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THE BASICS 06/2006

Food allergy and food intolerance

1. Introduction

Most people enjoy a wide variety of foods with no problems. For a small percentage of people however, specific foods or components of food may cause adverse reactions ranging from a slight rash to a severe allergic response.

Adverse reactions to foods may be due to food allergy or food intolerance. While about one in three people believe they are "allergic" to certain foods, the true prevalence of food allergy is only about 2% of the adult population. In children, the incidence is higher at 3-7% although the majority of children outgrow food allergies by the time they start school.

2. What is the difference between food allergy and food intolerance?

An adverse reaction to food is often mislabelled as a food allergy. In many instances, it is caused by something else - perhaps microbial food poisoning, a psychological aversion to a food or an intolerance to an ingredient in a food.

Food allergy is a specific form of intolerance to a food or food component that activates the immune system. An allergen (a protein in the offending food, which in the majority of people will not produce and adverse reaction) sets of a chain of reactions in the immune system including the release of antibodies. These antibodies trigger the release of body chemicals, such as histamine, which give rise to various symptoms such as itchy skin, a runny nose, coughing or wheezing. Allergies to foods or food components are often inherited and usually identified early in life.

Food intolerance involves the body's metabolism but not the immune system. A good example is lactose intolerance, in which certain individuals lack the digestive enzyme lactase, which breaks down the milk sugar.

3. Food allergy

3.1. What happens in an allergic reaction?

The immune system generally protects the body from harmful foreign proteins by generating a response to eliminate them. Allergy is essentially "immunity gone wrong" where a normally harmless substance is perceived as a threat -an allergen- and attacked by the body's immunological defences. In a true allergic reaction, the body produces antibodies (a protein that specifically binds to another protein called antigen -in this case the allergen- to deactivate it and remove it from the body). The class of antibodies known as immunoglobulin E (IgE) reacts with the allergen and this in turn triggers a reaction with the mast cells (tissue cells) and basophils (a type of blood cell). Mast cells are found below the surface of the skin and in the membranes lining the nose, respiratory tract, eyes and intestine. A substance called histamine or other substances such as leukotrienes and prostaglandins are released from mast cells and they cause allergic responses such as those set out in the table below. The adverse reactions are immediate and are usually localised. Some allergic reactions take several hours or even days to develop after exposure to a foreign protein. These are often called "delayed hypersensitivity reactions".

Fortunately, most allergic reactions to food are relatively mild but a small number of people experience a severe and life-threatening reaction called anaphylaxis. An anaphylactic reaction can sometimes occur within a few minutes of exposure and immediate medical attention is necessary. Peanuts are well known for causing "anaphylactic shock", a serious condition in which blood pressure drops rapidly and the sufferer could die from cardiac arrest unless adrenaline is administered rapidly in order to open up the airways.

Runny nose or nasal congestion
Asthma (difficulty breathing)
Breathing difficulties
Swelling of the lips, mouth, tongue, face and/or throat (angioodema)
Urticaria (hives)
Rashes or redness
Itching (pruritus)
Abdominal cramps
Anaphylactic shock (severe generalised shock)

3.2. Which people are at risk from food allergy?

Family history is one of the best predictors of the likelihood of any problems with a food allergy. Infants with one allergic parent have about twice the risk of developing food allergy than infants whose parents do not have allergies. If both parents are allergic, the risk increases four to six times. Breast-feeding has been reported to reduce the risk of food allergy compared with formula-fed infants. For babies born into families where close relatives suffer from allergies, it seems that exclusive breast-feeding for 4-6 months may provide a degree of protection.

3.3. What is the prevalence of food allergy?

Estimates for the prevalence of food allergy are much lower than are perceived by the public. While up to one in three people claim to have a food allergy, the actual incidence is quite low. Only a few studies indicate the true prevalence of a food allergy, where an allergic reaction is confirmed in a double blind, placebo-controlled food challenge.

From these studies, it has been estimated that food allergy occurs on average in about 1-2% of the adult population. The prevalence is higher among young children, with estimates from 3-7%. Fortunately, 80 to 90% of these individuals outgrow their sensitivities by the age of 3 years. Whilst childhood allergies to egg and cow's milk may disappear, allergies to nuts, legumes, fish and shellfish tend to endure throughout the lifetime of the individual.

