Food Allergy and Intolerance

Key points:

  • Food intolerance is the general term used to describe a range of adverse responses to food, including allergic reactions, adverse reactions resulting from enzyme deficiencies, pharmacological reactions and other non-defined responses
  • An allergic reaction to a food can be described as an inappropriate reaction by the body's immune system to the ingestion of a food
  • Allergic reactions to foods vary in severity and can be potentially fatal
  • Any food has the potential to cause an adverse reaction. Foods that commonly induce adverse reactions include milk, gluten containing cereals, nuts, peanuts, eggs and shellfish.
  • Symptoms of food intolerance can overlap with those of more serious conditions, making use of appropriate diagnostic procedures is particularly important.

Most people can eat a very wide range of foods without any problems although they may have likes or dislikes that influence what they choose. However, some people react badly to certain everyday foods and eating them may cause uncomfortable symptoms or, in rare cases, a severe illness.

There are many different reasons for unpleasant reactions to food. Apart from food poisoning, the main causes are referred to as food intolerances. This term includes a number of different types of reaction including food allergies, which are reactions that involve the body’s immune system. But most food intolerances are not true allergies, although they may cause uncomfortable or distressing symptoms.

It is not known exactly how many people in the UK have a food allergy. Population studies in the UK using conventional testing procedures suggest that between 1 and 2 people in 100 (1-2%) have a food allergy that can be diagnosed reproducibly, whereas as many as 30 in 100 (20-30%) ‘believe’ themselves to be allergic or intolerant to one or more foods.

It is important that people who think they suffer from a food allergy or other food intolerance seek professional advice from their GP before changing their diet dramatically and risking it becoming unbalanced. Dietary change prior to tests can make diagnosis more difficult, for example in the case in coeliac disease.

What is the definition of food intolerance?

Food intolerance is the general term used to describe a range of adverse responses to food, including allergic reactions (e.g. peanut allergy or coeliac disease), adverse reactions resulting from enzyme deficiencies (e.g. lactose intolerance or hereditary fructose intolerance), pharmacological reactions (e.g. 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). Food intolerance reactions are usually reproducible adverse responses to a specific food or food ingredient, which can occur whether or not the person realises they have eaten the food. This is the basis of the ‘gold standard’ testing procedure, the double blind placebo controlled challenge, in which neither the subject nor the operator knows which test contains the allergen, and which is the placebo.

What is the definition of food allergy?

An allergic reaction to a food can be described as 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. The Food Standards Agency estimates that around 10 people a year in the UK die from severe allergic reactions to food. 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 peanut, milk and egg proteins). This component is termed the allergen. The immune system then typically produces immunoglobulin E (IgE) antibodies to the allergen, which trigger other cells to release substances that cause inflammation. Allergic reactions to food are usually localised to a particular part of the body and symptoms may include stomach upsets, rashes, eczema, itching of the skin or mouth, swelling of tissues (e.g. the lips or throat) 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 people in the UK. It is more common in children than adults especially those under the age of three, 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 (i.e. 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 to related proteins in other cereals such as barley and rye), but delayed reactions can also on occasion occur in response to a range of other foods, including milk and soya.

Why don't all people 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. This process is known as the development of tolerance. The mechanisms that underpin this process are not fully understood and work is underway to establish whether, for example, the timing of the introduction of foods is of importance in prompting normal tolerance.

It is also unclear why most childhood allergies disappear after 12-24 months (e.g. milk or egg allergy) whilst others are present for life (e.g. 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 very difficult to totally exclude a food or ingredient in studies designed to investigate this.

How common is food allergy and intolerance?

Food allergy is more common in children, especially those under the age of three, than in adults. It is estimated to affect 1 in 25 of the population. Although 20-30% of people perceive themselves to have an intolerance or an allergy to one or more foods, the true prevalence is thought to be much less than this, although the precise number is uncertain. Some reports suggest that reproducible food intolerance affects no more than 5-8% of children and less than 2% of adults.

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.

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

Lactose intolerance results in abdominal symptoms such as bloating and diarrhoea in response to test doses of lactose. It is a condition seen in older children and adults, particularly those of non-caucasian origin. The severity of symptoms varies between individuals and most people with this condition can consume moderate amounts of milk and milk products, particularly with meals; complete avoidance of milk and milk products is rarely necessary as most people still produce some lactase enzyme. Yogurt is usually better tolerated than ordinary milk. This is thought to be due in part to the fact that an enzyme very 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.

Hard cheeses such as Cheddar contain only trace amounts of lactose and so are well tolerated. Having milk as part of a meal is also more likely to be tolerated and most people can consume 200ml without adverse effects, and so can benefit from the nutrients, particularly calcium, provided by these foods. For people who are very sensitive, lactose reduced milks are now widely available. Although it is not possible to induce the enzyme once levels have fallen, there is some limited evidence that it is possible to develop a tolerance to lactose by gradually modifying the profile of bacteria that reside in the large bowel in favour of ones that cope with the lactose without causing 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. 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 (e.g. some yoghurt or a small glass of milk), particularly if these are consumed as part of a meal. People of Northern European descent on the other hand, i.e. 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 the UK is relatively very low, estimated to affect no more than 2-5% of older children and adults to varying extents, some symptoms being very mild.

