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Food allergyThis factsheet is for people who have, or suspect that they have a food allergy. Most people have a bad reaction to a food at some point, and some may worry that an allergy may be responsible. Around one in five people think that they have a food allergy - however, true or 'classic' food allergy is estimated to affect only 1 in 70 adults. Food allergies in young children are more common than in adults, but most children outgrow these by school age. True food allergiesWith true or 'classic' food allergies, a problem food triggers the immune system, producing a response to the food that can cause severe and even life-threatening symptoms. It's important that if you think you have a true allergy to a food you get it diagnosed by a doctor, so that you can avoid it and prevent further allergic reactions. Having a tendency to allergies (atopy) is partly inherited. If eczema, asthma, or hay fever run in your family, you're more likely to have allergies. Is it really a food allergy?Food allergy is sometimes confused with a food intolerance or food poisoning, which do not involve an immune response. These can make you feel ill, and may be dangerous, but they are not usually harmful in the same way that a true food allergy might be (see Symptoms and Food intolerance for more details). Food and the immune systemThere are two components involved in a true allergic reaction. The first is the production of immunoglobulin E (IgE), an antibody that circulates in the blood attacking things such as germs. The second is the mast cell, a type of cell that is found in the body at places where allergic reactions occur, including the nose, throat, lungs and skin. If you have a food allergy, your body mistakes certain foods as harmful, and produces IgE antibodies. This is known as sensitisation and may not cause many, if any, symptoms. However, the next time you eat the food, the IgE is ready to react with the food. This causes mast cells to release chemicals such as histamine, which leads to a range of physical symptoms that is called an allergic reaction. SymptomsThe first symptom of an allergic reaction to food is often itching and swelling in the mouth, tongue and throat. You may also get some or all of the following symptoms:
Allergic reactions usually happen within a few minutes of eating food that you are allergic to, but they can take several hours to develop. Some people develop a severe, whole-body allergic reaction called anaphylaxis, or anaphylactic shock. This is a rare but potentially fatal allergic reaction where the symptoms develop all over the body, causing swelling, loss of consciousness, low blood pressure and breathing problems. Problem foodsIn adults, the most common food allergies are to peanuts, tree nuts (such as walnuts and hazelnuts), fish and shellfish. Foods which commonly trigger allergies in children include cows' milk, eggs, peanuts, soya, wheat, tree nuts, fish and shellfish. If you have one food allergy, you are more likely to react to other foods (cross-reactivity). For instance, if you are allergic to prawns, other shellfish may affect you. People with hay fever may also have a cross-reaction to certain foods. Your GP or a state registered dietitian should be able to give you advice on which foods to avoid. Diagnosing a food allergyIf you think you have a food allergy, you should contact your GP who will take a case history of your reactions to try and find out if you have one. The more details you can give, the better. Some examples of questions you might be asked are listed below.
If an allergy seems likely, your doctor may refer you to a specialist allergy clinic. Here you will have some tests to help confirm or rule out a food allergy. Elimination dietsUnder the supervision of an allergy specialist, you remove the suspected allergy food from your diet and replace it with another food. For example, if your doctor suspects an egg allergy, you would replace egg with a different food. If the allergy symptoms go away, a diagnosis can usually be made. To confirm this diagnosis, you may be asked to reintroduce the allergy food back into your diet. However, this is not safe if you've had severe allergy symptoms in the past, except under medical supervision at hospital. You shouldn't cut food groups from your diet without first getting medical advice, as you could risk missing out on important nutrients. Skin prick testsThese are sometimes used to help diagnose a food allergy. An extract of the suspect food is put on a small patch of skin, usually on your forearm or back, and a very small, fine scratch is made. If redness and swelling develops around the scratch, the test is positive for that food allergy. However, you can develop a positive reaction to this type of test without having allergic symptoms when you eat the food. These "false positives" mean that the test is not always completely reliable, so it is usually used in combination with other tests. Again, these tests may not be safe if you've had severe allergic reactions in the past so should be carried out by an allergy specialist or doctor in hospital. Blood testsAnother method of testing for food allergies is the RAST (radioallergeosorbent) test. This measures food-specific IgE in a blood sample. RAST tests may also produce false positive results, but the advantage is that this test can be used to confirm a diagnosis for people with severe food allergies. This is because a sample of your blood is tested, so there is no risk of the suspect foods causing anaphylaxis. It is also useful if you have extensive eczema or if no skin is available for a skin prick test. Unproven 'allergy tests'Numerous tests that claim to detect allergies are available by mail order, through health food shops, sports centres and complementary practitioners. These include hair testing, electro-acupuncture, and various kinds of blood tests, including cytotoxicity testing. However, there is no scientific evidence to prove that these tests have any value in diagnosing or managing food allergy. They can be expensive, and may give a worrying false diagnosis - giving either a false positive, or false reassurance. For this reason, the Royal College of Pathologists recommends that you do not use them. Managing food allergiesSome people outgrow their food allergies, especially children. However, this is unlikely with allergies to nuts. The most reliable treatment for food allergy is to avoid the problem food. A registered dietitian can help you identify and remove a problem food from your diet and replace it with alternatives, to make sure you don't miss out on essential nutrients. Your dietician can explain what you need to look for on food labels. Symptoms of mild food allergies, such as a rash or runny nose, may be treated with antihistamines, which are available without a prescription from pharmacists. Emergency treatmentPeople with severe food allergies who are at risk of anaphylaxis need to take precautions. Despite reading menus and ingredients lists carefully, it is easy to eat a problem food by accident. A tiny amount in a dish can still trigger a severe reaction.
Food intoleranceThis type of response to a trigger food may develop over a number of hours or days. Food intolerances are sometimes caused when you aren't producing enough of the natural digestive chemicals to break down a particular type of food, although often the cause remains unknown. Symptoms of food intolerance include diarrhoea and flatulence. The most common food intolerances are to lactose (the sugar found in milk) and gluten (this is called coeliac disease). Food additives, such as preservatives and colourants, can also cause skin rashes and wheezing in some people. Food poisoningThis is a reaction to food contaminated by bacteria, viruses, parasites and naturally occurring toxins. For more information, see the separate BUPA factsheet, Food hygiene. Food poisoning symptoms can sometimes be similar to a food allergy. However, unlike food allergies, food poisoning is likely to affect most or all of the people who eat the contaminated food. Further information
Sources
Published by BUPA's health information team, healthinfo@bupa.com, November 2006.
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