What is food allergy?

The term food allergy is often mistakenly used to encompass all adverse reactions to foods and hence includes delayed intolerances as well as immediate-onset (IgE-mediated) true allergic reactions.

Top of page

Who is at risk for the development of food allergy?

 The development of food allergy depends on a complex interaction between environmental, dietary and genetic factors. It is not fully understood as to why food allergy has risen over the last 2 decades. The ‘hygiene hypothesis’ suggests that the developing immune system is better protected against the development of allergy if it is ‘stimulated’ by immune ‘challenges’ found in ‘dirty’ environments. Such environmental factors may include large families, farm animal and product (e.g. unpasteurised milk) exposure and possibly fewer courses of antibiotics. Children commonly develop more than just a single allergic disease (e.g. eczema, asthma, hayfever)and if food allergic, frequently become allergic to more than just one single food. This is often referred to as the ‘Allergic March’. Early onset eczema and a family history of allergies are the greatest risk factors for the development of food allergy. Children with a single food allergy are at risk for the development of additional food allergies with the association between egg allergy and the subsequent development of peanut allergy being particularly strong. The presence of egg allergy in childhood may also be a risk factor for the subsequent development of asthma.

Top of page

Which are the commonest food allergies?

Food allergy is most prevalent during the first few years of life and affects between 6-8% of children in the UK and USA. Although approximately 25% of adults suspect that they are food allergic, true IgE-mediated food allergy rates for adults are less than 3%. The specific food/s to which individuals are allergic varies with age: cow’s milk, hen’s egg, peanut, tree nut, sesame, soya, wheat and kiwi are responsible for the majority of food-induced allergic reactions in young children. Fin-fish, shellfish, tree nut and peanuts are common causes of food allergy in adulthood.

Tree pollen-allergic children and adults may develop allergic reactions to cross reactive foods, this is known as the Oral Allergy Syndrome (OAS). The OAS results in troubling symptoms such as lip and throat tingling and a ‘metallic taste’ when eating cross reactive fruits e.g apple, pear cherry.

Although food allergy in general is thought to be on the increase, the increasing prevalence is allergen-specific and has been best described for peanut allergy (the prevalence of peanut allergy in the UK has nearly doubled over the past decade in UK, and now approximates 1.8%. There are also early reports to suggest an increase in the prevalence of less ‘traditional’ food allergens such as kiwi and sesame allergy.

Top of page

Will my child outgrow their food allergies?

The majority of food allergies present during early childhood. However, food allergy may present in older children if the food is not commonly eaten in early childhood e.g. molluscs and shellfish. The chance of outgrowing food allergy is allergen-specific.  For example, egg and milk allergy are outgrown in at least 85% of children by the age of 5-7 years, whereas peanut, tree nut and sesame allergy are infrequently outgrown (less than 25%).

Top of page

What are the typical symptoms of food allergy?

Food-induced allergic reactions may present with symptoms and signs varying from a few transient hives to life-threatening or fatal anaphylaxis. The most commonly involved organs are the skin (e.g. hives), gastrointestinal tract (e.g. vomiting), and upper and lower respiratory tracts (e.g. itchy eyes and or wheezing). Food-induced allergic reactions may also be associated with non-specific emotional and behavioural changes – adults may report a ‘feeling of impending doom’ and children may become ‘suddenly quiet’ or ‘clingy’. The severity of allergic reactions to the same (or different) food allergens is not predictable. Food allergic patients are at risk of both co-reactivity and cross-reactivity.  For example, 20-30% of egg allergic children will develop co-reactivity to peanut whereas 50% of peanut allergic children will develop cross-reactivity to tree nuts.

Top of page

How do you diagnose food allergy?

A diagnosis of food allergy should only be made by a doctor who is skilled in this field as the correct diagnosis will facilitate avoidance of the relevant food allergen/s and the implementation of a personalised emergency plan. Ruling out an incorrect diagnosis will allow for the expansion of an unnecessarily restricted diet. The Doctor will always obtain a rigorous clinical history prior to ordering allergy tests. Allergy testing should never be performed in the absence of a clinical history. In young children it is important to realise that food allergy may masquerade as a ‘food aversion’. For example, babies with egg allergy may refuse - rather than react to - egg, thus asking, “does your child have egg allergy?” to the parents may be misleading, even if the parents answer “no”.  The history should aim to establish if the child can eat an age-appropriate quantity of the food.  For instance, a non-allergic 5 year old child should be able to tolerate a peanut butter sandwich, a whole egg, a whole slice of bread, or a full glass of milk.  Only if these criteria are fulfilled can it be said that the child is tolerant to that food. The Doctor will also perform a physical examination to determine if your child growing well and to exclude signs suggestive of nutritional failure such as iron deficiency and rickets. The respiratory and dermatological examination should also seek to document the presence or absence of concomitant allergic conditions such as asthma and/or eczema.

