There is an enormous amount of controversy surrounding food allergy and food intolerance, and they are not trivial clinical problems as is so often suggested. The avoidance of a specific food can mean the difference between life and death to someone allergic to peanuts. On the other hand, eliminating some common foods can have very beneficial effects on diverse conditions such as migraine, hyperactivity, asthma, atopic eczema, hives, irritable bowel syndrome and Crohn's disease.

What are the different types of reactions to foods?

We consume about a ton of food and drink a year and it is astonishing how well we cope with such a large range of foods, additives, colours and preservatives. The gut contains the most amazing immune system that can keep the body protected in the main from allergic and immune reactions to the variety of "foreign" proteins that we eat. Whereas the cause of immediate food allergy is well recognised, for example - acute swelling of the tongue and lips in children who have peanut allergy, the mechanisms for most of the delayed reactions to foods, food intolerance, are unknown.

There are a number of ways in which foodstuffs can cause adverse effects on the body.

Enzyme Defects

When the wall of the intestine lacks lactase, the enzyme that breaks down milk sugar, diarrhoea can result.

Pharmacological agents

Many foods contain chemicals that can act as a drug on the body; caffeine in coffee and phenylethylamine in some cheeses. Both chemicals have been associated with migraine.

Interaction of foods with drugs

Some antidepressant drugs such as monoamine inhibitors can interact with foods containing amines such as cheese, resulting in an increase in blood pressure. Other foods (citrus) can interact with anti-rejection drugs (cyclosporin) to alter the levels adversely for the patient.

Toxin

A number of foods contain naturally occurring toxins such poisonous mushrooms, or lectins in uncooked beans. Bacterial spoilage can also lead to the release of toxins.

Psychological factorsPeople can have an aversion to a food because of the look, taste, past associations or through conditioned responses.

When all the above causes of food reactions have been excluded, Food Hypersensitivity can be considered. The reactions under this heading can be divided into acute and delayed reactions.

Acute food allergy

Where the time interval between ingestion of the food and the appearance of symptoms is short, the culprit food can usually be identified from the patients history. Blood tests (for specific IgE) and skin prick tests can confirm the clinical suspicion. Typically, when the food reaches the lining of the mouth or stomach, there is immediate swelling leading to nausea and vomiting and occasionally, anaphylaxis and death. The amount of food needed to produce symptoms can be very small.

The top eight culprits for acute food allergic reactions are: egg, milk, wheat, fish, shellfish, soy, tree nuts and peanuts (peanuts belong to the legume family and are not true nuts).

Anaphylaxis. This is the most severe form of food allergy and can be lethal. The risk factors are a previous history of a severe reaction and poorly controlled asthma. Some deaths from asthma may be due to food an aphylaxis. Peanut allergy seems to be on the increase and recent estimates suggest that more than one per cent of the population are allergic to peanuts or tree nuts. Peanuts are the cause of the majority of food related deaths from anaphylaxis and a third of all cases of anaphylaxis in the community.

Relationship between pollen allergy and reactions to foods. Patients who are allergic to silver birch pollen which pollinates in April, can find that eating apples causes a tingling in the mouth. This is because there is an identical protein in both the pollen grain and in the apple. Rather like getting "hay fever of the mouth". This is called the "Oral Allergy Syndrome". Other links can be seen between a food and non-food such as latex with allergy to avocado, banana and kiwi. Latex allergy is now present in approximately seven to 10 per cent of health care professionals. Kiwi is a relatively new food and there is an increase in kiwi allergy in children.

Diagnosis. The allergic antibody - IgE - can be measured in the blood and its presence shown by skin prick testing where a minute amount of the soluble allergen is pricked into the skin which leads to a small localised wheal and erythematous reaction at the site. Skin tests must be interpreted carefully as false positives are common - that is the skin test is positive but the subject can eat the food without ill effect. False negatives are less common.

Delayed food intolerance

Sufferers from delayed reactions to foods usually do not know which foods are involved as the interval between ingestion and symptoms may be hours or even days later. The foods are usually eaten several times a day and meal sized portions may be required to trigger responses. The exact mechanisms of these delayed reactions are not fully understood.

The range of clinical reactions. A wide range of symptoms can result from Delayed Food Intolerance. A patient with food induced migraine may also have achy joints, irritable bowel syndrome, fatigue and problems with memory and concentration. The medical profession finds it difficult to manage patients with too many symptoms and they can become "heart-sink" or "thick-note" patients. What is wonderful is that a significant proportion of these people can be transformed to be fit, healthy and active people following a suitable food elimination diet. This could represent a significant saving to the health budget.

The range of symptoms that occur in Delayed Food Intolerance is wide and multiple symptoms are usual. The conditions listed in Table 1 have been shown to be either caused or made worse by food intolerance.I shall highlight a particular set of diseases related to the gut and mention other syndromes in less detail.

Cows milk allergy

The incidence of allergy to milk in children is approximately two per cent. Probably the bulk of these children will have immediate rather than the more difficult to diagnose delayed reactions, thus suggesting that the true incidence is likely to be much higher.

There is a variation in the quantity of milk it takes to elicit a reaction and also the time after which symptoms occur. This can vary from minutes to hours and even days. A study from Melbourne has highlighted the timing of the various reactions that children have to milk.

In about a quarter of children, the reaction is immediate and the amount of milk needed to cause symptoms is small. Half the children needed larger amounts of milk and had symptoms two to 24 hours later. The last group took between one and five days to react and required repeated ingestion of the food to induce symptoms. These latter children often had severe symptoms. What presents a major problem is that this last group of children did not have any evidence of immune response to the food.

Some recent research has shown that children with acid reflux (gastro-oesophageal reflux) and constipation have signs of inflammation in the oesophagus suggesting an allergic mechanism and can also respond to the exclusion of milk in their diet with clearing of symptoms.

There is no doubt that cows milk hypersensitivity can lead to chronic problems in children. A child who was a colicky baby, then vomited the cows milk feeds, went on to develop glue ear and then a chronic cough - could be helped by eliminating cows milk products. An adult with post gastroenteritis irritable bowel syndrome with wind, bloating, pain and diarrhoea given a period without milk products, could be a changed person. Many more examples exist.

Gastro-intestinal disease

There are a number of gut diseases where diet has been shown to be beneficial. These include coeliac disease, irritable bowel syndrome and Crohn's disease.

Coeliac disease. The patients sensitivity to gluten is shown by white blood cells infiltrating the lining of the small intestine which leads to an inflammation of the lining of the gut and a reduced absorption of minerals and vitamins. The treatment is a diet eliminating gluten - for life. This was thought to be a paediatric condition but is now commonly seen in adults.

Irritable bowel syndrome (IBS). This is one of the commonest conditions seen in a gastroenterology out-patient department, comprising almost 50 per cent of the patients attending. A number of studies have shown that about half the patients with this condition will respond to an elimination diet with resolution of their symptoms, no longer needing symptom suppressing drug therapy. Thus a quarter of all patients attending a gastroenterology department could be significantly helped by diet and perhaps not drugs.Not only that, but many of the symptoms associated with BS such as headache, fatigue, aching joints and thrush may also clear on the diet.

This form of treatment, the elimination diet, could represent a significant saving to the NHS budget if a well trained dietician could be placed in designated gastroenterology clinics to manage such patients. To test whether the patient has an "external" cause for their IBS, whether it be food intolerance, bugs in the gut or candida is surely worthwhile when the alternative is symptom suppression by drugs with no clear diagnostic hypothesis.

Crohn's disease. Although an inflammatory bowel disease such as Crohn's disease might be regarded as less likely to respond to the elimination of foods, sufferers have been shown to improve on such a regime. Long term improvement whilst avoiding culprit foods has been documented. Considering the morbidity associated with this condition, this simple approach must be worthy of further investigation.

The many faces of food intolerance

The conditions outlined in Table 1 represent a significant percentage of the patients who attend the medical profession. The child with milk allergy can present in so many different ways and to so many different specialists. There are many other patients who have chronic conditions where the value of elimination diets has been shown. Chronic brittle asthma, chronic cough, chronic rhinitis: they may all benefit. Atopic eczema and urticaria (hives) - distressing symptoms for which drug treatment is almost the rule. But, diet can and does help.

Severe migraine. Cut out the 4Cs': cheese, chocolate, citrus, coffee - and red wine.

Some children with migraine and epilepsy will also respond to an elimination diet with relief of both the migraine and the epilepsy. Children with just epilepsy do not respond. There are now four well designed clinical trials that show the benefit of an elimination diet in hyperactivity. Interestingly, the diet in these children can also help relieve symptoms of rhinitis, asthma, abdominal symptoms ("bellyaching") and aching legs.

The problem of diagnostic blood tests in food intolerance

There is a lack of any reliable and validated laboratory test for food intolerance.This has left the diagnosis open to abuse and made the assessment of prevalence so very difficult. What is crucially needed is research into the mechanisms of food intolerance so that tests can be devised to help the many patients with these conditions.The paucity of allergy services in the UK (and especially clinics with skills in food intolerance), coupled with the lack of diagnostic tests must make it even more difficult for the patient to get a good and competent management scheme for their condition.

The cost of Food Intolerance to the National Health Service

In one sense, the patients with these syndromes have cost the NHS large sums for their investigation and "treatment" but have not been "cured". The classical treatment is pharmacological and this aims to suppress symptoms rather than remove the cause. The Irritable Bowel Syndrome patient can be a success story if they happen to find an out-patient department with an interest in food intolerance. This is unlikely. The "treatment" by diet is not costly, and when successful, the patient has little further need for out-patient specialist support.

What a saving to the NHS exchequer if at least some of these conditions were at least investigated in this way.

Professor Jonathan Brostoff is a Senior Research Fellow, and Professor Emeritus of Allergy and Environmental Health, at Kings College London, and co-authorof The Complete Guide to Food Allergyand Intolerance (Bloomsbury)jonathan.brostoff@kcl.ac.uk.