Food Allergy or Food Intolerance?

Food Allergy

Food allergy is caused when the body mistakenly makes an antibody (IgE) to 'fight off' a specific food. When the food is next eaten (or sometimes is just in contact with the skin, or is smelt) it triggers an immune system response which results in the release of histamine and other naturally occurring chemicals in the body. These chemicals cause various symptoms, depending on where in the body they are released. For example, in the gut they may cause abdominal pain, vomiting and diarrhoea; in the skin, itching and swelling (rash or nettlerash); in the upper airways, a runny nose or sneezing; in the lower airways, a wheeze or cough. Very rarely the immune system chemicals are released throughout the body, causing a 'systemic' reaction (such as anaphylaxis).

These 'true' allergy reactions are rare. Normally symptoms arise within a few minutes of eating the offending food, although they may be delayed by up to a couple of hours. The symptoms are usually those of 'classic' allergy such as rashes, wheezing, itching, severe gut symptoms or (very rarely) sudden collapse. The rapid onset of symptoms, which always occur in response to the same food, mean that most people will already recognize that the food causes them a problem. There are reliable blood tests (IgE or RAST tests) and skin tests, available through the NHS that can show quite accurately the presence or absence of food allergy.

The foods that most commonly cause allergy are milk and eggs, nuts (including peanuts) and seeds, shellfish, fish, wheat, soya and some fruits such as citrus and kiwi.

Food Intolerance

Food intolerance is much more common. The onset of symptoms is usually slower and may be delayed by many hours after eating the offending food; the symptoms may also last for many hours, even into the next day. Intolerance to several foods or a group of foods is not uncommon, and it can be much more difficult to decide whether food intolerance is the cause of chronic illness, and which foods or substances may be responsible.

The symptoms caused by intolerance are much more variable and can include fatigue, bloating, irritable bowel, joint pains, rashes, nettlerash, eczema, migraine and various other symptoms.

Food intolerance can have a number of different causes; some people may make abnormal antibodies (IgG, different from the IgE that causes allergy) which mistakenly 'fight off' the food when they eat it; others may be lacking in an enzyme that is required for proper digestion of the food. Some people seem to be upset by chemicals that occur naturally in the food, and which don't affect other people. This means that there is no easy blood or skin test to identify intolerance (except for one or two well-researched intolerances such as gluten intolerance and lactose intolerance). Some tests have been developed that may be helpful in identifying intolerances, but none of them give a definite answer; they can only guide us about what might be causing a problem, but this then needs careful checking.

In a few people, underlying conditions such as low levels of digestive enzymes, the presence of the wrong mix of bacteria in the gut, parasites or excessive yeast levels, can make food intolerances worse. In these cases treatment for the underlying condition may be required in order to allow symptoms to improve. You should always seek advice rather than attempting to diagnose or treat these problems yourself.

Updated 4/2008