Top of page

Allergy Testing

There are only two scientifically validated allergy tests for the investigation of IgE-mediated food allergy; the Skin Prick Test (SPT) and specific IgE antibody testing. Tests should be performed to those ‘candidate food allergen/s’ identified on clinical history. Allergy testing is performed to ensure that the history correctly identified the cause of the index food reaction. Testing should also establish if the patient is co or cross-allergic to other food allergens, particularly if the patient has yet to eat these foods or if the foods were previously tasted but ‘disliked’. Testing should also seek to identify allergens which may influence the control of other allergic disorders; for example, pet dander allergy may exacerbate asthma which in itself is a risk factor for more severe food-induced allergic reactions. If a detailed history is unable to clearly identify a candidate allergen, then a screening allergen panel may be required. Screening panels should not be open-ended and should include allergens which are relevant to the patient’s age, allergic condition and geographical location. In practice, testing to peanut, tree nuts, egg white and cow’s milk will account for the majority of food allergies in young children. Wheat, soya and kiwi can be added to these panels, if not previously tolerated or eaten. In older children and adults, fin-fish, shellfish, kiwi and sesame should also be added to the panels, unless an unequivocal history of tolerance to these foods is obtained.

Allergy tests are able to predict for the likelihood of future allergic reactions if accidental exposure were to occur, however, this testing cannot predict for the severity of these reactions. The use of allergy test predictive values significantly reduces the need for diagnostic dietary investigations. Supervised incremental oral challenge tests remain the gold standard investigation for the diagnosis of food allergy.

Top of page

Ongoing management of food allergy

The main strategy for the prevention of food allergic reactions is avoidance of the known allergen. This may prove difficult unless patients, and caregivers, are trained to identify the relevant food/s. Dangerous scenarios for accidental food reactions include nursery/school environments, restaurants, meals which are accompanied by the intake of alcohol and commercial air flights. The patient and caregivers must be trained to identify allergy symptoms and empowered to commence early, and appropriate, emergency treatments if required. All patients with food allergy require a written emergency plan. The emergency plan must clearly identify the patient and their medical contacts, their food allergies (and asthma if present) and steps to be taken in the event of an accidental exposure. The emergency plan, and medications, should be accessible at all times.

The majority of food-induced allergic reactions settle spontaneously, or after the administration of an antihistamine. Adrenaline delivered via the intramuscular route remains the drug of choice for moderate to severe food-induced allergic reactions. As it is difficult to predict the likely severity of future food-induced allergic reactions, all patients should at the very least, be considered for the prescription of a self-injectable adrenaline device. Wheezing is a common food-induced allergic symptom, particularly in children.  Therefore, all food allergic patients with concomitant asthma should have a Salbutamol inhaler and Corticosteroids included in their emergency plan. Corticosteroids may also provide protection against delayed ‘bi-phasic’ anaphylactic reactions which typically occur four hours post allergen exposure. Specific weight and/or age related doses of these medications are available from the UK Resuscitation Council.

The common food allergens are excellent sources of nutrition; food-allergic children are often allergic to more than one food. Unsupervised elimination diets, particularly if performed to more than one food, may therefore result in nutritional complications. The nutritional management of individuals with food allergy cannot be stressed enough, and is a task best performed by a dietitian.  The ongoing assessment of the food allergic patient is important and aims to re-evaluate the patient’s allergic status, nutritional status, emergency plan and psychological wellbeing.

Top of page

List of common food allergies

Table 2: Overview of Immune Mechanisms of Food Hypersensitivity reactions

IgE mediated
Gastrointestinal Oral allergy syndrome, gastrointestinal anaphylaxis symptoms such as vomiting, pain, diarrhoea
Cutaneous Urticaria, angioedema, pruritus, morbilliform rashes and flushing
Respiratory Acute rhinoconjunctivitis, wheezing, coughing and stridor
Generalized Anaphylaxis
Mixed IgE and cell mediated
Gastrointestinal Eosinophilic oesophagitis, Eosinophilic gastroenteritis
Cutaneous Atopic eczema
Respiratory Asthma
Cell mediated
Gastrointestinal Food protein–induced enterocolitis, proctocolitis and enteropathy syndromes. Celiac disease
Cutaneous Contact dermatitis, dermatitis herpetiformis
Respiratory Food-induced pulmonary hemosiderosis (Heiner syndrome)

Are there any guidelines regarding teenagers with food allergies?

Food Allergy Canada has some great videos on their YouTube channel. You can also watch a selection here.

Top of page