Food allergy and intolerance: what are the facts?

In the second article in our series on nutrition, Judy Buttriss explains the science behind the most common food allergies and intolerances

A casual glance through the tabloid press and weekend supplements could easily leave you thinking that we have a major food allergy epidemic in Britain. But is this really true? The reality is that most people can eat a wide range of foods without any problems, although they may have likes or dislikes that influence what they choose. However, some people--a small minority--react badly to certain everyday foods and eating them may cause uncomfortable symptoms or, in rare cases, a severe illness. Next month's article will focus on food intolerance myths, so in this one I will concentrate on the different types of food intolerances.

What is the definition of food intolerance?

Food intolerance is the general term used to describe a range of reproducible adverse responses to a specific food or food ingredient which can occur whether or not the person realises they have eaten the food.^{1 2} This general term includes allergic reactions that by definition involve the immune system (such as peanut allergy or coeliac disease); adverse reactions resulting from enzyme deficiencies (such as lactose intolerance or hereditary fructose intolerance); pharmacological reactions (such as caffeine sensitivity); and other non-defined responses. Food intolerance does not include food poisoning from bacteria and viruses, moulds, chemicals, toxins, and irritants in foods, nor does it include food aversion (dislike and subsequent avoidance of various foods). (See figure.) True food intolerance is estimated to affect 5-8% of children, most of whom outgrow the condition, and less than 1-2% of adults. However, the paradox is that as many as 20% of adults believe that they are food intolerant.³

What is the definition of food allergy?

An allergic reaction to a food is an inappropriate reaction by the body's immune system to the ingestion of a food that in the majority of individuals causes no adverse effects. Allergic reactions to foods vary in severity and can be potentially fatal. In food allergy the immune system does not recognise as safe a protein component of the food to which the individual is sensitive (such as some proteins in peanuts). This component is termed the allergen. The immune system then typically (see below) produces immunoglobulin E (IgE) antibodies to the allergen, which trigger other cells to release substances that cause inflammation. Allergic reactions are usually localised to a particular part of the body and symptoms may include asthma, eczema, flushing, and swelling of tissues (such as the lips) or difficulty in breathing. A severe reaction may result in anaphylaxis (as with severe peanut allergy), in which there is a rapid fall in blood pressure and severe shock. Food allergy is relatively rare, affecting an estimated 1-2% of children and less than 1% of adults (typically 0.2-0.5%), and is often wrongly used as a general term for adverse reactions to food.

Are there different types of allergy?

There are two well defined mechanisms via which allergic reactions to food (that is, reactions that involve the immune system) can occur. Most cases of food allergy involve the production of antibodies known as immunoglobulin E (IgE) and are known as IgE mediated allergies. Symptoms develop quickly and can vary in severity, but the severest form of this type of reaction is anaphylactic shock. The other recognised mechanism is a delayed response (taking hours or even days to develop), which involves a different immune system component, T lymphocytes (T cells). The best defined example of this type of reaction is coeliac disease (sensitivity to the protein, gluten, found in wheat and other cereals), but delayed reactions can also on occasion occur in response to a range of other foods, including milk and soya.^{1 2}

Why doesn't everyone develop allergies?

Under normal circumstances a baby rapidly becomes tolerant (non-responsive) to the many proteins that it encounters in the early days and months of its life.¹ The mechanisms that underpin this process are not fully understood. It is also unclear why most childhood allergies disappear after 12 to 24 months (such as milk allergy) while others are present for life (such as peanut allergy). Another aspect that is poorly understood is the relative importance of diet in the development of allergic diseases, although it is recognised that diet can aggravate existing conditions such as asthma and atopic dermatitis (many other factors may also be involved). Similarly, the benefit of dietary restriction in the treatment for these conditions is uncertain, particularly among adults. This is partly because it is difficult to totally exclude a food or ingredient.

Which foods are the most common causes of allergic reactions?

The majority of allergic reactions to dietary components are caused by a small number of foods, namely cows' milk, hens' eggs, peanuts, tree nuts, soya beans and soya products, fish, shellfish and gluten containing cereals--for example, wheat (which causes a delayed response known as coeliac disease)^{1 2} Citrus fruits can also be a cause. In children, it has been estimated nine out of 10 reactions are in response to milk, eggs, soya, peanuts, tree nuts, or wheat gluten. Many of these reactions are outgrown in early childhood, and the majority of allergic reactions in adults result from sensitisation to shellfish, fish, peanuts, and tree nuts. It is unusual for food allergy to begin in adulthood.

How do the symptoms and severity of food allergy and other forms of food intolerance compare?

Food intolerance reactions vary considerably in the severity of the associated symptoms and the length of time for which they persist. For example, peanut allergy is often a life long affliction and can cause severe, even life threatening, anaphylactic reactions to tiny amounts of peanut protein. Cows' milk intolerance may be severe in early life, but typically disappears as the child grows older. The majority (about 90%) have outgrown the intolerance by the time they go to school (typically by the age of 3 years). Similarly, egg intolerance is usually a temporary phenomenon associated with early childhood.^{1 2}

Coeliac disease (gluten sensitivity) is normally life long and requires adherence to a diet that excludes all gluten, but in some people the disease is mild and goes undiagnosed as the individual is not aware of any symptoms.

