Estimates of the prevalence of food intolerance vary widely because studies of the problem are very difficult to devise. A major reason for this is the lack of a simple test. Excluding foods from the diet to see if symptoms clear, and reintroducing them again (food intolerance test) to see if symptoms are provoked is at present the only valid method. The condition is probably affects about 5% to 15% of the population.

In contrast, food aversion and food phobia are psychological avoidance responses. Phobias may relate to a wide range of environmental exposures, and food is no exception. Eating disorders are common manifestations of emotional distress, and food aversion or phobia due to an inappropriate perception of food allergy is a well-known variant of this. An examination of the social and emotional stresses that the patient was experiencing at the time of onset of the illness can help the physician or dietitian to determine if food aversion may be the cause. Addressing these underlying issues is then the correct way to deal with the condition.

Causes of food intolerance

Food intolerance does not have a single cause. The term is used to describe an adverse reaction to a food where in which standard allergy tests (skin tests and allergy blood tests) are negative. It is diagnosed by a food intolerance test in which a range of foods, those most likely to cause the condition, is avoided to see if symptoms disappear.  If they do, foods are then reintroduced in turn to establish which cause symptoms.

In the majority of cases of food intolerance, no mechanism can be identified to explain why symptoms occur. It seems likely that some kind of abnormal immune response is to blame. In other words; most cases of food intolerance seem to be due to allergy of a type we don't understand.

In the minority of cases in which a mechanism can be shown, one of the following four causes can be shown to apply.

Symptoms of food intolerance

Food intolerance causes a wide variety of symptoms.

For the large majority of cases, those not due to enzyme defect, drug interaction, pharmacological or toxic reaction, there are several characteristic feature.

Symptoms are not immediate
The time relationship between eating the culprit food and the start of symptoms depends on many factors. When a food is being consumed regularly and there has been no period of avoidance (e.g. for several days) then there is no obvious relationship between the time of eating and the time of symptoms. 

However, when the food has been avoided for several days and is then eaten again, symptoms are typically strong and start within a few minutes or within an hour or two of reintroducing the food. They may then last for a few hours, or may continue for a day or so.

When there has been no period of avoidance, each intake of the food is more likely to temporarily relieve rather than worsen symptoms. This is similar to the way that a further alcoholic drink ('the hair of the dog') temporarily relieves the symptoms of alcohol addiction. Whether this is due to a similar mechanism is not known.

Symptoms are usually multiple
In food intolerance a very much wider range of symptoms occurs than in the case of food allergy and multiple symptoms are usual. The conditions listed have been shown by clinical studies to be either caused or made worse by food intolerance.

So a typical food intolerance sufferer may suffer migraine and unexplained fatigue (central nervous system symptoms) abdominal pain, bloating and frequent diarrhoea (gastrointestinal system symptoms) unexplained muscle and joint pains (musculoskeletal system symptoms) and unexplained nasal congestion and discharge (upper respiratory symptoms).

Symptoms may have been considered all in the the mind
Doctors use the term somatisation disorder to explain the patient with a wide range of apparently unrelated symptoms who has no evidence that an underlying disease is present. In other words all the tests that have been done are normal. The term implies an illness is that is psychosomatic i.e. it is caused by psychological distress manifesting itself through the soma (body) as opposed to through the psyche (mind).

There is, of course, no doubt whatever that some people do have psychosomatic illnesses. However there is also the suspicion that some of the symptoms of somatisation may be the symptoms of allergic responses that are beyond our understanding, responses capable of inducing both bodily and mental symptoms.

Withdrawal symptoms
If the culprit food is one that is eaten regularly (e.g. wheat, milk, potato), and it is then inadvertently or deliberately omitted from the diet, withdrawal symptoms may occur. This can explain why not missing breakfast stops some migraine sufferers from getting mid-morning migraines.

Careful elimination and challenge testing of that patient's regular breakfast foods then usually identifies the food intolerance.

The reintroduction of an avoided food causes symptoms to return, usually within an hour or two.  This is the basis for the elimination diet that is necessary to diagnose the condition.

Elimination diet test

The only acceptable way of diagnosing or confirming a 'food intolerance' (as opposed to a 'food allergy') is by an elimination diet.

Food avoidance
The test is based on the simple observation that if all likely or possible trigger foods are avoided at the same time, and if food intolerance has been the cause of the patients symptoms, then symptom clearance occurs.  Such a diet is known as an elimination diet, a range of possible trigger foods being disallowed at the same time.  From research studies we know that a wide range of possible foods can trigger delayed food reactions and it is therefore not unusual for an elimination diet to exclude 20 or 30 suspect foods.  In occasional cases, especially when  symptom clearance is anticipated but does not occur with a less strict elimination diet, a few foods diet can be tried.  Certain situations, for example severe arthritis or Crohn's (inflammatory bowel) disease may justify the use of an elemental diet, a complete liquid food comprising only dietary 'elements' (simple amino acids, sugars etc.) the theory being that no immune system recognition of the food source is then possible.

If this period of food avoidance dramatically clears all the symptoms, the recurrence of symptoms on re-challenge is then more obvious, the period of avoidance appears to heighten both the rate and the briskness of the symptom response.  Experience suggests that attention to detail is needed during this elimination phase.  For example, if sensitivity to corn (maize) is the cause of symptoms (derivatives of corn include corn starch, corn flour, dextrose and other food additives), symptoms may not disappear until all forms are excluded.  This may require the simultaneous avoidance of toothpaste and (where possible) medications whilst on the test diet.  This kind of test is difficult to do on your own.  The assistance of a doctor or dietitian with experience of this kind of testing is invaluable.