3.4. Which foods are the common causes of food allergy ?

While allergic reactions can occur with any food or food component, some are more common than others. Common food allergens include cow's milk, eggs, soya, wheat, crustacean, fruits, peanuts and treenuts, such as walnuts.

3.4.1. Cow's Milk protein allergy

Cow's milk protein allergy is more commonly found in infants and children, particularly when there is a family history of allergy. It occurs in 0.5-4% of infants but its prevalence decreases with age. The most common symptoms are vomiting and diarrhoea, although the range of adverse reactions can differ from one person to another. Fortunately, reactions to cow's milk protein are generally short-lived and the incidence in older children and adults is much lower.

The allergenicity of cow's milk can be reduced by different dairy processing treatments. Heat treatment denatures some of the milk proteins reducing their allergenicity. For this reason, some milk-sensitive individuals can tolerate sterilised or evaporated milk products but not pasteurised milk. Other dairy processes, such as enzymatic digesting of proteins to peptides can also reduce allergenicity potential of whey proteins. In fermented products, like yoghurt, and in cheeses, the structure of the milk proteins remains largely unchanged and hence they retain their allergenicity.

Once a diagnosis of allergy to milk protein has been confirmed, it is important to ensure that a healthy balanced diet is maintained, especially during a child's growth and development. Dietary advice from a registered dietician is essential to ensure optimal intakes of essential nutrients such as calcium, magnesium, vitamins A, D and B2 and B12. The consumption of sardines and salmon with bones (canned) and dark green cooked vegetables like broccoli, will help to maintain calcium intakes.

3.4.2. Peanuts and treenuts

Nut allergy is an important condition because it starts at an early age, is lifelong and can be fatal. Peanuts, also known as ground nuts or monkey nuts, and treenuts, such as almonds, brazil nuts, hazelnuts and walnuts can give rise to symptoms even with minimal contact through intact skin or by inhalation. In its mildest form, treenuts allergy can be limited to a rash, sickness and headache to swelling of the tongue and lips, whereas both treenuts and peanuts allergy in its extreme form it can cause anaphylactic shock. The potential severity of the symptoms of allergic reaction to nuts dictates that sufferers have to avoid carefully any contact with nuts and to carry adrenaline (to counteract the severe allergic reaction) at all times.

3.4.3. Other common food allergens

Other foods that are more likely to be associated with allergic reactions include fruits, pulses (including soya beans), eggs, crustacean (crab, crayfish, lobster and shrimps), fish and vegetables, sesame seeds, sunflower seeds, cottonseed, poppy seeds and mustard seed. The allergenic capacity of some food allergens is destroyed by cooking and food processing, when the proteins are denatured. Newer processing techniques, such as high-pressure treatment of foods, fermentation and enzyme treatment, can help to reduce the allergenicity of some food proteins. Moreover, allergens can be removed from oils by refining. Some of the unresolved problems of food allergy are concerned with the presence of low amounts of a given allergen in processed foods or recipe dishes served out of home.

4. Food intolerance

Food intolerance may invoke similar symptoms to a food allergy (including nausea, diarrhoea and stomach cramping) however the response does not involve the immune system in the same way. Food intolerance occurs when the body can't digest a food or food component properly. While people with true food allergies generally need to eliminate the offending food altogether, people with an intolerance can often tolerate small amounts of the food or food component without symptoms. Exceptions to this are those who are sensitive to gluten and sulphite sensitive individuals.

4.1. What are the most common causes of food intolerance?

The two most common causes of a food intolerance are lactose and gluten.

4.1.1. Lactose intolerance

Lactose is the sugar found in milk. Normally, the enzyme lactase, which is present in the small intestine, is sufficient to digest the lactose into simpler sugars (glucose and galactose), which are then absorbed into the bloodstream. When enzyme activity is low, the lactose remains undigested and it is transported to the large intestine, where it is fermented by the bacteria in the gut. This can give rise to symptoms of flatulence, pain and diarrhoea.

