Food allergy and intolerance

If you want to give your GP a stressful moment, tell him that you suspect you have a food allergy or intolerance - few subjects in medicine have been the cause of more controversy! Much of what we know about allergy comes from the study of reactions to airborne allergens, for example pollens and the house dust mite. In the past doctors and medical scientists have tried to use this knowledge to understand food allergy. However it is now becoming clear that this was a mistake, what happens in the gut is quite different from what happens in the lung. The nature and extent of the food allergy problem has therefore been seriously underestimated as the available tests do not fully explain what is happening.

When eating a food causes symptoms, these may occur either immediately after eating the food, or else they are delayed by hours or even a day or so.

A reaction to a food that occurs in the first hour is usually caused by an immune reaction, and this condition is correctly called food allergy (see below). In this condition, symptoms occur within an hour of eating the food and a skin prick or a blood test is positive to that food. Food allergy is fairly uncommon (less than 2% of the population). Reactions that are delayed in onset, with symptoms arising hours or even a day or so after eating the food, are much commoner, but the causes are less easy to understand. This condition in which allergy skin prick and blood tests are negative is called food intolerance (see below). This is much more common (perhaps 20% of the population) although most sufferers are unaware of the connection between common foods and their frequent, medically unexplained and distressing symptoms.

Food allergy

A child who gets a blotchy, itchy rash and swelling of the lips, within minutes of eating a peanut is allergic to peanut. This is an example of an immediate food reaction; correctly called a food allergy. If this child has a blood test for peanut antibody (immune globulin E or IgE test), it will be found that the child has an excessive amount of this antibody and this confirms the allergy. A skin prick test to peanut in the same child will show a positive result. These two tests are the same as those that confirm allergy to grass pollen in a hay fever sufferer, or allergy to house dust mite in some asthmatics.

Food allergy reactions cause a limited range of symptoms. Diagnosis is based on the history of what happened when the food was eaten, supported by evidence from skin tests and blood tests. The blood test looks for the presence of a specific antibody (an immunoglobulin E or IgE antibody) to the food being tested.

Reactions may be almost instantaneous, particularly where symptoms in the lips, tongue, mouth or throat are involved. Anaphylaxis is the name given to a generalised allergic response that may include an all-over rash, collapse, severe wheezing and sometimes throat or tongue swelling. It is the most serious form of immediate food allergy; in the UK a few deaths (usually three or four, the same number as die through being struck by lightening) occur each year from this cause. Therefore sufferers of all but the mildest variety are advised to carry an adrenaline injection to use should the need arise.

Symptoms of food allergy

Sufferers of immediate food reactions often know (or in the case of children, their parents know) which foods are involved. Symptoms may involve the gastrointestinal tract (oral allergy syndrome, enterocolitis), skin (eczema, urticaria), respiratory system (rhinitis, asthma), or symptoms may be generalised (anaphylaxis). Tiny amounts of the specific food may trigger these responses. Such reactions involve a heightened immune response to a specific food peptide fragment, a response that involves the production of a specific antibody to that peptide fragment. It is the existence of this mechanism that categorises the medical condition termed food allergy.

Asthma About 5 - 10% of asthmatics are affected by allergy to foods or food additives
Urticaria 'Nettle Rash' or 'Hives' - 10 - 15% of cases may relate to food allergy
Laryngeal Oedema Throat Swelling - food allergy is often involved
Eczema Elimination diet trials suggest that anyone with persistent eczema has at least a 50% chance that one or more foods are involved. Some will be example of food intolerance (allergy tests negative - see below) although true food allergy (with positive allergy tests) may also be present.
Rhinitis (nasal allergy) 30% of cases may involve food hypersensitivity although most will be of the food intolerance (allergy tests negative - see below) type.
Oral Allergy Syndrome Itching and swelling of lips, tongue and throat (common in hay fever sufferers) food allergy is involved in the majority of cases
Anaphylaxis Allergic Collapse - food allergy is a common cause, especially to peanuts and tree nuts (brazil nut, hazel nut, almond etc.)

Over 90% of true food allergy is caused by only eight foods; egg, peanut, tree nuts, milk, soya, fish, shellfish and wheat. However a wide range of other foods have occasionally been implicated, and recently there has been an increase in allergy to foods recently introduced to our diet, for example kiwi fruit and sesame seeds.