Once an elimination diet has been followed for a sufficient length of time to secure a convincing period of symptom relief (perhaps as long as 28 days, but often only 7 to 14 days is needed) the previously hidden food 'allergy' will have become effectively unmasked, the first eating of the food after the period of avoidance producing symptoms within an hour or two thereby showing up a clear relationship between the eating of the food and the precipitation of symptoms that would not have been apparent when the food was being regularly consumed.  

Food reintroductions 
These need to be undertaken with the supervision of a doctor or dietitian skilled in this approach.  The patient will be instructed which foods to test, and in what form and quantity.  Sometimes the first reintroduction might be with a tiny amount, and if no reaction is observed within a few hours, a larger amount is then tested.  The order of testing is important, closely related foods may contain similar allergens and should be separated.  For example potato should not be tested the day after tomato, to which it has a close botanical relationship.  Patients will be asked to record symptoms for a period of hours after the test.  Some foods may be tested just once, others may be tested several times over a day or so before being cleared as non-reactive.  In some cases, physiological measurements taken before and after ingestion, and depending on the expected symptom response, may be helpful.  For example, a simple breathing test known as the peak flow rate, measured before and after food challenge, may help assess for wheezing and a grip strength meter may show up a temporary flare in arthritis.  Sharp increases in weight have been recorded on the morning following positive food challenge in some patients, presumably indicating a marked and temporary retention of fluid.R  A particularly useful and simple measurement is the pulse rate, a sharp increase in the pulse rate within 30-60 minutes of eating the food commonly heralds the occurrence of other symptoms.R

As testing proceeds, the patient gradually builds up the list of tolerated foods and the persistence of a symptom-free state between positive food challenges continues to confirm that progress is on expected lines.  Where doubt exists about the reactivity of any particular individual food, then this food is set aside for a re-test later.  Testing normally proceeds with the sequential introduction of individual 'natural' food items first.  When this stage is complete, the patient then tries out more complex food items, looking out for unexpected reactions that may signify problems with food additives etc.  Contrary to expectation, many patients find that their problem is associated with intolerance to natural foods to a greater extent than with food additives.  Patients with food additive intolerance are often those who find they have a sensitivity to synthetic chemical substances generally; for example they may find that they encounter symptoms associated with the breathing of volatile chemical fumes.

Drawbacks
It should be clear that this form of testing is cumbersome.  It requires a co-operative patient who possesses the ability for careful self- assessment.  Being subjective, the method cannot exclude responses that arise out of positive expectation; the so-called placebo response.  However, given that many carefully conducted clinical trials have established that this form of 'food allergy' both exists, and is extremely important in a range of medical conditions and given that there is no validated and acceptable form of objective test, this method of testing is the only alternative.

With careful patient selection, and the correct identification of an appropriate elimination diet, quite remarkable clearance of multiple and previously unexplained symptoms can occur.  Physicians with experience of elimination diets often remark that the improvements in the well-being of patients that can often be achieved by using this approach will far exceed the degree of relief that can be achieved in similar patients by using drugs to suppress their symptoms.

Other tests

It would be very helpful for many people suffering common conditions such as migraine and irritable bowel syndrome if there was an easier test than the elimination diet test.  One result of this has been the proliferation of tests and clinics that offer to 'diagnose' your food intolerance.  Some use measurements of muscle strength (Kinesiology) or electrical activity (Vega test - see left) when you are in close contact with the food.  Some clinics will even offer to test a sample of your hair or urine through the post.  None of these tests has any rational scientific basis and none has been properly compared with the results of elimination diet and sequential food challenge.  On study undertaken at the University of Southampton investigated to see if the test showed any degree of accuracy when compared to the standard method - skin prick testing.  The study concluded that the Vega test was not at all accurate when used in this way.

Before the results of such tests are accepted, they need to be confirmed by an elimination and challenge test.

Blood tests

A food intolerance is a food sensitivity that does not show up with the standard food allergy tests. This means that the RAST blood test that looks for IgE antibody to the food will be negative.

A new kind of blood test measures a different antibody called IgG (IgG ELISA test).  A recent controlled and 'blinded' study at the University of Manchester showed that for patients with irritable bowel syndrome (IBS), a change of diet that was based on the results of the IgG ELISA test was significantly more effective in reducing the IBS symptoms than a change of diet that was arbitrary (i.e. a sham diet - the patient was told to eliminate the wrong foods).  However, the outcome was far from perfect, patients were improved but not cured.  The study was unable to assess if troublesome foods were missed or if non-troublesome foods were avoided.

Other tests use a blood sample and examine the effects of dilute quantities of the food on the white blood cells (Nutron Test, Cytotoxic Test, ALCAT Test and FACT test).  Whilst these tests may provide pointers to the involved foods, none can be fully relied on to identify the cause of the patients symptoms.  In particular, all these tests are capable of identifying problem foods in people with no symptoms.  So in people who do have symptoms, positive food tests do not necessarily identify the cause of symptoms.  So if these tests are used, the results need to be confirmed, ideally with the help of a doctor or dietitian skilled in the management of food allergy and intolerance.

Before the results of any of these tests are accepted, they need to be confirmed by an elimination and challenge test.

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