How food affects drugs

In order to work properly, drugs must be present in the correct amounts at the receptor sites of the cell membranes. A number of factors can affect the way in which a drug works and, therefore, its effect on the body. Food is one of the most important of these factors. It can affect the absorption of a drug, its movement through the blood, its metabolism and its excretion.

Food and drug absorption

When drugs are taken orally, their absorption is influenced in different ways by the presence of food in the digestive tract. For example, food causes increased secretion of hydrochloric acid (HCl) in the stomach which can result in the destruction of some penicillin antibiotics that are taken by mouth. For this reason, penicillin should always be taken 1 hour before or 3 hours after meals.

Calcium in milk and dairy products tends to bind with tetracycline antibiotics to form non-absorbable compounds, with the result that the drug is lost in the stool. This means that these foods should not be eaten at the same time or within a few hours of taking the medication. However, the instruction 'Not to be taken with milk products' which sometimes appears on medicine labels can be misleading as it does not mean that you cannot consume any milk products while you are on the medication.

Iron salts (such as ferrous sulphate) can also bind with tetracycline antibiotics and result in the loss of the antibiotic. Therefore, anyone using an iron supplement as well as a tetracycline should take them at least two hours apart.

On the other hand, the absorption of some drugs can be increased when they are taken with foods. For example, foods with a high fat content increase the absorption of Griseofulvin, an antibiotic and antifungal drug. Iron supplements are also absorbed better when they are taken with vitamin C or foods rich in vitamin C, such as orange and guava juice. Interestingly, although some drugs (such as aspirin) are absorbed better on an empty stomach, they should nevertheless be taken with food - or at least milk - because of their irritating effect on the stomach lining.

Food and drug metabolism

Certain reactions inside the body can chemically change a drug (`metabolize' it) so that it is quite a different compound by the time it leaves the bloodstream. For example, some drugs combine with body proteins as they move through the bloodstream, which has the effect of preventing these drugs from entering the cells where they were intended to act. Others are broken down into smaller chemical components by the liver, the kidneys and the digestive tract. The metabolism of specific drugs can be influenced by the following in food: protein content, alcohol content, charcoal broiling, certain types of vegetables, caffeine intake, dietary fibre and the times food is eaten in relation to drug intake.

Food and the excretion of drugs

When metabolized by the body, some food alters the pH of the urine making it either more or less acidic - which in turn affects the excretion of drugs and therefore, the duration of their effects. For example, if excretion is decreased, more of the drug will be reabsorbed into the blood and its effects will be prolonged.

As an illustration, vitamin C makes the urine more acidic, thereby reducing the excretion of acidic drugs like phenobarbital and aspirin. When taking an antibiotic for a urinary tract infection, it is beneficial to acidify the urine, because this will result in decreased excretion of the antibiotic.

What drugs should you take when?

In order to minimize the adverse effects of foods on drug absorption, metabolism and excretion, the timing of drug administration in relation to food intake is very important. This is why it is essential to adhere to any such instructions which may accompany a prescribed drug. For example:

• If rapid absorption of the drug is desired, the doctor/pharmacist may specify on the prescription that the drug should be taken before meals.
• Drugs that irritate the stomach (such as indomethacin, phenobarbitone) should be taken when the stomach is 'cushioned' with food. In this case, the prescription should specify that the drug is to be taken after (or in the middle of) a meal.
• Fat-soluble drugs should not be taken with a fatty meal, since the medication will be absorbed with the fat content in the food; because fat is digested slowly, the drug may not be absorbed into the bloodstream as rapidly as it should be.
• Acetylsalicylic acid, Bisacodyl, Erythromycin and tetracyclines should not be taken with milk (see above).
• Ampicillin, Cloxacillin and Erythromycin-based drugs, as well as stearate and Penicillin G should not be taken with fruit juices or carbonated beverages.

Food Allergy Effects on the central nervous system (CNS)

This is a controversial area and one which tends to attract a lot of media attention. Basically, though, there are several effects on the CNS which can be linked to food allergy:

1. Migraine

The tendency to suffer from this severe form of headache runs in families. The headache is often proceeded by an 'aura' such as visual hallucinations in the form of flashing lights. Generally, only one side of the head is affected and the pain is usually accompanied by nausea. In most cases the migraine continues for 8-24 hours; sufferers often find it impossible to function normally while it lasts. In recent times the long-held belief that certain foods such as chocolate, wines, yeast extracts, hard cheeses, milk and eggs are linked to migraine headaches, has been proved beyond doubt and several offending compounds have been identified.

