There are at least five reasons why some people are unable to tolerate certain foods, of which allergy is only one (see below). In true food allergy the body's immune system is directly involved, whereas in food intolerance other factors play a role. The manifestations of the reaction - such as sudden vomiting or diarrhoea - may be identical in both cases.
The most obvious example of food intolerance 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 incidental additives can make foods 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. A significant number of people come into the world without the enzyme lactase, which is essential for the digestion of the milk sugar lactose. As a result, they invariably suffer from flatulence and diarrhoea when they drink cow's milk. The majority of the population obviously have the necessary enzyme and can drink milk without any ill effects.
A third - and very interesting - possible cause of intolerance is our psychological reactions to foods. It has been shown that if you are convinced, because of a past experience, that you have a bad reactionto a certain food, you will usually avoid it. But if you do eat it again the chances are it will affect you in the same way, sometimes eventhough it cannot be proved that you are in any way allergic to it. One survey of 580 adults found that two-thirds of them avoided one ormore foods because of their taste or texture or due to some past reaction.
Next come foods that produce reactions which seem very much like true allergic responses. For example, mature cheese may contain histamine or tyramine (which will cause reactions) while foods such as strawberries and shellfish may trigger off certain cells in the body which are directly involved in allergic reactions. These are not true allergic reactions because they do not involve the immune system and naturally occurring chemicals within the foods - not the foods themselves - are responsible for triggering off the reactions.
So much for food intolerance. What we are really referring to when we talk about 'food allergy' are those reactions which have a truly allergic basis and have to do with the body's immune (defence) system.
Potential problem foods
The following are some of the foods which most commonly cause reactions in sensitive individuals:
Foods rich in histamine: Cheese, chocolate, avocados, bananas, tomatoes, sauerkraut, broad beans, canned figs, soy sauce, meat and yeast extracts, smoked or pickled fish, liver, alcoholic beverages (except whisky, gin and vodka).
Foods rich in tyramine: French cheeses such as Camembert, Brie, Gruyere and Roquefort; Cheddar cheese, brewer's yeast, herrings, chianti.