What causes lactose intolerance and how common is it?

Lactose intolerance occurs in individuals who lack or have low levels of the enzyme lactase, which is needed to digest the sugar lactose (found in milk) to its constituent sugars (glucose and galactose) in readiness for absorption in the small intestine. It is a condition seen in older children and adults. In the absence of lactase, undigested lactose passes into the large intestine causing the characteristic symptoms of diarrhoea, wind, and general discomfort. In about 70% of the world's population, a reduction in lactase production after early childhood is the norm. When milk is consumed, symptoms are typically experienced to varying degrees in people of Asian, African, Jewish, and Hispanic descent. Nevertheless, the majority of affected individuals can still tolerate moderate amounts of dairy products (such as a glass of milk), particularly if these are consumed as part of a meal. Complete avoidance is rarely necessary, as most people still produce some lactase enzyme. Hard cheese contains little lactase and so is well tolerated. Yogurt is usually better tolerated than ordinary milk, possibly because an enzyme similar to human lactase is present in the bacteria used in the manufacture of yogurt (the bacterial culture), although other factors are likely to be of relevance too. For people who are very sensitive, lactose reduced milks are now widely available.

People of northern European descent, on the other hand--that is, the majority of the British population, usually retain the ability to produce lactase throughout their life, presumably as a result of genetic inheritance. As a direct result, the prevalence of lactose intolerance in Britain is relatively low, affecting an estimated 2% of older children and adults to varying extents. However, in some people lactose intolerance is a complication of gastrointestinal infections.

How common is gluten sensitivity (coeliac disease)?

The prevalence of clinically proven coeliac disease is one to three cases per 1000 people. However, recent advances in the methods for diagnosing gluten sensitivity, based on blood measurements rather than an intestinal biopsy, have enabled screening of the general population and the identification of unrecognised cases. As a result, the European consensus is a total prevalence of three to four cases per 1000 people. Superficially there seems to have been an increase in the prevalence of coeliac disease, but increased awareness and these new blood tests probably account for most, if not all, of the increase.¹

Coeliac disease is usually a life long condition requiring a strict gluten free diet, and the main organ affected is the small intestine. Ingestion of gluten activates immune cells in the small intestine, which trigger inflammation and local damage. This disrupts the normal processes used to digest and absorb foods. As a result, untreated coeliac patients lose weight, develop deficiency syndromes such as anaemia, and experience symptoms such as diarrhoea. Gluten is found in wheat, barley, and rye, which means that many dietary staples such as bread, many breakfast cereals and foods, such as pizza and pasta, can no longer be eaten. Oats were thought to trigger reactions, though this is looking less and less likely. ^1,2

How common is peanut allergy?

As peanut allergy is one of the few allergies that is typically life long, its prevalence is estimated to be greater in adults (0.5-1.0% of the population) than in children (0.5%). It remains unclear whether sensitisation to peanuts can occur before birth, but as a precaution the government's advisory committee known as COT (Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment) has recommended that pregnant women who are atopic (predisposed to allergic reactions), or for whom the father or any sibling of the unborn child has an atopic disease--for example, allergic asthma or eczema--may wish to avoid eating peanuts and peanut products during pregnancy. There is no justification for the avoidance of peanuts if there is no history of allergy in the parents or brothers and sisters of the new baby.

All people who are known to be sensitive to peanuts should carry preloaded adrenaline syringes and (with the exception of very young children) be trained in their use. Those caring for children at risk of anaphylaxis, including schools, must be trained in the use of adrenaline and have access to supplies. Even a slight delay in the administration of adrenaline can be fatal.

Is migraine caused by food allergy?

It is likely that some of the headaches and migraines experienced by some people are provoked by food. However, there is unlikely to be a single food that is a common cause. Various mechanisms may be involved, but allergy is not a likely candidate. Coffee, chocolate, and alcoholic drinks are possible triggers for some people, but will be without effect in others.

What should consumers look for on labels?

By law, the majority of packaged food products have to carry a full list of the ingredients they contain, in descending order of weight in the final product.¹ Manufacturers are increasingly deciding, voluntarily, to include information on the presence of small quantities of allergens associated with severe reactions, even when it is not required by law. This information can help consumers identify whether or not a food contains an ingredient that they need to avoid. In addition, where there is a risk that traces of an allergen, such as nut protein, may be present although nuts are not an ingredient, this is generally highlighted at the end of the ingredients list.

Judy Buttriss, science director, British Nutrition Foundation
Email: j.buttriss@nutrition.org.uk


studentBMJ 2001;09:357-398 October ISSN 0966-6494

1. British Nutrition Foundation Task Force. Adverse reactions to food. Buttriss J, ed. Oxford: Blackwell Science, 2001.
2. British Nutrition Foundation. Food allergy and intolerance. Briefing paper. London: British Nutrition Foundation, 2000.
3. Young E, Stoneham MD, Petruckevitch A, Barton J, Rona R. A population study of food intolerance. Lancet 1994;343:1127-30.