2. The Allergic Tension-Fatigue Syndrome

Many allergic children have pale faces with dark rings under their eyes, giving them a tired look. They may be difficult to rouse in the mornings and tend to concentrate poorly at school, also especially in the mornings. A large number are rather irritable in general and may be unable to fall asleep easily at night. It has been found that there is a link between this syndrome and an excessive intake of milk, cola drinks and chocolate. Cutting these problem-causers out of their diet can improve the behaviour of these children quite dramatically.

3. Hyperactivity

Dr Ben Feingold, an American allergist, has become well-known for drawing attention to the apparent connection between behaviourial problems and the presence of food additives in the diet. The story of how he reached his conclusion is a long one, but inessence depended on his observations that some adult patients reacted adversely to aspirin. He felt that in addition to causing their asthma, aspirin also appeared to change the patients' behaviour, making them more restless.

If this was so, he reasoned, then the effects in children diagnosed as hyperactive might be even more striking. Hyperactive children typically rush about in purposeless and often destructive activity, often putting their parents under considerable strain. It was thought that one solution might be to remove salicylic acid (used in aspirin) and its derivatives from their diet. (Several foods also contain natural salicylates.In Dr Feingold's view, these preservatives served no essential purpose and would not be missed if children ate foods that were free of them. He then went even further. The yellow food dye, tartrazine, has a similar effect to aspirin in some sensitive asthmatics, so he suggested that it might play a similar role in hyperactivity. From there it was only a short step to the elimination of all artificial colouring and flavours from the food of hyperactive children. Considerable success has been claimed in using this diet (known as the Feingold Diet) in the management of these children.

Unfortunately, however, no properly controlled study has been able to confirm this theory, which has resulted in much controversy in many countries.

Food additives and contaminants continue...

Pesticides and consumer protection

Food law is very clear on the subject of the excessive use of pesticides and penalties for side-stepping the regulations are such that it pays growers to adhere closely to the letter of the law. Most pesticides may not be sprayed over food crops within stipulated times of harvesting (which vary according to the type of pesticide) and

this aspect is particularly strictly monitored by the Departments of Health and Agriculture.

It is perhaps worth mentioning that it is in any case not in growers' interests to use excessive amounts of pesticides on their crops, purely from an economic point of view, since agricultural chemicals are extremely expensive. Nor is it worth a grower's while to risk his reputation - and his livelihood - by careless practices which can easily

be detected.

`Preventing contamination'

The use of pesticides is only one area of food production where the problem of contamination has to be guarded against. In fact, a host of measures are enforced to prevent the poisoning of food by microorganisms. A good example is the law that forbids fresh meat and processed ready-to-eat meat products to be sold side by side, as fresh meat is highly contaminated by bacteria. These are of course destroyed when the meat is cooked, but can in the meantime contaminate the processed product. A case in point was the well- publicised outbreak of salmonella poisoning in North Wales in July 1989 which was traced, according to the press, to a shop selling both fresh and cooked meats. About three hundred cases resulted, with

three deaths.

Likewise, it is against the law to sell a 'blown' can since it could contain the deadly toxin, clostridium botulinum.

Health inspectors visit all food manufacturers and retail food outlets on a regular basis; samples of products are passed on to government laboratories where they are analysed; anyone found to be contravening the health regulations is taken to court. A case in point is the `boerewors scandal' of a couple of years ago which highlighted the use of prohibited animal off cuts in the production of this traditional sausage; the culprits were brought to book and fined heavily.

It should be added that a good deal of 'informal' policing also takes place amongst food producers themselves, who are always on the lookout for malpractices by their competitors in the marketplace! While this should in no way be regarded as a substitute for controlled monitoring, it undoubtedly contributes to consumer protection. One important regulation which is however flouted fairly often is the law stating that people who serve or handle unwrapped food behind shop counters must always wear either plastic or rubber gloves while doing so. (They are in fact required to wear hair nets as well.) Since the hygienic aspects of foodhandling are extremely important, consumers should make a practice of reporting sloppiness in this area, either to store managers or to their local Department of Health.