Oral allergy syndrome

This is a milder form of food allergy in which some or all of the foods in a particular list that includes certain fruits (peach, apple, plum, nectarine, cherry) vegetables (tomato, raw celery, raw carrot, parsley) and nuts (hazel, almond) cause problems to some hay fever sufferers. The condition is known as the oral allergy syndrome.

In the most common form, there will be a localised swelling or itching in the lips, mouth tongue or throat immediately after contact with the food. Fresh fruit or raw vegetables normally cause these symptoms; reactions to the same foods when cooked are less likely but can sometimes occur. In most cases, progression to a severe reaction is extremely unlikely. In a few people, symptoms in the mouth or throat may be caused by other foods such as nuts, shrimp or egg, and for these the risk of progression to a more severe generalised reaction is greater. People with oral allergy syndrome caused by fruits or vegetables often have an associated allergy to certain pollens, and may get hay fever when these pollens are in season.

A number of pollen allergies may be connected with the condition, although the commonest related allergy is to birch pollen. It has been estimated that as many as 75% of birch-allergic patients may be affected, although those with the mildest form (a sensation in the lips or tongue after eating raw apples) may not have realised that there was an allergic problem. Other pollens involved include grass pollen and certain weed pollens.

Adults appear to develop this condition more often than children, local reaction to fruits and vegetables being the most frequently encountered kind of true food allergy in sufferers over the age of ten. Children appear more likely to suffer a more widespread kind of allergy (e.g. rash, vomiting or wheeze) as a response to foods such as egg, nuts or seafood to which they are allergic and in them, the oral allergy syndrome and reactions to fruits or raw vegetables is less common.

Peanut allergy

Peanut allergy is of particular importance as it presents early in life, is often severe, and usually does not resolve. It is increasing in prevalence and a recent US survey estimated that 1.1% of the population was allergic to peanuts and/or tree nuts. Peanut accounts for the majority of food-related anaphylactic fatalities and 30% of all cases of anaphylaxis in the community. Individuals who are allergic to peanuts are frequently also allergic to tree nuts. In a UK study, 19 of 47 (40%) peanut allergic patients were also allergic to tree nuts; those most commonly implicated being brazil, almond and hazel.

There have been many explanations put forward for the increase in peanut allergy. It is known that it is more common in children who had eczema in early life, and it is possible that sensitisation may have occurred because both eczema creams and nipple creams used by breast-feeding mothers commonly used to contain peanut oil.

Conditions that may mimic food allergy

Confusingly, reactions indistinguishable from true food allergy, but with negative skin prick and blood tests, can sometimes occur when histamine is released directly from foods causing a 'histamine rush'. Scombroid food poisoning, a kind of false food allergy, is due to the early spoilage by bacteria (putrefaction) of certain foods, most often fish belonging to the genus scombroidae (e.g. tuna). The action of the bacteria can cause the release of sufficiently high levels of histamine to induce symptoms in whoever eats the food. Although individuals vary in their level of sensitivity to histamine, provided the dose is sufficiently high, all those who eat the food will be affected. Diagnosis of this condition is mainly by suspicion, although this becomes heightened when skin prick test or blood test results fail to confirm that a true food allergy to the suspect food is present.

A number of other foods may cause minor 'food allergy' symptoms, even though skin prick or RAST blood test to that food may not show evidence of a true allergy. Such foods may contain histamine or other biologically active 'amine' compounds such as tyramine. Food additive intolerance may also sometimes occur, and this may act by releasing amine compounds from foods. Another range of chemical compounds in foods known as 'salicylates' are related to aspirin. Avoidance of foods containing these compounds occasionally helps patients suffering from chronic allergic conditions such as rhinitis, nasal polyps or urticaria, especially when sensitivity aspirin as a drug is already evident. Avoidance of salicylate-containing foods together with avoidance of food additives is also thought to be helpful in childhood hyperactivity (ADHD - attention deficit, hyperactivity disorder).

Food intolerance

Whereas the cause of 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. In most case this appears to be a kind of delayed allergy although the cause is not fully understood. In a few cases delayed allergy is not the explanation and one of the following three causes can be shown to apply.

Enzyme defect - 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.

Pharmacological - 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.

Toxic - 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.

