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What
is food intolerance?
Not all 'allergic' reactions to
food are simple to prove or easy to understand. Food allergy, in which symptoms occur
within an hour of eating the food, and in which a skin prick or a blood test for is positive, is
fairly uncommon (less than 2% of the population). However food intolerance,
with its delayed onset of symptoms and with negative allergy tests, is much more common
affecting about 20% of the population.
Excluding foods from the diet with relief of symptoms and then reintroducing them again with
symptom recurrence is at present the only proven way of diagnosing this.
Causes of
Food Intolerance
Whereas the cause of immediate
food allergy is an abnormal immune response, the exact mechanism for the majority of delayed
food reactions encountered remains unknown. Food
intolerance is diagnosed by the avoidance of a range of
foods to see if symptom clearance occurs. If so, foods
are then reintroduced in turn to establish which cause symptoms:
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Food
Intolerance
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A range of potential culprit foods is avoided and then
reintroduced singly to establish the reactive food or foods.
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The
majority of these delayed reactions to foods diagnosed in this way, appear to be a kind of
delayed allergy (an abnormality of immune system response to the
food) although as yet the cause of this is not fully understood.
There are a few exceptions to this, for example where
one of the following four causes can be shown
to apply:
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Enzyme defect
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Lactose (milk sugar) intolerance causes some of the gut
upsets that may occur with milk. This is caused
by deficiency (sometimes temporary, sometimes permanent) of the enzyme lactase.
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Pharmacological
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Certain foods contain naturally occurring, pharmacologically active ingredients such as
caffeine in coffee, or phenylethylamine in certain cheeses.
These can produce symptoms such as headaches or urticaria in susceptible individuals.
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Drug interaction
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Certain antidepressant drugs (monoamine oxidase inhibitor
drugs - MAOI) can produce serious adverse effects when foods containing certain amine compounds (such
as pickled herrings or anchovies) are eaten.
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Toxic
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A number of foods contain naturally occurring toxic compounds. For example inadequately
cooked kidney beans contain compounds called lectins that can cause a toxic effect on the
blood.
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Allergy Clinic