The sweetener scare

The heart of the problem of food additive legislation is how tointerpret data which gives results that do not translate simply into an immediate threat to public health. A prime example of this is the controversy that erupted over the compounds used in artificial sweeteners, both in the USA and in this country.

Artificial sweeteners have recently come under considerable scrutiny in this country. This follows widely publicized research in the USA after the cyclamate scare in 1970, when the Food and Drug Administration banned this compound due to the fact that laboratory tests on rats which had consumed huge amounts of cyclamate indicated that it may be carcinogenic. Then in 1977, saccharin came under suspicion when the FDA questioned the safety of that compound. The concern resulted from the restrictions imposed by the Delaney Clause (see above) and the hysterical public response it provoked is still causing repercussions today.

However, as it turned out, the quantities eaten by the rats were so high that they probably would have died of obesity long before if they had taken a comparative amount of sweetener in the form of sugar - meaning in effect that valid assumptions could not be made from this data. It should therefore be stressed that there is no evidence that, taken in reasonable quantities, any artifical sweetener is harmful to human health.

In this country the law with regard to artifical sweeteners states that any of some twenty permitted compounds may be used in food products separately or together, but that the name of the sweetener must be given, followed by wording stating that the compound is an artificial sweetener. Next to this, the producer must state whether or not sugar has been added as well. Certain sweeteners provide small amounts of kilojoules and the Foodstuffs, Cosmetics and Disinfectants Act states that this must be indicated on the label as follows: . no
permitted sweetener shall be described as low energy', 'low kilojoule', 'non-nutritive', 'artificial' or words with a similar meaning, unless the energy value of the equivalent of 1 teaspoonful of sucrose is not more than 5kJ'.

Food additives and contaminants

This much-publicised aspect of food production is well provided for in food legislation. A number of specific regulations exist to control any additives and contaminants that may be present in foods, either by direct addition or through indirect absorption. The adding of

colourants is an example of direct addition. Colouring can only be added to a food if the legislation for that food permits it, and only one or more of the colours listed in the regulations can be added. It may not be added in greater quantity than prescribed and it must comply with the standard of purity laid down in the regulation.

An example of indirect absorption is the maximum amount of lead that may be present in food, irrespective of the source. If you're about to argue that there shouldn't be any lead in your food at all, consider that the word 'source' includes the soil in which the plant is grown, the food consumed by an animal used for meat, or equipment in which a food is processed.

Chemicals- cause for concern?

An increasingly large number of additives and processing methods are used to produce the food we eat every day and understandably, public concern is once again on the increase. The amount of chemical substances used in food processing is possibly the major source of worry. Yet the use of these chemicals has been given the green light by legislators, or they would not be present in just about every food on our supermarket shelves.

Why are chemicals necessary in foods? In the simplest terms, because food itself is nothing but chemicals. In fact, the common foods we eat every day are such complex mixtures of chemicals that it would take a brash scientist indeed to assert that the composition of any single food is completely known. Even mother's milk - the only product designed by nature exclusively as a food for mankind - is known to contain more than 140 chemicals!

Perhaps it is worth pointing out that many of the chemicals which man produces are exact copies of those already found in nature. Just one example of this is vitamin C, a vital nutrient which of course occurs abundantly in foods such as citrus fruit and tomatoes. The vitamin C produced by man is identical to the natural chemical, both in terms of chemistry and efficacy, and its widespread use is the only reason scurvy is no longer the scourge of many populations whose normal diets do not provide sufficient of/this essential vitamin.

Interestingly, only a small proportion of pest resistance in plants is manmade. Plants develop their own defence mechanisms against pests and these include the use of natural toxins. Indeed, breeding crops for pest resistance may actually increase their natural toxin levels. For example, it is known that a pest-resistant potato had to be removed from the marketplace in the USA when its natural solanine and chaconine concentrations were found to have reached highly toxic levels. Similarly, a pest-resistant celery had to be discarded when workers handling it suffered outbreaks of dermatitis. It was belatedly discovered that the concentrations of 8-methoxypsoralen (a carcinogen) had inadvertently been increased ten-fold! Unfortunately, since new `disease-resistant' crops are not subjected to the same regulatory scrutiny imposed on man-made pesticides, we may never know how the risks and benefits of reducing pesticide use versus natural toxin consumption ultimately balance out.