Although most people of northern European descent produce enough lactase throughout their lives, among the non-white races and people from the Middle East, India and parts of Africa and their descendants, lactase deficiency is a very common phenomenon. In reality, around 70 per cent of the world's adult population do not produce enough lactase, and therefore has some degree of lactose intolerance. In Europe, lactase deficiency is present in about five per cent of white people and a much larger proportion (50 to 80 per cent) in the ethnic groups.

The quantity of milk and dairy products that leads to symptoms of intolerance varies widely. Many individuals who have low intestinal lactase activity can drink a glass of milk without experiencing discomfort. Similarly, hard cheeses, which are low in lactose, and fermented milk products such as yoghurt are usually well tolerated. This could explain why cultured milk products and yoghurts are widely consumed in areas of the world where lactase deficiency is common. Furthermore, the consumption of lactose-containing foods as a part of a meal and the reduction of the amount of lactose-containing foods at any one time can improve tolerance in sensitive individuals.

4.1.2. Gluten intolerance:

Gluten intolerance is an intestinal disorder that occurs when the body can't tolerate gluten (a protein found in wheat, rye, barley and oats, although the role of the latter is controversial and is currently the subject of research). The prevalence of the condition, sometimes called coeliac disease or gluten-sensitive enteropathy, is underestimated. Serological testing detects otherwise undiagnosed disease in 1 in 100 individuals in the European population (although there are of course regional varieties).

Coeliac disease is a permanent condition that can be diagnosed at any age. If a sufferer consumes a gluten-containing food, the lining of the small intestine becomes damaged and less able to absorb essential nutrients such as fats, protein, carbohydrates, minerals and vitamins. Symptoms include diarrhoea, weight loss weakness, irritability and abdominal cramps. In children, symptoms of malnourishment including growth failure, can occur. Currently, the only help for coeliac patients is a gluten-free diet. Gluten-free food lists are usually available from local dietetic and coeliac disease information centres and associations. When gluten is removed from the diet, the intestine gradually repairs itself and symptoms disappear.

Research efforts are underway to identify the exact nature and sequence of amino acids in the gluten that result in coeliac disease, and this knowledge may, in the future, be an important application of biotechnology in the development of cereal crops that do not cause intolerance.

5. Food additives and adverse reactions?

While food additives pose no problems for most people, a small number of people with specific allergies may be sensitive to certain food additives such as certain colours and sulphites.

As all food additives must be clearly labelled, those with specific sensitivities and those who believe they have sensitivity to a food additive can readily avoid any that may pose problems for them.

6. How is food allergy or food intolerance diagnosed?

Food allergy or food intolerance can be properly diagnosed with scientifically sound methods of testing. If someone thinks that they may be suffering from an allergic response to certain food substances, the first step is to go to a doctor to ensure that the symptoms are not due to another condition and who can refer the patient to a dietician or a specialist on allergy.

The first step to a reliable diagnosis is reached by getting a detailed history of the patient's and family's medical background. Special attention will be given to the type and frequency of the symptoms, along with when the symptoms occur in relation to eating particular foods. A complete physical examination of the patient will also be undertaken. Then, the following main detection methods are used:

6.1. Skin tests

On the basis of past diet history, foods suspected to cause allergic reactions are included in the panel used for skin tests. The value of these types of tests is very controversial and the results are not 100 per cent reliable. The tests involve placing on the skin extracts of a particular food, which are then pricked or scratched into the skin to look for a reaction of itching or swelling.

6.2. Food elimination diets

The principle of the elimination diet is that a single or combination of suspect foods may be removed from the diet for around 2 weeks prior to a food challenge. If the symptoms disappear during this period, suspect foods are added back to the diet, one at a time, in small but gradually increasing amounts until a normal consumption pattern is achieved. Once all the suspect foods have been checked out, those foods causing problems can be avoided.

6.3. RAST (radioallergosorbent) tests

These tests involve mixing small samples of the patient's blood with food extracts in a test tube. In a true allergy, the blood produces antibodies to fight off the foreign protein, which can be detected. The test can only be used as an indication of an allergy and does not determine the extent of the sensitivity to the offending food.