How common is gluten sensitivity (coeliac disease)?

Until recently, it was though that coeliac disease affected about one in 1500 people in the UK. However, evidence published in February 2004 suggests that about 1 in 100 people have the condition. According to the Food Standards Agency, this figure is based on blood tests in children and hasn't been confirmed by biopsies. So coeliac disease might not be as common as the research suggests. However, many people with coeliac disease don't realise they have the condition and it’s estimated that only 1 in 8 people have been diagnosed.

What type of diet do people diagnosed as coeliac have to follow?

Coeliac disease is usually a life-long condition requiring a life-long and 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. It is now considered that provided there has not been contamination with other cereals during milling, moderate amounts of oats can be tolerated by most adults with coeliac disease though the picture is less clear for children.

Which foods are the most common causes of allergic reactions and food intolerance?

Many allergies are not triggered by foods at all, but by pollen, animal fur or house dust mite. 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 e.g. wheat (which causes a delayed response known as coeliac disease). Citrus fruits can also be a cause. In children it has been estimated that 9 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 common in peanut allergy and what is the current advice for pregnant women?

The most recent figures (published in 2008) indicate that in children aged 3 years the prevalence of peanut allergy is 1.2%. Peanut allergy is one of the few allergies that is typically life-long, so its prevalence in adults is likely to be similar.

In August 2009, the UK government issued new advice on peanut consumption during pregnancy, breastfeeding and early life. This followed a major review of the scientific evidence, conducted by the British Nutrition Foundation on behalf of the Food Standards Agency, which was then considered in detail by the Government’s advisory committee, the Committee on Toxicity. The advice prior to 2009 concerned avoidance of peanuts where there was a family history of allergy.

The new advice is as follows:

During pregnancy and whilst breastfeeding:
The revised advice states that if mothers would like to eat peanuts or foods containing peanuts during pregnancy or breastfeeding, then they can choose to do so as part of a healthy balanced diet, unless they are allergic to peanuts themselves.

When introducing peanut into a child’s diet:
General advice is that all mothers should try to exclusively breastfeed their baby for the first 6 months of life. The revised advice states that if a mother chooses to start giving her baby solid foods before 6 months of age, she should not introduce peanuts or other potentially allergenic foods (such as other nuts, seeds, milk, eggs, wheat, fish or shellfish) before six months of age. When these foods are eventually introduced, they should be introduced one at a time so that any allergic reactions can be identified.

The revised guidance additionally advises that where a child already has another kind of allergy (e.g. diagnosed eczema or a diagnosed allergy to foods other than peanut), or if there is a history of allergy in the child’s immediate family (parents, siblings), then mothers should talk to their GP, health visitor or medical allergy specialist before giving peanut to the child for the first time, because these children are at higher risk of developing peanut allergy.

All people who are known to be sensitive to peanuts should carry pre-loaded 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. Also all pre-packed foods sold in the UK have to clearly show on the label if they contain one of the following items (known to contain common allergens) as an ingredient (or if one of the ingredients contains or is made from one of these):

  • peanuts
  • nuts (almonds, hazelnuts, walnuts, Brazil nuts, cashews, pecans, pistachios, macademia nuts and Queensland nuts)
  • eggs
  • milk
  • crustaceans (including prawns, crab and lobster)
  • mollusc (including squid, mussels, cockles, whelks and snails)
  • fish
  • sesame seeds
  • cereals containing gluten (including wheat, rye, barley and oats)
  • soya
  • lupin
  • celery
  • mustard
  • sulphur dioxide / sulphites (preservatives used in some foods and drinks) at levels above 10mg per kg or per litre

This information can help consumers identify whether or not a food contains an ingredient that they need to avoid. As well as the ingredients list, many food products have a statement or an allergy advice box on the label saying they contain a particular ingredient, such as gluten or milk. There might also be a picture of the ingredient. But this type of statement is not compulsory on food labels and so it is always important to check the ingredients list too.

Sometimes small amounts of an allergen can get into a product by accident, even though food producers take great care to stop this happening. If there is a possibility that this could happen in a factory, the food label might say something such as ‘may contain nuts’.

Fact sheet written by  the British Nutrition Foundation and provided by MIMS

British Nutrition Foundation
Imperial House, 6th Floor
15-19 Kingsway
London WC2B 6UN
Tel: 020 7557 7930
Web site:

Last reviewed: 2009

Register or Subscribe to MIMS

GPs can get MIMS print & online and GPonline for free when they register online – take 2 minutes, and make sure you get your free MIMS access! If you're not a GP, you can subscribe to MIMS for full access.

Register or subscribe

MIMS bulletins

News and updates straight to your inbox.

Prescribing Update: Fortnightly news bulletin
Urgent prescribing updates
Spotlight: Disease-themed monthly round-up

Sign me up

MIMS Dermatology

Read the latest issue online exclusively on MIMS Learning.

Read MIMS Dermatology

Mobile apps

MIMS: access the full drug database and quick-reference tables on the go

MIMS Diagnosis and Management: concise information on signs and symptoms, investigations and diseases

Promo Image

Clinical calculators

Handy calculators and conversions for primary care.