To a large extent, the assumption that nature is benign has become ingrained in consumers' minds and even embodied in our regulatory codes. Biological materials are regarded as innocent until proven guilty, while man-made ones are assumed guilty until proven innocent. Without a proper body of balanced scientific scrutiny to back them up, assumptions such as these are not only simplistic - they could bedangerous.

However, solutions to problems such as the above will no doubt be found as more and more research is done in this area; there is no doubt that biotechnology will have an increasingly dramatic impact on the agriculture of the future. The genetic improvement of agricultural crops so that they become more resistant to disease, viruses and drought is clearly an important goal in a world already struggling to feed its billions of inhabitants. While the development of pest-resistant plants may never totally replace the need for agricultural chemicals, it could certainly help to decrease our dependence on them and so lessen the level of contamination to the environment - one of the majorconcerns of our times.

Food Allergy Effects on the respiratory (breathing) organs

Food allergy may be associated with hayfever, bronchitis, asthma, and even some forms of pneumonia.

Hay fever

Hay fever with spasms of sneezing and watery nasal discharge may be caused by certain foods, but more common causes are house dust and animal danders (or dandruff). Once again, cow's milk is often responsible, especially in very young children, although it can cause problems in older children and adults as well.

Asthma

The role of food allergy as a cause of asthma may be underestimated because of difficulty with diagnosis. It is possible that food allergens can cause the typical narrowing of the airways associated with asthma, but only several hours after food has been eaten, so that the relationship between a common food and the beginning of symptoms may not be obvious. The most likely causes include milk, eggs, wheat and soya products. Wheezing due to cow's milk usually begins early in an allergic person's life, but the cause-effect relationship often goes unrecognised for long periods. A diet which eliminates the allergen is necessary to confirm the diagnosis.

Recurrent pneumonia

One form of recurrent pneumonia has been shown to be due to cow's milk allergy. This illness, known as Heiner's Syndrome, is what doctors now suspect when children who drink cow's milk suffer from re-current or chronic lung disease. The problem is caused by the fact that they have a very high level of antibodies to the milk in their blood; most symptoms disappear within a few days on a milk-free diet.

Food allergy or food intolerance?

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.

How is food allergy diagnosed?

The diagnosis of food allergy is often based more on a careful evaluation of the patient than on laboratory tests. A person's history and symptoms may have characteristic features and lead to the recognition of food allergy when other conditions which cause similar symptoms have been excluded. To confirm the diagnosis, the patient will be put on a diet which excludes the suspected food or foods. If the symptoms disappear, the food will be reintroduced in order to confirm the diagnosis. Skin tests and a test called RAST (which detects circulating IgE antibodies) are sometimes helpful, especially in younger children. Unfortunately they are far from being totally accurate as diagnostic aids.

There are two major problems in identifying an allergenic food. Firstly, patients may react to not one but several different foods and secondly, the foods they eat may contain a wide range of allergenic ingredients. Bread, for example, may contain wheat, soya, yeast and milk - all of which are potential allergens. Ideally, to get around these problems an `exclusion diet' consisting only of water and a mixture of basic nutrients, should be used for diagnosis. The major drawback is that a diet like this could rapidly lead to malnutrition, especially in babies.

Many specialists prefer to use a so-called `oligoallergenic diet' (see below), which consists of well-defined unprocessed foods that generally produce few allergic reactions. Patients are put on this diet for a number of weeks and if their symptoms improve, they can gradually reintroduce foods one at a time, while watching for the reappearance of symptoms. As tell-tale reactions occur, the responsible food is recognised and eliminated. Since children tend to outgrow their food allergies, the offending food may be reintroduced each year to see whether the child has developed a tolerance to it. Very impressive results have been obtained in treating two common allergic conditions - atopic eczema and migraine - using oligoallergenic diets.

Oligoallergenic diets used to confirm the presence of food allergy

Diet 1 Diet 2

Stewed lamb Turkey

Rice and rice flour Potatoes and potato

Cabbage Carrots

Sweet potato Apples/pears

Milk-free margarine Olive oil

Milk substitute Soya milk

The 'double-blind challenge test'

Some authorities believe that the only way to diagnose food allergy is to use what is known as the 'double-blind challenge test'. In this test, dried foods in powder form are put into opaque capsules. Neither the doctor nor the patient must know what food is contained in the capsules. Patients are observed for up to 72 hours after taking the capsule and careful observation is made of any occurrence of symptoms following each 'challenge'. The major drawback of this method is that it bypasses the mouth, where many symptoms may begin. It is also not possible to use the test in babies and it is a very time-consuming procedure.