6.4. Double-blind, placebo-controlled food challenge tests (DBPCF)

In this allergy test, a suspected allergen (e.g. milk, fish, soya) is placed in a capsule or hidden in a food and is fed to the patient under strict clinical conditions. These tests allow doctors specialised in allergies and food intolerance to identify the most common foods and food components causing adverse effects.

Some other forms of food allergy testing are unproven and may not have any value.

7. What can people do to prevent food allergy and food intolerance?

Once a thorough examination has helped to accurately identify the offending foods or food components, the only way to prevent the allergic reaction in sensitive individuals is to eliminate the food or food component from the diet or from the environment. In the case of food intolerance, limiting the food to smaller servings may be sufficient to avoid symptoms. Reading the ingredient information on food labels and knowing which foods trigger the allergies, intolerance or asthma are the best defences.

Professional help from a registered dietician can help ensure that no nutrients are excluded from the diet when food changes and substitutions are made. Asking about ingredients and cooking methods when taking food outside of the home can help to avoid known problem foods. When eating away from home, it is important to explain your situation and special needs to your host or food server. If necessary, ask to speak to the chef or manager of a café or restaurant.

If in doubt, play it safe and stick to plain foods such as grilled meats or prepare and carry foods you have prepared yourself. Always have a rapid-response plan and if you are, or someone else is, experiencing a severe food-allergic reaction, call immediately for medical support or an ambulance.

8. What are the responsibilities of food manufacturers and retailers?

Food allergy is now recognised as an important food safety issue and the food industry must take the greatest care to assist those who suffer allergies to select a suitable diet with confidence.

Manufacturers need to practise due diligence in considering the use of major serious allergens as ingredients, in warning of the presence or potential presence of such allergens in products, and in preventing adventitious cross-contamination of products with allergens present in other manufactured products.

These issues are addressed through the use of Good Manufacturing Practices (GMP) including the implementation of Hazard Analysis and Critical Control Point (HACCP) systems, which involve close co-operation with suppliers of the raw materials and at other points in the food supply chain. The proper development of recipes and controls ensures that the food product, as prepared, contains only the ingredients specified in the formulations. Precautions are also taken to prevent cross-contact of materials during storage, handling and processing of foods, especially where shared production equipment is used.

9. What is happening in the area of food labelling of potential allergens ?

The European Union is in the process of considering appropriate allergen labelling, and in the meantime many national organisations have produced guidelines that encourage further adoption of GMP and the provision of consumer information.

The Codex Alimentarius Commission, the EU Commission and other international organisations, are setting out scientific criteria for the selection of allergenic foods for labelling. Peanuts, tree nuts, crustacean, fish, soya beans, cow's milk, eggs, and wheat, as well as sesame (EU Commission) have been selected as major allergens.

While there are no specific provisions under EU food legislation that require potential food allergens to be labelled, the general requirements are that all ingredients added to food must be declared on a product's ingredient list. At the moment, there are certain exceptions to this general rule:

  • Ingredients that are included in the "25% rule" do not require to be labelled in the ingredient list. This is the case for compound ingredients (an ingredient with a common name but composed of several ingredients), which constitutes less than 25 per cent of the final food;
  • "Carry-over" ingredients, such as some additives that do not have any technological function in the end product, but are carried over into a food by one of its ingredients;
  • and some foods (e.g. some cheeses, most alcoholic drinks).

On a voluntary bases the major critical allergens are already declared on the ingredient lists of certain manufacturers and retailers, even if present in very small amounts. In addition, food producers use labels such as, 'may contain', on products in which small amounts of a potential allergen may adventitiously be present. However, in response to the repeated request from consumers for better information about the foods they purchase, the Comission has issued a proposal for an amendment to the food labelling Directive 2000/13/EC. The proposal will abolish the "25% rule" meaning that all ingredients intentionally added will have to be labelled. The proposal will also establish the obligation to label those ingredients recognised in the scientific literature as liable to cause allergies. The amendment is intended to ensure better information on the contents of foodstuffs in order to enable consumers with allergies to identify allergenic ingredients that may be present.

Some manufacturers and retailers provide lists of products free from specified allergens to consumers in the form of leaflets, carelines and websites.


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