Food intolerance refers to any adverse reaction to food or food additives. Food allergy is only one such reaction and refers specifically to food intolerance in which the body’s immune (defence) system is directly involved.

Some doctors prefer to use the term food intolerance only for adverse reactions to food in which the body’s immune system is not involved (non-immune or non-allergic).

There are different types of non-immune reactions to food. The most obvious example is simple toxicity. Contaminants (substances which get into food during the growth, harvesting, processing, packaging or storage of food) are occasionally harmful. In severe cases these contaminants can make food poisonous and anyone eating them will suffer ill effects. Much less obvious are the adverse reactions of a few people who lack the enzymes necessary to digest certain foods.

A common example is lactose intolerance. This is due to the lack of the enzyme lactose, which is essential for the digestion of the milk sugar lactose. Most people have adequate levels of the necessary enzyme and tolerate milk.

Another example of non-allergic food intolerance is the reaction caused by naturally occurring chemicals in food, or by food additives (chemical reaction). These reactions are similar to true allergic responses and may be mistakenly labelled as food allergy.

Food additives include a variety of substances, such as preservatives, flavouring agents, colouring agents, etc. Well-known examples are tartrazine, monosodium glutamate (MSG), sulphur dioxide and benzoates. Chemical reactions to food or food additives are not true allergic reactions because they do not involve the immune system.

Another interesting cause of a non-allergic reaction is psychological reactions to foods. It has been shown that if one is convinced, perhaps by a past experience, that one cannot tolerate a certain food, one tends to avoid it. Should one eat it again in the future, the chances are that one will experience, that one cannot tolerate a certain food, one tends to avoid it.

Should one eat it again in the future, the chances are that one will experience ill effects.

Is true food allergy common?
True food allergy is less common than popularly believed. It is estimated that only between 1% and 4% of the general population suffers from a definite food allergy. Food allergy tends to be more common in children (up to 6%) than adults. In selected groups, such as children with eczema, the prevalence of food allergy may be as high as 25%.

What are the symptoms of food allergy?
The symptoms of food allergy may be confined to the digestive system only (the most common sort), or involve other parts of the body as well.

Symptoms of the upper part of the digestive system, which come on rapidly, include swelling of the lips, and itching and redness around the mouth. Sometimes the mouth becomes intensely itchy and the uvula (the small hanging fold of flesh at the back of the throat) may become swollen. Foods which most often cause such reactions are eggs, nuts, shellfish, citrus fruits and berries.

Reactions of the lower part of the digestive tract may take a little longer to develop such as nausea and vomiting; other common symptoms are stomach cramps (colic) and diarrhoea.

Other systems which may be involved in food allergy are the skin, respiratory (breathing) organs and the central nervous system.

Skin symptoms include eczema and urticaria (hives or nettle rash) characterised by raised red and itchy wheals.

The respiratory organs are less commonly affected than the digestive tract or skin. Symptoms of the respiratory organs include asthma and hayfever.

The effects of food allergy on the central nervous system (CNS) is a controversial area and one which tends to attract a lot of media attention. CNS disorders which have been linked to food allergy include migraine, the allergic tension-fatigue syndrome and hyperactivity. The tendency to suffer from migraine runs in families.

Several foods have been shown to trigger migraine: chocolate, red wine, yeast extracts, hard cheeses, milk and eggs.

Children with the allergic tension-fatigue syndrome have pale faces with dark rings under their eyes, giving them a tired look.

They may be difficult to rouse in the mornings and tend to concentrate poorly at school, especially in the mornings.

These children tend to be irritable and to sleep badly at night. It has been found that there is a link between this syndrome and an excessive intake of milk, cool drinks and chocolates. Elimination of these foods from the diet may improve the symptoms of these children quite dramatically.

How is food allergy diagnosed?
The diagnosis of food allergy rests more on a careful evaluation of the patient than on laboratory tests.

The diagnosis is easy when the reaction occurs soon after a new food is introduced. If a commonly used food is involved, the diagnosis is more difficult. In this situation, a variety of tests can be used.

The final mainstay of diagnosis remains the demonstration of relief of symptoms on removal of a given food item and recurrence of symptoms on its re-introduction (elimination-challenge testing).

Before this is undertaken, skin tests and a test called RAST (which detects IgE antibodies in the blood) may help to pinpoint the foods that should be tested in the withdrawal-relief and challenge procedures.

In general skin tests and RAST are helpful, especially in children, but they are far from being totally accurate in the diagnosis of food allergy.

How is food allergy treated?
Avoidance of the offending food is the mainstay of treatment. At the same time it is essential to provide a balanced diet which contains enough protein, calories, minerals and vitamins. Close co-operation between the patient, the doctor and a qualified dietician is important to ensure this.

Strict avoidance of offending foods is the key to successful treatment. When dietary measures do not appear to work, this may be due to one of three reasons:

 
• the diet is not strict enough; or,  
• the patient is eating hidden sources of the offending food;  
• the reaction might be due to food additives, such as colouring agents,  
• or to other naturally occurring chemicals, such as salicylates and histamine;  
• the diagnosis of food allergy is not correct.

If the diagnosis of food allergy is correct and the recommended diet is not working, medication may have to be added to the treatment. The doctor will usually decide on the appropriate medication, depending on the patient’s symptoms.