Food Allergy Effects on the skin

Everyone is familiar with skin rashes, which can be due to a variety of causes. Rashes which are caused by food allergy are distinctive and may be diagnosed as either `atopic eczema' or urticaria'.

Atopic eczema

This skin disorder affects about 5% of all young children and is often caused by food allergy. It may show up as a dry and scaly or rather `weepy', itchy rash on the child's cheeks, neck, inside the elbows and behind the knees. The rash is usually followed by an outbreak of asthma or hay fever. Doctors treat atopic eczema by first putting the child on a diet which excludes known allergens (such as wheat and dairy products), which often solves the problem. Obviously, if the eczema flares up again when these foods are reintroduced, this confirms the relationship. Special tests can also be done to identify problem foods. Egg white and wheat are two of the most common ones and cow's milk and soya often cause trouble as well.

Urticaria

You can identify urticaria - also known as 'hives' or 'nettle rash' by raised red (and itchy) welts which usually start on the face and body and spread to arms and legs. Shellfish, nuts, tomatoes, milk and eggs

are typical culprits. Other possible causes are food additives - such as tartrazine, benzoates, quinoline yellow, BHA (butylhydroxyanisol) and BHT (butylhydroxytoluene) - and drugs such as antibiotics and aspirin. Urticaria may occur in adults or children and is not more common in any particular age group. It often occurs in people who have never had any other prior allergic condition in their lives.

How is food allergy treated?

As mentioned above, the treatment of food allergy begins with eliminating the culprit from the diet. At the same time, it is essential to provide a balanced diet which contains enough proteins, calories, trace elements and vitamins. Close cooperation between the patient, the doctor and a qualified dietitian is just as important; patients and their families should be aware that even small amounts of the offending food are sufficient to produce symptoms. A knowledge of food families (see below) is also important. For example, someone who is allergic to peanuts may react adversely to any member of the legume family.

Food Families

Legumes			Nightshade	Cereal	Lily
Beans			Potato	Barley	Asparagus
Peas			Tomato	Maize	Chives
Soya			Peppers	Oats	Garlic
Lentils			Eggplant	Rye	Leeks
Peanuts			Chili	Rice	Onion
				Wheat
Ground			Cashew	Nuts
Melon			Cashews	Brazil
Cucumber			Mango	Walnut
Pumpkin			Pistachio	Pecan
Squash
Honeydew

Very restricted diets should not be used for long periods and foods of the various groups should gradually be reintroduced, so as to build up a permanent diet which excludes only the allergens.

When recommended diets do not appear to work, this may be due to one of two reasons: either the patient is not following the diet strictly enough, or he or she is reacting to additives, colouring agents, salicylate or histamine-rich foods or to other naturally occurring chemical agents.

Foods that contain salicylates

* Citrus and most other fruits (except banana, mango, pawpaw and peeled pears

* Most herbs and spices and many vegetables (except potatoes, peas, beans, cauliflower, cabbage, brussels sprouts, lettuce, celery, onion, asparagus and garlic)

* Tea, some coffees, fruit juices, most alcoholic beverages (except whisky, gin and vodka)

Recent years have seen the development of a very safe drug called sodium cromoglycate (sold under the trade name of Nalcrom), which prevents mast cells from releasing their chemical substances. It is thought that it may act in the wall of the intestines by decreasing the digestive tract's permeability to large food molecules. Another anti-allergic drug, ketotifen (Zaditen), may work in the same way and also has the benefit of some antihistamine effects. These drugs may, therefore, interfere with the processes which produce symptoms in food allergic people. Both drugs must be taken before meals. They both show great promise but research is still continuing in this area.

