergy, and milk the least [6]. However, this does not mean that milk as a trigger of IgE-mediated allergy in adults should be discounted. Although it is less common in adults, it does occur and can provoke severe allergic reactions [7]. Both milk and wheat are known triggers of non-immune-mediated conditions such as lactose intolerance or non-celiac wheat sensitivity, with symptoms provoked by wheat carbohydrates (fructans). Many individuals often gain symptomatic relief through the exclusion of wheat for the management of Irritable Bowel Disease. This may result in a failure to investigate further to rule out IgE-FA or a differential diagnosis such as Food Protein Induced Enterocolitis Syndrome.

Alongside the wheat allergen Tri a 19 (omega-5-gliadin), someone reporting symptoms to wheat should also be tested for the wheat lipid transfer protein (LTP) Tri a 14, especially as symptoms due to LTP allergy very often only manifest in the presence of co-factors. There is increasing evidence that LTP allergy, traditionally seen in Southern Europe, is probably much more widespread than first thought. Therefore, where co-factors are reported but wheat is not the culprit, it is helpful to test for sensitisation to other LTPs such as the peach allergen Pru p 3. LTP allergens might also be involved in Cannabis allergy, another plant allergy that can give rise to reactions to plant foods. The most common Cannabis allergen linked to food allergy is Can s 3, the LTP allergen and so, where relevant, it is important to determine the likelihood of cannabis exposure, as well as alcohol consumption, especially if severe reactions to wheat or other plant foods have been reported [8].

The most prevalent new-onset IgE-mediated plant food allergy in adults is driven by pollen exposure. Pollen-food syndrome (PFS) is a highly prevalent adult-onset food allergy, affecting many adults who have often had seasonal allergic rhinitis since childhood but often only develop PFS in adult life. Unlike wheat, fruits and vegetables are often discounted as being a likely cause of food allergy. An itchy mouth when eating raw fruits may be considered to be irrelevant by the sufferer and frequently thought to be due to pesticides. The allergens involved in PFS include profilins and PR-10 proteins, and these heat-labile allergens are the reason why PFS sufferers can often tolerate cooked plant foods. However, there is good evidence to suggest that some PR-10 allergens can re-fold on cooling, which may account for their ability to cause moderate or severe reactions, for example, to soy milk.

The increasing popularity of gluten-free and plant-based diets has led to an increased use of seeds and legumes in composite foods. Many legumes and seeds are in gluten-free products, health/protein bars, shakes, and faux meat products, often those not previously identified as a trigger of food allergy and therefore not labelled as an allergen. The European Anaphylaxis registry data showed that anaphylaxis was reported to many legumes, most commonly to soy which, although considered to be a food mainly provoking childhood allergy, affected proportionately a far greater number of adults [9]. After soy, the most common triggers were pea, lupin and lentils—foods increasingly used in a wide range of composite products. Seeds such as sesame, sunflower or pumpkin seeds are increasingly reported to provoke anaphylaxis, but those which have not previously been a common cause of IgE-FA such as linseed/flaxseed or hemp seed are also appearing as triggers in case reports.

In conclusion, the paper from Neyer and colleagues suggests that confirmed adult-onset IgE-mediated allergy to wheat is rare in Europe. However, avoidance of wheat and gluten-containing foods is common, and it is still important to ensure correct testing to exclude IgE, non-IgE and non-immune-mediated food hypersensitivity, and that appropriate dietary management and support are available. The same is true for other foods, especially milk, so as to avoid the potential adverse health consequences of adopting a gluten-free or milk-free diet in the absence of a diagnosis and associated dietetic support. This study also reflects a growing need for more studies on adult food allergy. It is vital that the allergy community recognise that IgE-FA can occur in adults of any age and to foods they have been consuming their whole lives.

Isabel Skypala developed the concept for the editorial, wrote half of the manuscript, and edited the whole version ready for publication. Angela Simpson co-wrote the first part of the editorial and devised one of the figures. Clare Mills co-wrote the first part of the manuscript and contributed one of the figures. All authors reviewed the completed manuscript and approved the final version for publication.

The authors declare no conflicts of interest.