What is a food allergy?
Food allergy is an immunological reaction to a food protein that is either immediate (occurring seconds to minutes after eating or touching the food item) or delayed (occurring hours or days later). Allergic reactions may occur when the affected person eats or touches a tiny amount of the responsible protein.
Food allergy is most common in young babies (4%), who often outgrow their allergies. About 2% of adults also suffer from an allergy to one or more foods. The tendency to food allergy runs in families.
Food allergy and the skin
Skin conditions due to food allergy include:
- Some cases of anaphylaxis: serious reaction with urticarial rash, difficulty breathing and circulatory collapse.
- Some cases of acute urticaria: a hive reaction occurring soon after eating the responsible food.
- Some cases of contact urticaria: swelling and redness confined to the area touching the food.
- Mucosal contact urticaria: short-lasting irritation and swelling confined to mucosal surfaces, particularly mouth and lips.
- Pollen-food allergy syndrome: symptoms are triggered by eating specific raw foods in people with certain pollen allergies
- Tick bite-induced red meat allergy
- Some cases of atopic eczema: exacerbations of eczema after eating certain foods.
- Some cases of allergic contact dermatitis: touching a food such as fig results in dermatitis in areas in contact with it.
- Some cases of photoallergic contact dermatitis: touching a food results in dermatitis in areas in contact with it and then exposed to the sun.
- Protein contact dermatitis: contact urticaria followed by dermatitis.
- Some aphthous ulcers, e.g., due to chocolate or mint allergy
- Dermatitis herpetiformis (gluten): an uncommon blistering rash associated with coeliac disease.
Other symptoms from a food allergy
Food allergy may result in the following symptoms.
- Hay fever (rhinitis) — sneezing, running/itchy eyes
- Asthma — wheezing, difficulty in breathing, cough
- Nausea and vomiting
- Heartburn, reflux
- Colic, cramps, diarrhoea, constipation
- Irritable bowel syndrome (IBS)
- Anaphylaxis — low blood pressure, shock
What foods cause allergic reactions?
About 90% of systemic allergic reactions are due to the following food items:
- Tree nuts.
There are numerous other foods and plants that occasionally cause allergy. Some are listed here.
- Seeds and nuts such as cashew
- Fruit, including fig, kiwifruit, chestnut, lemon, lime and mango
- Corn and maize
- Garlic, leek and onion
- Vegetables, including artichoke, asparagus, celery, lettuce, parsnip
- Legumes (chickpeas, beans, peas, lentils)
- Herbs such as mint, parsley, chives
- Spices such as balsam of Peru and vanilla
- Red meat (in tick bite-induced red meat allergy)
How is food allergy diagnosed?
Food allergy is diagnosed by taking a careful history of the symptoms and their relationship to food, supported by examination findings and the results of tests. Unfortunately, neither history nor tests are entirely reliable in everyone.
- A false positive means the test was positive, but the patient is not allergic to the test substance.
- A false negative means the test was negative, but the patient is allergic to the test substance.
The main tests for food allergy are:
- Prick tests
- Specific IgE blood tests (RAST)
- Patch tests, in some cases of dermatitis
- Skin biopsy
- Antigliadin/antiendomysial blood test for gluten sensitivity/coeliac disease and dermatitis herpetiformis.
Other reactions to food
Not all reactions to food are allergic in origin. Intolerance can cause similar symptoms to allergy, including urticaria and dermatitis. But the reaction often depends on how much is consumed. These reactions are classified as follows.
- Allergy-like intolerance, that is, symptoms consistently arise from a specific food, but tests are negative. Symptoms are often due to FOD-MAP sugars in various foods.
- Phototoxic contact dermatitis to a plant such as lime or parsley.
- Irritants, such as irritant hand dermatitis due to handling vegetables and fruit; stinging from acidic compounds.
- Chemical intolerance, such as urticaria induced by salicylates and amines.
- Food toxins, such as erythema in scombroid fish poisoning.
- Enzyme deficiency, such as porphyria cutanea tarda induced by alcohol, flushing from alcohol.
- Excessive ingestion, such as carotenoderma from excessive intake of deep yellow, orange or green coloured fruit and vegetables.
- Heavy metal toxicity from contaminants in food, such as mercury found in long-lived fish.
Skin conditions may also be due to deficiencies in the diet, sometimes because of attempts to reduce exposure to known or presumed allergens.
- Protein-energy malnutrition
- Vitamin B deficiency: pellagra
- Vitamin C deficiency: scurvy
- Vitamin D deficiency
- Zinc deficiency: acrodermatitis enteropathica and acrodermatitis enteropathica-like conditions
- Iron deficiency
- Biotin deficiency
What is the treatment for food allergy?
The mainstay of management is to identify which foods are responsible for reactions, and then to avoid them. Prescribed treatments depend on symptoms, and may include:
- Adrenaline injections for emergencies in case of anaphylaxis
- Antihistamines for urticaria and rhinitis
- Topical steroids for eczema
- Nasal sprays for rhinitis and inhalers for asthma
- Dapsone for dermatitis herpetiformis
Prevention of food allergy
It is not known how to prevent all food allergy. Recommendations regarding the introduction of peanuts to the food of infants have recently changed.
- Children with mild to moderate eczema should have peanut introduced around six months of age.
- Children with severe eczema, egg allergy, or both should have testing for peanut allergy followed by peanut introduction at 4–6 months of age based on these results.