In conclusion, it should be mentioned that there seems to be a general belief (whether true or false) that our entire environment is being progressively poisoned and that our health is suffering as a result. This is especially prevalent in the conviction that we are falling victims to the food we eat, in one way or another. As yet, there is no direct evidence of a general increase in obviously toxic reactions to our diet, but what is undoubtedly necessary is a great deal of research in the field of food allergy and food intolerance, as well as better diagnostic methods and better treatment. The future promises some exciting developments in a field that doctors have known about for centuries but largely ignored, and where they now have much ground to make up.

The symptoms of food allergy

If you have ever seen a victim of true food allergy after he or she has eaten an offending food, you will know that the reactions can be alarming! The lips swell up almost instantly and an itchy, raised red rash develops along with welts around the mouth. Sometimes the mouth becomes intensely itchy and the uvula (the small tongue at the back of the throat) may become swollen. The foods which most often cause this kind of reaction are eggs, nuts, shellfish, citru fruits and berries. Reactions in the lower part of the digestive tract take a little longer to develop but nausea and vomiting - often accompanied by sudden severe diarrhoea (sometimes with blood in the stools) - may occur within 30 minutes of eating seafood or eggs. This reaction may be followed by flushing, headaches, itching and palpitations which usually disappear after 2-4 hours. All in all, it can be a very unpleasant business!

Highly allergic people should carry an adrenaline inhaler with them at all times, but especially when eating out. If they feel an allergic reaction starting up, they should immediately take in between 3 and 6 puffs of the inhaler. This will prevent any tightness and swelling occurring in the throat. Immediately thereafter a rapidly acting antihistamine (such as Phenergan) 25mg tablet should be swallowed. In virtually all cases, this on-the-spot treatment will enable the affected person to reach medical attention in good time.

Foods may produce allergic reactions in other organs as well. For example, some people can have asthma attacks which may be just as severe as reactions caused by inhaling allergens such as dust or pollens. Allergy to foods may also worsen an existing allergic rash (urticaria) in other parts of the body. In one hospital study in Europe330 patients with recurrent urticaria (also known as hives or nettle rash) were asked what made their condition worse. Foods were said to worsen their skin rash in 30% of cases and 18% thought that drinks did the same. Fruits: vegetables, nuts and fish were the worst offenders.

The best way of looking at this aspect of food allergy is to divide the symptoms that occur into two groups - those that affect the digestive tract (by far the most common) and those that affect other parts of the body. It is important to stress that these symptoms are much more common in young infants and toddlers under 2 years of age than in older children and adults. In fact, there are few examples of food allergy persisting from infancy or early childhood into adult life and the vast majority of babies who are allergic to some foods outgrow their early sensitivity as their bowel matures.

Digestive symptoms

The immediate reactions which occur on contact with an allergenicfood have been discussed above, as have the other common reactions such as vomiting, diarrhoea, nausea and abdominal pain.

In studies on babies in London, doctors were struck by the fact that there seemed to be a link between putting the infants onto artificial feeds and the onset of colic and diarrhoea. It appeared to be mainly the large bowel that was affected and if the babies were taken off these feeds, their colitis (inflammation of the bowel) cleared rapidly. It was found that the major cause of the problem was cow's milk, though some of the babies had reactions to soya and beef proteins. To find out what was happening, a series of studies were done, using a tube with several well-positioned reflecting optics which allowed the inside of the large bowel to be seen clearly. What this showed was that there was noticeable inflammation of the wall of the colon, which disappeared when the offending food was withdrawn.

Food intolerance appears to be closely linked with the Irritable Bowel Syndrome (IBS) or 'spastic colon' which occurs in adults. In this uncomfortable condition, abdominal pain is accompanied by diarrhoea or constipation. In one research project 21 sufferers from IBS were put onto what is known as an 'elimination diet', consisting only of meat, fruit and water. Fourteen of them found that their symptoms cleared and wheat was identified as the most common cause of symptoms. Maize and dairy products were next on the list.

Reactions to food chemicals

In addition to chemicals which occur naturally in foods (for example, caffeine in coffee), there are numerous others which may be incorporated - either directly or indirectly - into food during the growing, storage or processing stages. These chemicals include flavouring, colourants and preservatives. The first two are the most common - because of their very wide distribution in foods, beverages and pharmaceutical products, practically everyone has some daily contact with one or more flavouring and colourant. However, the consumer is often surprised to learn that an everday food product contains either a synthetic colour or flavour or both.

