Ten years ago a food allergy or intolerance was hardly heard of. It was seldom linked to any medical condition, nor was much known about the impact of the food substances on an individual with an allergy or intolerance. Times have changed, and it seems like every second child is being diagnosed with some form of food allergy or intolerance. Is this a consequence of change in our diet to more refined and processed foods? Or is it purely an increased knowledge of these conditions and their potential impact on our health?
1. Definitions and Classification
Food hypersensitivity is the non-specific term which refers to any adverse (untoward) reaction to food or food additives. Food allergy is a type of food hypersensitivity in which the body's immune (defence) system is directly involved and overreacts to a particular protein in that food. There are several types of hypersensitivity reactions to food which are non-allergic, commonly known as food intolerances (a non-immune food reaction).
Many people who think they are allergic to a food may actually be intolerant to it. Some of the symptoms of food intolerance and food allergy are similar, but the differences between the two are very important. Food allergy reactions can be life-threatening, in some cases even in response to tiny amounts of the food, so people with this type of allergy must be very careful to avoid their food triggers. Food intolerances are not life threatening and the person may often tolerate small amounts of the triggering food, and have symptoms only on intake of larger amounts.
There are 2 main groups of food allergies, namely IgE-mediated (immediate type) food allergies, and non-IgE mediated (delayed type) food allergies. These types of allergies present quite differently as they have very different underlying mechanisms. IgE- mediated allergies are most commonly to milk, hen's egg, peanut and other nuts, fish, shellfish, soya and wheat. Non IgE- mediated allergies are most commonly to cow's milk and soya, but can occur to many other solid foods.
1. IgE-mediated (immediate type) food allergy
IgE-mediated food allergies are the most common type and present as "classical" food allergies with reactions typically within minutes to 2 hours of intake of the offending food. Different people need different amounts and forms of the food for symptoms to occur, for example in a milder allergy a person may need to eat a reasonable portion size before they have symptoms; in a severe allergy they may just need a tiny amount. In fact some people even react to the food in contact with the skin, e.g. after being kissed by someone who has just eaten the food, and some people react if the food particles are in the surrounding air, e.g. if someone is cooking using that food.
The diagnosis of food allergy rests on a careful evaluation of the patient together with laboratory tests.
Taking a good history may reveal reproducible symptoms involved in eating certain food types. This is usually more obvious in the immediate reactions then in the delayed type reactions.
In immediate type allergies skin prick tests and/or blood tests (which detect IgE antibodies to various foods in the blood) are useful. Remember that a positive skin or blood test does not necessarily mean the person is allergic. If the diagnosis remains uncertain, or to test whether someone has outgrown their food allergy, an oral food challenge may be necessary. In an oral food challenge, initially tiny then increasing amounts of the food in question are given under close supervision to check for reactions.
Laboratory tests are less useful for the delayed type allergies. For such allergies, the final mainstay of diagnosis remains the demonstration of relief of symptoms on removal of a given food item for a few weeks, and recurrence of symptoms on its re-introduction (elimination-challenge testing). A Dietician trained in allergies will need to be involved in this process. In a few cases, a biopsy specimen of the gut may be required.
At the moment, there is no "cure" for food allergies. Avoidance of the offending food(s) is the mainstay of treatment. The patient needs to be educated about food labelling and hidden sources of the food. At the same time it is essential to provide a balanced diet which contains enough protein, calories, minerals and vitamins. Close co-operation between the patient, the doctor and a qualified Dietician is important to ensure this.
For immediate-type reactions the patient needs to be educated on how to recognise and treat an allergic reaction, in case they accidentally eat the food they are allergic to. This requires an emergency action plan and emergency treatment. For milder reactions, an antihistamine is given. For more severe reactions involving the airway or circulatory system, a dose of adrenaline may be needed and can be life-saving. The doctor will select whether or not the patient needs an adrenaline pen to keep at home/school if they are at risk of severe reactions.
There are different types of non-immune reactions to food, known as food intolerances. These are generally not "dangerous" to the patient, but can cause irritability or discomfort. Intolerances are more common in adults than children, so in children, an allergy always needs to be excluded first.
Most people with food intolerance will tolerate small amounts of a certain food, only to become symptomatic if they have too much of it, for example wheat in people with irritable bowel syndrome . It can also build up over a period of days, after consuming various foods containing the same problem substance. Symptoms can occur immediately, or be delayed by up to 24 - 48 hours, which can often make it difficult to determine what is causing the food intolerance.
Food Allergies can be tested for in the following ways: