Food Allergy and Intolerance

Authored by , Reviewed by Dr Helen Huins on | Certified by The Information Standard

Food allergy and food intolerance are not the same thing. The following leaflet explores the difference between them, their causes, and how they are diagnosed and managed.

With a food allergy, there is an abnormal reaction of the body's immune system to a particular food. This can range from a mild reaction to one that is severe and life-threatening (anaphylaxis). Food intolerance happens because the body has difficulty digesting certain substances in food, or because certain substances have a direct effect on the body in some way. Food intolerance can cause symptoms such as diarrhoea, bloating and stomach cramps. However, with food intolerance there is no allergic reaction and the immune system is generally not involved.

The symptoms of food intolerance occur usually a few hours after eating the food. Allergic reactions usually happen much more quickly.

With an allergy, even a tiny amount of the food can cause an allergic reaction to take place. Some people with a severe allergy to nuts might experience an extreme form of allergic reaction (anaphylaxis):

  • After eating something made in a factory that also handles nuts; or
  • After kissing someone who has recently eaten nuts.

With food intolerances you need a lot more of the food to cause the symptoms.

Food intolerances are never life-threatening. Some allergies are, as they can cause anaphylaxis.

An allergy is a response by the body's immune system to something (called an allergen) that is not necessarily harmful in itself. Certain people are sensitive to this allergen and have a reaction when exposed to it. Some allergic reactions are mild and harmless, but others are severe and potentially life-threatening (anaphylaxis). (See separate leaflets called Allergies and Anaphylaxis for more information.)

A food allergy is an abnormal reaction of the body's immune system to a particular food. This tends to cause unpleasant and unwanted effects (symptoms).

There are different types of food allergy, depending on which part of the immune system responds. The acute, sudden-onset reactions tend to be caused by antibodies called immunoglobulin E (IgE). These are called IgE-mediated reactions. As a result of these reactions, various chemicals are made, including histamine. Histamine causes many of the allergy symptoms and this is why antihistamines can be used to treat some allergic reactions.

There are also non-IgE-mediated reactions which tend to be more delayed and less severe. In these reactions the symptoms are not caused by IgE. Instead other parts of the immune system, such as a type of white blood cell, called a T cell, cause the problem. Common reactions include skin problems such as rashes and eczema, and tummy (abdominal) symptoms and bowel disturbance.

It is also possible to have mixed reactions.

A food intolerance is not the same as a food allergy. Many people incorrectly use the words interchangeably. A food allergy is the body's immune system reacting abnormally to specific foods. No allergic reaction takes place with a food intolerance.

People with a food intolerance may have digestive symptoms such as diarrhoea, bloating and stomach cramps. These are quite common symptoms anyway. For example, other conditions, such as irritable bowel syndrome (IBS), can also cause these symptoms.

In food intolerance, the symptoms can be caused by various problems. Although the immune system is not involved, it does not mean that symptoms of food intolerance are unimportant or mild in nature. Food intolerance can cause considerable problems and significant symptoms.

Some examples are:

  • Lactose intolerance. This causes diarrhoea and tummy (abdominal) symptoms (bloating and pain) after milk is drunk or dairy products are eaten. It happens because of a deficiency of lactase, an enzyme in the body that digests the sugar in milk (lactose). Unless lactose is broken down, the body cannot absorb it. You can be born without any lactase, or only a low level, or you can develop lactase deficiency - often after an episode of infection of the gut (gastroenteritis). See separate leaflet called Lactose Intolerance for more details.
  • Direct effects of foods and additives. Certain foods and chemicals in foods may directly affect the body and cause symptoms. For example, some people find that the food additive monosodium glutamate (MSG) causes flushing, headache, abdominal pains and bloating. However, the exact way in which foods and additives may cause symptoms is often not clear.