In many cases the foods are those especially designed for children, for example sweets and various breakfast foods. Tartrazine is the most common colouring agent used and is found in a wide range of foods, beverages and medicines. The adverse responses to it are similar to those caused by aspirin and can include very severe reactions. Most commonly, it causes urticaria, asthma, nasal discharge and itching. The flavour-enhancer monosodium glutamate (MSG) is usually found in prepared meats, meat extracts and in Chinese food. People who are sensitive to MSG may have a reaction known as the `Chinese Restaurant Syndrome' after eating foods containing the offending compound. Sweating, a feeling of weakness and a tight feeling in the chest are the symptoms.

All this means that people who are sensitive to certain chemicals should be extra careful about reading all food labels carefully and should avoid those foods and drinks which contain possible problem causers. However, it is important to emphasise that naturally occurring chemical substances - such as caffeine in tea, coffee and cola drinks and theobromine in chocolate - may also have adverse effects such as insomnia, restlessness, abdominal pain and nausea in sensitive people. No additives are responsible for these reactions.

What is true food allergy?

It is important to stress at the outset that contrary to popular belief, true food allergy is not very common. Actual figures for the number of people affected are difficult to estimate, but in general it is thought that no more than between 1 and 4% of the population suffers from a definite food allergy. Interestingly, true food allergies are usually (but not always) seen in people who also have one of the other common allergic conditions, such as hay fever or asthma. Obviously many more than 4% of the population think that they have a food allergy but several careful studies suggest that as many as half of these patients have no allergic symptoms at all. However, they are often so firmly convinced of their allergy that it is difficult for a doctor to persuade them otherwise.

So how do we pinpoint true food allergy? To begin with, we have to look carefully at the mechanisms and symptoms involved.

How do food allergies work?

Studies of food allergy and food intolerance have always been linked and until recently, most attempts to find out how they worked concentrated on the immune system. The root cause of classical or true allergies - including those to food - is the production of Immunoglobulin-E (IgE) antibodies to the offending molecule or allergen (a substance which causes an allergic reaction). IgE is an antibody which normally does the job of protecting the body against parasite infestation, but in allergic people this mechanism has gone awry. When this happens, instead of having a protective role like that of other antibodies, IgE may produce harmful effects in the body. Abnormal amounts of it are produced in response to protein substances which are completely harmless to non-allergic people. IgE binds to cells rich in chemical substances, known as 'mast' cells.

Once an allergen reacts with the IgE molecule, a complex set of biochemical reactions is set in motion and as a result potent chemicals ⁴such as histamine, serotonin and prostaglandins are released into the body. These are what cause the intense itching, swelling and inflammation. A more serious effect occurs when really large amounts of these chemicals are released. They enter the circulation system,causing capillaries to enlarge and become more permeable, which in turn results in a drop in blood pressure. This reaction - known as anaphylaxis - may be fatal. Laboratory tests in people who have true food allergy show high levels of IgE, but this is not the case in food intolerance. In fact, the involvement of IgE is what makes the crucial distinction between the two.

The specific area where IgE is important is in the wall of the digestive tract. When an allergic person eats an offending food it could combine with IgE in the tract and set off local inflammation, which in turn makes the wall of the tract more permeable and allows undigested food molecules to get through. Once absorbed, these molecules can cause a whole range of symptoms.

Several recent studies have shown that patients with food allergy tend to have 'leakier' digestive tracts than do healthy people, although this is not always the case. When food molecules do enter the blood they usually form what are termed 'circulating immune complexes' (CICs) with antibodies. These CICs may cause inflammation and could account for the joint pains and migraine which some patients sufferfrom.

IgE and CICs are not the only immune pathways involved in food allergy. Another possible mechanism is via lymphokines' or `cytokines', which are hormone-like substances that form a communication link between different cells in the immune system. In the case of food allergy, an abnormal immune response to food molecules could lead to the release of lymphokines, followed by unpleasant symptoms such as headache, lethargy, dizziness, joint pains and nausea. Other factors, such as the incomplete digestion of particular food proteins, or certain enzyme deficiencies, may also be relevant in allergic people.

Experts believe that at least two of the defects mentioned above, plus certain environmental triggers, are necessary for the symptoms of food allergy to appear. In other words, the theory is that a combination of several factors could be responsible. If this is so, it could explain why no two victims of food allergy seem to have the same symptoms.