A positive result from an allergy test does not necessarily indicate a clinically significant allergy.
If the allergic reactions are not severe, there is usually no need to eliminate the offending food.
An anaphylactic or other generalised reaction warrants investigations in specialist care, guidance to an elimination diet, an adrenaline injector and allergy medication in reserve as well as assessment and counselling by a therapeutic dietitian as needed.
Epidemiology
The prevalence is 3-4%, but up to 20% of adults avoid certain foods due to symptoms.
Symptoms usually start in childhood. The majority of allergies to commonly used foods (such as milk) resolve before adulthood. If an allergy persists into adulthood it resolves more rarely.
Food allergy may also begin in the adulthood.
Symptoms
Oral allergy syndrome (OAS) is a common food-related allergy seen in adults. OAS typically develops in patients with an allergy to tree pollen (birch). Raw and unprocessed fruit and vegetables cause itching in the mouth and possibly swelling of the pharynx.
Trigger foods may exacerbate the symptoms of asthma and allergic rhinitis.
Gastrointestinal symptoms (nausea, abdominal pain, diarrhoea and vomiting) are possible.
Exacerbation of atopic dermatitis, urticaria or angio-oedema may occur rarely.
Anaphylactic reaction
On rare occasions, a particular food may result in an anaphylactic reaction in association with exercise alone. Wheat-dependent exercise-induced anaphylaxis is the most documented.
Causative foods
Any food or food ingredient can contribute to a food allergy.
Heating or freezing fresh vegetables changes their proteins and thus improves tolerability.
Cross-reactions with pollen are common.
Birch - fruit, root vegetables, pulses, nuts (significant in practice)
Grasses - cereals (appears primarily as a cross-reaction in skin prick tests)
The most common foods causing severe reactions include nuts and seeds as well as fruit and vegetables, but e.g. milk and wheat can also act as the causative foods.
New methods are available for the recognition of allergen components, i.e. proteins that share similar characteristics and cause cross reactivity. These components are either termed labile (allergenicity easily reduced by, for example, heating) or stable. The heat-stable components are more likely to cause severe reactions.
Certain medicines (e.g. anti-inflammatory drugs, beta blockers and ACE inhibitors) and alcohol may play a role in the development of an allergic reaction.
Investigations
Thorough history
Careful description of symptoms; the amount of food, and the length of time after ingestion, required for the symptoms to occur. Symptoms that develop within minutes after a small amount of the offending food are suggestive of significant IgE-mediated allergy.
A positive result (i.e. sensitization to an allergen) does not infer clinically significant allergy, but the results must always be viewed in relation to the symptoms.
Patients who have become sensitised to pollen often test positive for plants, spices, vegetables and cereals without having major clinical symptoms.
An allergen-specific serum IgE test is indicated if an SPT is not available or cannot be performed. The results should be interpreted in the same manner as those of an SPT. The more antibodies there are the more likely it is that the patient is symptomatic.
Allergen component-resolved diagnostics are performed in specialized care, e.g. when investigating whether the patient's symptoms are caused by severe peanut allergy or by cross-reaction with birch tree.
An elimination diet test at home: the suspected food is eliminated from the diet for 2 weeks. If the symptoms disappear, the food is reintroduced whilst observing for symptom occurrence. It may be necessary to repeat the test, particularly if the patient has gastrointestinal symptoms.
An open or double blind placebo-controlled food challenge is performed at an outpatient clinic.
The possibility of asthma must be borne in mind, and appropriate tests carried out, if the patient has severe allergic reactions (particularly if dyspnoea is present).
Investigations in specialist care are indicated
for a serious allergic reaction caused by an unidentified allergen
if the elimination diet is extensive and there is a risk of nutritional deficiencies.
Differential diagnosis
Intestinal symptoms: irritable bowel syndrome, lactose intolerance and coeliac disease
Biogenic amines (e.g. fish preserves) and histamines may sometimes cause symptoms suggestive of an allergy.
Eosinophilic gastroenteritis comprises a cluster of inflammatory intestinal diseases (may involve the oesophagus, stomach and, in some cases, the lower gastrointestinal tract).
Symptoms include food sticking in the oesophagus and symptoms similar to those of gastro-oesophageal reflux disease.
Diagnosis is made on endoscopic biopsies which show eosinophilic inflammation.
A history of atopy is present in up to 70% of patients, and the avoidance of certain foods is beneficial in some cases.
So far, the role played by food allergy is not fully understood.
If the symptoms are mild an antihistamine may be prescribed, to be used as required, whilst oral glucocorticoids are used for more severe symptoms, either as single doses or courses lasting for a few days.
The treatment of a serious allergic reaction is intramuscular adrenaline (epinephrine) Anaphylaxis. The patient must be given hands-on training on the use of the adrenaline autoinjector. The patient should always carry an autoinjector with him/her together with antihistamine and glucocorticoid tablets, which are used as adjunctive treatment.
An elimination diet is not prescribed if the symptoms are not severe. Avoiding certain foods as a precautionary measure is likely to be harmful.
Allergen-specific immunotherapy for pollen allergy (e.g. birch pollen) Allergen Immunotherapy may relieve the symptoms caused by cross-reactivity.
Various food allergen-specific therapies are being developed.
Muraro A, Werfel T, Hoffmann-Sommergruber K et al. EAACI food allergy and anaphylaxis guidelines: diagnosis and management of food allergy. Allergy 2014;69(8):1008-25. [PubMed]
Dellon ES, Liacouras CA. Advances in clinical management of eosinophilic esophagitis. Gastroenterology 2014;147(6):1238-54. [PubMed]