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A food allergy is an abnormal response of the immune system to an otherwise harmless food. It is currently estimated to affect between 2-5% of the population but it is becoming more common. Food allergy is often confused with other reactions to food such as:
adverse reactions which are abnormal responses to eating a food or food additive
food intolerance, a physiological response to a food or additive, which does not affect the immune system. This can be an individual reaction to food for no apparent reason, a response to the chemicals (for example caffeine), metabolic (such as lactose intolerance) or toxic (shellfish poisoning)
food aversion is a psychological reaction to food where the appearance, taste or smell of a food can cause a range of responses, including panic attacks, which may have a similar appearance to anaphylaxis (shock).
What foods are most likely to cause an allergy?
The most common allergens (things that cause allergy) are peanuts, tree nuts, fish and shellfish. Peanuts are the most common cause of severe allergic reactions. The next most common is milk, eggs, fruits, seeds (sesame), soy and cereal grains. Food additives and spices may also cause reactions. Some highly allergic individuals can have allergies to a wide range of food groups.
What happens in an allergic reaction?
The immune system mistakenly believes that the food is harmful to the body, and therefore tries to protect it. It sends out its own army (antibodies) to fight off the invader. These create special IgE antibodies against that food, so that every time the system is exposed to that food it reacts - sending out chemicals and histamines to protect itself. These chemicals are what cause the typical allergic reactions affecting the skin, breathing, gut and heart.
What are the symptoms?
A reaction can occur within minutes or a few hours after the food is eaten and may lead to many different symptoms including hives, swelling around the mouth, asthma, diarrhea, vomiting, eczema and anaphylaxis (a severe adverse reaction involving the major body systems).
How are food allergies diagnosed?
Adverse reactions to foods are not always allergic reactions. For example, you can be hypersensitive or have intolerance to some foods and have similar symptoms to someone with an allergy without actually having an allergy. For an accurate diagnosis, you need to see an experienced doctor or an allergy specialist. Diagnosing and treating food allergy presents quite a challenge - there are no foolproof laboratory tests and inappropriate elimination diets may have serious nutritional consequences.
Getting an accurate medical history for the affected person and their family is very important to making a diagnosis. The most common tests are skin testing, RAST blood testing and/or elimination diet.
An elimination diet involves removing the suspected food from your diet for a period of one to weeks (short-term). Total avoidance is very important. You should keep a food diary of everything you consume and record any symptoms. Reintroduce the products one at a time, noting any symptoms. This process needs to be supervised by your GP or a registered dietitian who may also be helpful in designing a diagnostic elimination diet. Long-term elimination diets are not recommended unless a dietitian is assessing the diet and ensures that all required nutrients are provided. If you have a family history of allergy, or if an allergy is suspected, you shouldn't reintroduce "allergenic" food without careful medical supervision.
If there is doubt about the diagnosis of food allergy, a food "challenge" can be undertaken where neither the person nor the doctor knows what the food is. The patient receives a series of capsules or liquid containing either the food or placebo and every reaction is documented. Food challenges are performed in hospital.
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