Definitions and Causes
Food allergy is often mistaken for food intolerance. It is important to note that allergy is only one of a number of possible reasons for food intolerance.
Food intolerance can be defined as a condition where particular adverse effects occur after eating a particular food or food ingredient. Genuine food intolerance is different from psychologically based food aversion, where a person strongly dislikes a food and believes that a food produces a particular reaction.
A genuine food allergy is when a specific immune reaction occurs in the body in response to consuming a particular food. Allergies often run in families, and people who are allergic to some foods may also be allergic to other environmental factors, such as house dust, animal fur and pollen.
A true allergic response involves an altered or abnormal tissue reaction to an antigen. An antigen can be a protein, a substance bound to a protein, a food additive or less commonly, a polysaccharide. The antigen combines with an antibody and produces an immune response, which results in cell damage and the release of histamine. The immune system plays an essential role in our bodies in protecting us from the invasion of harmful substances. An allergy occurs when the mechanism operates inappropriately in response to a harmless substance such as a particular food protein.
Food intolerances, other than allergies, can occur for a variety of reasons including:
Non allergic histamine release
The signs are very similar to an allergy and include headache, swelling, urticaria, vomiting and diarrhoea. A substance called histamine is released (it is also released in true allergic reactions) in response to foods such as shellfish or strawberries.
Metabolic defects
A lack or deficiency of enzymes responsible for the digestion of food can cause many types of food intolerance. For example, a deficiency in lactase, the enzyme responsible for digesting milk, causes intolerance to milk.
Coeliac disease is a gut intolerance to a protein found in wheat, called gluten, it would not be considered an allergy. The symptoms of coeliac disease are controlled by following a gluten-free diet. It is unknown exactly why or how gluten harms the gut, although it is now thought to be an abnormal immunological response rather than an enzyme deficiency. It is still not considered to be a food allergy in the true sense of the definition.
Pharmacological effects
Some food substances can act like drugs, particularly if taken in large quantities. The most familiar of these substances is caffeine, found in tea, coffee, chocolate and cola drinks. A large intake of caffeine can cause tremor, migraine and palpitations. Other pharmacologically active substances found in food include histamine, tyramine, tryptamine and serotonin, which may be consumed in foods such as red wine, cheese, yeast extract, avocados and bananas. In susceptible people, these foods can trigger urticaria, facial flushing and headaches.
Food intolerance of unknown origin
Reactions can be provoked by many foods and food products which we cannot be clear about. They may or may not be allergic reactions. Food additives, particularly tartrazine and sodium benzoate, can provoke urticaria, rhinitis and asthma. Yeasts can provoke a number of reactions in some people, particularly skin disorders.
Common Causes of Food Intolerance
The most common food intolerances, in order of frequency are milk, eggs, nuts, fish/shellfish, wheat/flour, chocolate, artificial colours, pork/bacon, chicken, tomato, soft fruit, cheese and yeast.
Whilst not all food intolerances are related to meat and dairy products, it can be seen from the above list that vegetarians, and particularly vegans, will suffer less from food intolerance because they already eliminate some of the most common causes of intolerance.
Symptoms
The most common symptoms of allergy include asthma, gastro-intestinal symptoms (nausea, vomiting, and diarhhoea), eczema, urticaria (hives), rhinorrhea (heavy discharge from the nose), and angio-oedema (swelling of the blood vessels). Other more long-term symptoms include can depression, anxiety, fatigue, migraine, sleeplessness and hyperactivity in children.
Treatment
As it is sometimes quite difficult to distinguish between a genuine food allergy and a food intolerance, treatment is often similar. The first step is to diagnose the food intolerance. This should not be done without medical supervision as some reactions to food intolerance can be dangerous.
Sometimes the cause of a particular food intolerance is obvious, by the immediate effect that occurs on eating a particular food. In this case the treatment is simply to avoid that particular food. In most cases the suspected food is more difficult to track down. A diary kept of foods eaten and symptoms experienced can sometimes help detect the offending food or foods. Other factors such as the weather, menstrual cycles and difficult relationships can affect the symptoms. Sometimes simple exclusion diets are advised where record keeping suggests a particular food may be the cause. So, for example, milk, egg or wheat may be avoided to see if symptoms improve.
Other more restrictive diets may be advised, which only include a limited amount of foods which rarely cause a reaction. These diets are usually called exclusion diets. The idea of an exclusion diet is to identify an allergy or intolerance, by limiting the food to a very small choice, checking for symptoms and then very gradually introducing test foods to see if there is a reaction. An exclusion diet should not be followed without sound nutritional advice.
You can seek the help and advice of The Vegetarian Society if you have any problems with your vegetarian or vegan diet, relating to diagnosis or treatment of a food allergy or intolerance.
Further Reading
(available from our Online Store)
· The Food Allergy Plan, Dr Keith Mumby, Urwin Paperbacks.
· The Allergy Connection, Barbara Paterson, Thorsons.
· Food Intolerance Fact and Fiction, Dr Juliet Gray, Grafton Books.
· What's wrong with John? - and the sequel - What's right with Bill?, Clare Fretwell, Paperback, £9.99 each.

Further Information

· Coeliac Society, PO Box 220, High Wycombe, Bucks, HP11 2HY.
· National Society for Research into Allergy, PO Box 45, Hinckley, Leicestershire.
· The Leisure and Lifestyle Directory - for all commercial products and services related to vegetarianism.