What is the definition of food intolerance?

Food intolerance is the general term used to describe a range of adverse responses to food, including allergic reactions (e.g. peanut allergy or coeliac disease), adverse reactions resulting from enzyme deficiencies (e.g. lactose intolerance or hereditary fructose intolerance), pharmacological reactions (e.g. caffeine sensitivity) and other non-defined responses. Food intolerance does not include food poisoning from bacteria and viruses, moulds, chemicals, toxins and irritants in foods, nor does it include food aversion (dislike and subsequent avoidance of various foods). Food intolerance reactions are usually reproducible adverse responses to a specific food or food ingredient, which can occur whether or not the person realises they have eaten the food. This is the basis of the ‘gold standard’ testing procedure, the double blind placebo controlled challenge, in which neither the subject nor the operator knows which test contains the allergen, and which is the placebo.

What is the definition of food allergy?

An allergic reaction to a food can be described as an inappropriate reaction by the body's immune system to the ingestion of a food that in the majority of individuals causes no adverse effects. Allergic reactions to foods vary in severity and can be potentially fatal. The Food Standards Agency estimates that around 10 people a year in the UK die from severe allergic reactions to food. In food allergy the immune system does not recognise as safe a protein component of the food to which the individual is sensitive (such as some peanut, milk and egg proteins). This component is termed the allergen. The immune system then typically produces immunoglobulin E (IgE) antibodies to the allergen, which trigger other cells to release substances that cause inflammation. Allergic reactions to food are usually localised to a particular part of the body and symptoms may include stomach upsets, rashes, eczema, itching of the skin or mouth, swelling of tissues (e.g. the lips or throat) or difficulty in breathing. A severe reaction may result in anaphylaxis (as with severe peanut allergy) in which there is a rapid fall in blood pressure and severe shock. Food allergy is relatively rare, affecting an estimated 1-2% of people in the UK. It is more common in children than adults especially those under the age of three, and is often wrongly used as a general term for adverse reactions to food.

Are there different types of allergy?

There are two well-defined mechanisms via which allergic reactions to food (i.e. reactions that involve the immune system) can occur. Most cases of food allergy involve the production of antibodies known as immunoglobulin E (IgE) and are known as IgE-mediated allergies. Symptoms develop quickly and can vary in severity, but the severest form of this type of reaction is anaphylactic shock.

The other recognised mechanism is a delayed response (taking hours or even days to develop), which involves a different immune system component, T-lymphocytes (T cells). The best defined example of this type of reaction is the autoimmune disease, coeliac disease (sensitivity to the protein, gluten, found in wheat and to related proteins in other cereals such as barley and rye), but delayed reactions can also on occasion occur in response to a range of other foods, including milk and soya.