Food allergies
Food allergies are adverse immune responses to food proteins, which can manifest in various forms and affect multiple functions around the body. Some are obvious and immediate and like other allergies linked to IgE reactions; others involve different immune responses and can be delayed and less easy to spot. They most often occur initially in children but the more complex forms can also be identified for the first time in adults.
IgE-mediated food allergy:
- This type involves immediate hypersensitivity reactions, often leading to symptoms like urticaria (hives), mouth ulcers, and at worst life-threatening anaphylaxis.
- Common allergens are peanuts, shellfish, tree nuts, fish, soy, milk, wheat and eggs.
Non-IgE-mediated food allergy:
- The most common example is cow’s milk allergy in infants, with the most severe outcomes including enterocolitis (marked by vomiting and diarrhoea), and proctocolitis (griping with mucus or blood in the stool). Other reactions may be delayed, though again tend to start with gut inflammations. More systemic consequences can include atopic dermatitis, oedema, cough, wheezing, nasal congestion and wider immunological disturbances. These allergies have even been linked to psychological and neurological symptoms.
- Apart from cow’s milk other allergens include wheat, soy and grains, fish and eggs, with considerable regional variations in culprit foods around the world.
Mixed type food allergy.
- Examples are various gut disorders marked by prominent involvement of eosinophil blood cells in the inflammation. The most studied is eosinophilic oesophagitis, associated with perhaps vague stomach symptoms in infants and toddlers, with heartburn, chest pain, and swallowing difficulties in older children and adults. It is also associated with psychological symptoms of depression and anxiety and with coeliac disease.
The prevalence of food allergies is increasing, particularly in children. Fortunately milk and egg allergies are often outgrown; however peanut and tree nut allergies tend to persist.
The two most notorious food allergens are gluten (found in wheat, rye and barley) and cow’s milk. The first highlights the complexity of some food-related allergies. Gluten is a protein which fragments on digestion to produce gliadin, a gut wall irritant for everyone (it is a testament to the resilience of a healthy digestion that this irritant is mostly accommodated without ill effects)! However this accounts for gliadin generating two different types of allergic response. ‘Non-coeliac gluten sensitivity’ is the term used to describe most problems reported with eating gluten, seen by many people especially as they get older. The symptoms may be due to disturbances of the innate immune mechanism (see our intro to immunity). The other gluten-related allergy, coeliac disease is an autoimmune condition involving disturbances of acquired immunity. Wheat contains other non-gluten constituents that can be allergenic in their own right: wheat allergy is an IgE-mediated condition. In coeliac disease, a total gluten-free diet should be strictly followed, whereas wheat allergy only requires wheat elimination. In non-coeliac gluten sensitivity occasional trials of gluten reintroduction can be done. To reinforce the potential impact of gluten on wider health, gluten-free diets have been shown to alleviate ADHD symptoms, even without any evidence of coeliac disease.
Cow’s milk allergy illustrates a simple explanation of the origin of food allergies in babies. After birth the digestive and immune defences in the gut are not fully formed: the infant is set up to ingest only milk from their mother. Many instead have an early introduction to milk from the cow. This includes a complex protein (ß-lactoglobulin), a nutrient more suited to the development of the grass-digesting rumen in the calf. This may arrive before the gut can easily make the protein safe and before the backup immune system is also ready to effectively defend against it. Allergic reactions arise from the failure of these core processing tools.
Allergies could also be associated with other baby foods (like gluten products) especially if these are introduced too early. However after the first 3 months or so, it makes sense to increase food diversity in children’s diets to reduce allergy risks and it is now accepted that controlled exposure of peanuts and other dangerous allergens to infants at 4-6 months old can successfully head off future allergies.
Other likely risks for developing food allergies lie in modern processed foods. For example food additiveshave been associated with compromised intestinal mucosa and changes in the intestinal microbiota (eg an increase of Proteobacteria), with consequent inflammation, increased oxidative stress, and changes in the immune system. Also the composition and way many ultra-processed foods are produced induces the flavoursome ‘Maillard reaction’ between carbohydrates and proteins. In recent years, it has been postulated that the build-up of these metabolites in the body could promote food allergies, through more inflammation, the induction of new IgE-responses, and further reduction in the diversity of the intestinal microbiome.
Food elimination
Usual allergy tests, skin-prick testing, patch testing, and serum IgE tests, as well as the various alternative allergy test methods, are all notoriously unreliable in identifying food allergens. The only sure way to check for these is by carefully managed food eliminations and rechallenges. There are two reasons to consider these.
To help diagnose the condition, with a short-term closely observed exclusion of one or more suspect food ingredients, this followed by a ‘rechallenge’ with these ingredients; and the cycle possibly repeated for maximum clarity.
As a longer term measure to relieve the condition once the culprit foods have been identified. In this case focus should be on ensuring that potential nutritional deficiencies are addressed. As some food allergies self-correct after abstinence there should be occasional rechallenges, so that the elimination does not go on longer than necessary.
In the first case it is absolutely critical that the elimination is 100%, to rule out ongoing allergic reactions, although this also means that it need not take too long. If for example a milk allergy is suspected, the initial elimination should completely exclude all dairy products (including from goats and sheep in this first stage), and entails for example careful reading of prepared food ingredient lists to rule out ‘dairy solids’, casein, lactose or any other reference to dairy content. In the case of gluten all wheat, rye and barley products need to be avoided, including beer, and although oats does not have gluten itself it does have a closely related protein avenin that can occasionally cross-react. In practice rice, maize (‘corn’), buckwheat and other grains are safe to take.
To rule out inadvertent contaminations it is advisable in all elimination tests to avoid food that is not made at home from clearly identified starting ingredients.
If the exclusion is absolute then it is usually possible to see symptom change within a week in babies and young children, maybe up to a month in adults. Rechallenge should then be done under close observation, as occasionally the reaction can be severe (and this clearly would confirm that the food was an allergen). An old trick where the reaction may be less clear is to take the pulse before and within 15 minutes after the challenge. Any food intake will raise pulse rates a little: if it jumps by 20 beats a minute or more it could point to a significant reaction.
One approach to diagnostic elimination where the food culprit is less clear is to exclude all at once the six foods that account for 90% of cases (milk, egg, wheat, soy, peanut, tree nuts). Clearly this does risk nutritional deficiencies in the long term so this approach should only be used to see if there is any allergy to the foods. Only then is it worth redoing the eliminations food by food to identify the cause.
There is one reason for caution in conducting food-elimination diets. In some cases of hypersensitivity (such as atopic dermatitis) there is a potential risk of exacerbations to a culprit food on rechallenge after its elimination, especially in children. It will be wise to have to hand any emergency measures that might have been used in the past and if there is a history of alarming anaphylactic events not to proceed at all without expert advice.
For a deep dive into this topic check out these key research papers