Symptoms of food intolerance

Food intolerance can cause a surprisingly wide variety of symptoms. However certain features concerning the timing and occurrence of symptoms are helpful when trying to identify possible causes.

The time relationship between eating the food and getting symptoms depends on many factors. After four or more days of deliberate and scrupulous avoidance of the food, symptom onset ranges from almost immediately after eating the food to several hours. However time relationships are quite different when the food is being consumed regularly and has not been excluded. For example, if a troublesome food has been avoided for a number of days and is then eaten again there may be a brisk and clear-cut symptom response lasting a day or so. If however a further amount is eaten a day or so later (once the effects of eating the earlier amount have worn off) there may be no noticeable reaction. This is described as masking, a kind of immunity that may be caused by the fact that some of the troublesome food has not been fully eliminated from the bowel.

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

This 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. Suspect foods are then introduced one at a time, symptoms observed, and appropriate measurements taken (e.g. a pulse test). If this period of food avoidance dramatically clears all the symptoms, the recurrence on re-challenge is then more obvious, the period of avoidance also appears to heighten both the rate and the briskness of the symptom response.

Five to ten days of scrupulous avoidance most reliably produces this response, effectively unmasking the hidden allergy, so that the first eating of the food after the period of avoidance usually produces symptoms within an hour or two.

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.

The full range of symptoms that scientific studies have linked to food intolerance is surprisingly wide. The following list is just a selection of the possible symptom responses to foods; headaches, fatigue, migraine, hyperactivity, joint and muscle pains, irritable bowel, irritable bladder, rhinitis, asthma, urticaria, eczema, itching.

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) 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.

Other tests use a blood sample and examine the effects of dilute quantities of the food on the white blood cells (Nutron, Cytotoxic, ALCAT and CAST Elisa tests). Some measure the production of immune globulin G (IgG Elisa Test). Whilst these tests may provide pointers to the involved foods, none has been objectively assessed to confirm the level of accuracy. Therefore if these tests are used, the results need to be interpreted, ideally by a doctor or dietitian skilled in the management of food allergy and intolerance. Before the results of such tests are accepted, they need to be confirmed by an elimination and challenge test.

Double-blind challenge test

At the other extreme, some doctors have suggested a more rigorous test for food intolerance where small test quantities of suspected foods are hidden in capsules made to look identical to placebo (dummy) capsules that contain an inert substance in place of the food. The patient then has to take a series of such capsules, neither patient nor doctor knowing which ones contains the suspect food and which ones contain the placebo. This test is called the double-blind placebo- controlled food challenge (DBPCFC) and has been regarded by some as the 'gold standard' against which all other tests should be compared.

Whilst this test is a valid one for food allergy, when testing for food intolerance the test has many flaws, the main one being that the standard DBPCFC test does not employ an adequate elimination diet phase. As usually performed, the subject undergoes the food challenge tests after just an overnight fast. Unfortunately when dealing with food intolerance (as opposed to food allergy) the patient may be 'immune' to immediate symptom response unless an adequate period of strict elimination has preceded the food challenge.

An examination of the many scientific studies that have properly confirmed the existence of food intolerance shows that all have used elimination diets, and have followed this with double-blind placebo-controlled challenge tests. Studies that have utilised DBPCFC tests without elimination diet (the so-called 'gold standard) and have used this approach in conditions that involve food intolerance have either identified no food reactions at all, or have identified only the immediate food reactions (food allergy). As a result, the medical scientific community has been divided about the role of food intolerance in common medical conditions.

RAST blood test

Last, but not least, amongst tests that may cause confusion if food intolerance is suspected are tests intended for food allergy. One UK supermarket and several chains of chemist shops are now offering allergy blood tests (RAST tests) in conjunction with a private pathology laboratory. Although such tests are helpful in the diagnosis of food allergy (the immediate kind), a negative result does not rule out the possibility of food intolerance. For this and other reasons, people using these tests should have them interpreted by a doctor or dietician in conjunction with a case history.

Recommended reading:

Complete Guide to Food Allergy and Intolerance
Jonathan Brostoff and Linda Gamlin
Bloomsbury; ISBN: 0747534306

Dr Michael Radcliffe, Clinical Allergy Consultant, Royal Free NHS Trust and a Research Fellow in Allergy at the University of Southampton

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Next review: 27 November 2021