Food Allergy or Food Intolerance?
Food allergy is quite uncommon and normally causes symptoms within a few minutes of eating the offending food or being in contact with the relevant substance. It is caused when the body mistakenly makes an antibody (IgE) which 'fights off' the food when it is eaten (or sometimes is just in contact with the skin, or is smelt). The symptoms are usually those of 'classic' allergy such as rashes, wheezing, itching, severe gut symptoms or (very rarely) sudden collapse. Most people will therefore already recognize that the food causes them a problem. There are reliable blood tests and skin tests, available through the NHS, that can be used quite accurately to confirm the presence or absence of food allergy.
Food intolerance (non-allergic hypersensitivity) 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. Chronic conditions such as Arthritis, Eczema, Irritable Bowel Syndrome, ME (Chronic Fatigue Syndrome), Rheumatoid Arthritis, Migraine and Ulcerative Colitis are sometimes linked to allergy or intolerance to foods or other substances. However, it is very rare for true allergy to be an issue in these conditions. In some people, an intolerance might be a factor; in others, it may just exacerbate their underlying condition. In many people, allergy and intolerance will play no part.
Food intolerance can have a number of different causes. Some people may be lacking in an enzyme that is required for proper digestion of the food. Others seem to be upset by substances that occur naturally in the food (e.g. histamine or salicylates) which don't affect other people. There may be increased sensitivity to natural components such as caffeine, or to food additives. This means that there is no easy blood or skin test to identify intolerance. Some tests have been developed that may be helpful 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.
The most accurate way of identifying whether food intolerance is contributing to a chronic illness is an Elimination and Challenge Diet. This should only take place after a consultation with a qualified specialist, ideally after referral from your General Practitioner. A dietitian can provide an elimination diet tailored to your specific needs. Elimination diets must be followed strictly and for the correct period of time, to be effective.
If your symptoms diminish or disappear with the removal of certain food items, and these symptoms reappear with the reintroduction of the food, then you have proved the cause is dietary. If no change or improvement occurs in your condition while on the elimination diet, then food intolerance is not relevant to your condition and you should return to normal eating.
Having performed an elimination diet and found the cause of your symptoms, the only treatment is to avoid the offending food(s). However, avoiding a large number of foods is not sensible, as good nutrition is vitally important in chronic illness, and because repeatedly eating a small number of foods tends to make intolerances worse. Your dietitian will be able to ensure that your diet is not excessively limited, and that it contains all the necessary vitamins and minerals.
Often, some foods will cause more noticeable symptoms, and will be avoided, but others will only cause quite minor upset and can continue to be eaten, accepting that there will be some slight symptoms when the food is eaten. In this way, as one's condition improves, the more minor intolerances often settle over time.
What should I do if I think I may be reacting to a food?
Although some tests for food intolerance may be scientifically reliable, their relevance to food related symptoms is scientifically unproven.
The Gold Standard, and only way, to ascertain which foods cause adverse reactions, is by accurately recording the times and duration of all symptoms, illness or stress, as well as everything you eat and drink. This includes all prescribed medicines and other supplements, all sweets, nibbles and even licking out the mixing bowl when cooking!
This record diary should be continued for 2 weeks and should be representative of your normal diet. Use a new page each day. Ideally, it should be analysed by a registered dietician or nurse with nutritional training.
It is helpful to keep food packaging for reference by the health care professional.
Video on this subject
Last updated: March 2012