    There is controversy about whether certain additives (E numbers) are responsible for various symptoms, especially in children. Also, there is uncertainty as to whether certain foods can make symptoms worse in some people who have conditions such as IBS, migraine and eczema.
  • Food avoidance. Some people, especially children, may develop physical symptoms such as feeling sick and diarrhoea if they eat, or are offered, foods they have come to dislike. Strictly speaking this is not a true food intolerance. It may, however, be difficult to distinguish between avoidance and intolerance in children.

Also, many people incorrectly assume symptoms they have are due to food intolerances when in fact their symptoms are not due to food. Consequently, people commonly cut things out of their diet that they believe to be the cause of their problems. In itself this can lead to further problems.

In summary, food intolerance often ends up being a rather vague term which is sometimes difficult to clarify.

Some people get a type of allergic reaction to certain foods that only causes symptoms in the mouth and throat. It tends to cause itching, tingling, and swelling of the mouth, lips and throat. Fresh fruit, vegetables and nuts commonly cause this. It can be confused with an extreme form of allergic reaction (anaphylaxis).

It has the potential to be serious, as swelling in the mouth and throat can affect the ability to breathe; however, this is rare. The symptoms start within minutes of eating and tend to settle completely within an hour. Often if you have this condition, you can eat cooked versions of these foods with no problem, but the raw food gives you symptoms. This is because the proteins causing the allergic reaction are destroyed by the cooking process. Oral allergy syndrome usually affects people who get hay fever, and may also be called pollen food syndrome.

Note: an ambulance should be called immediately if you feel faint, have difficulty breathing or feel like your throat is closing up.

It is very difficult to know how common true food allergy really is. Studies tend to come up with very different figures. depending on how they are set up. They suggest that in the developed world anywhere between 1 and 17 people out of 100 have a food allergy. People tend to report food allergy more often than it is confirmed by formal testing. Many people mistakenly believe they have food allergies. Also, children can 'grow out' of allergies - for example, to cow's milk. The number of people with food allergies seems to be rising. However, the severity of these allergies varies.

Food allergy

Symptoms of food allergy due to an IgE allergic reaction include:

  • Tingling in the mouth.
  • Swelling of the lips, tongue or throat, which in severe cases can make it difficult to swallow or breathe.
  • A rash and/or itching of the skin - the rash may be raised, blotchy and red or the skin may be flushed.
  • Wheezing or feeling breathless.
  • Feeling sick or being sick (vomiting).
  • Tummy pain.
  • Diarrhoea.
  • Sneezing or a runny nose.
  • Itchy, reddened, watery eyes.
  • Swelling of the face around the eyes.
  • Feeling light-headed.

These symptoms tend to occur immediately, or shortly after, eating the trigger food.

Symptoms of food allergy due to a non-IgE allergic reaction include:

  • Atopic eczema (classical eczema) that is seen in allergy-prone (atopic) families - often those with hay fever and asthma too.
  • Vomiting that is generally effortless (gastro-oesophageal reflux).
  • Loose and/or frequent stools (faeces).
  • Constipation.
  • Blood or mucus in the stools.
  • Redness around the back passage (anus).
  • Tiredness.
  • Being pale (pallor).
  • Poor growth. This is often seen in children with a non-IgE food allergy. Young children, particularly those with digestive symptoms, should be regularly weighed. Weight should be plotted on a growth chart. This allows medical professionals such as GPs and health visitors to see if growth is slow.

These are all possible symptoms of a food allergy. They range from the mild reactions and  chronic symptoms to the most severe and life-threatening ones.

Food intolerance

Symptoms of a food intolerance vary widely and can include:

  • Bloating and tummy (abdominal) pain.
  • Diarrhoea or loose stools.
  • Skin rashes and itching.
  • A runny nose.
  • Feeling tired.
  • Headaches.

Some of these symptoms are the same as those which can be caused by a non-IgE food allergy. So this shows how complicated it can be to determine whether you have an allergy or an intolerance.

Foods that most commonly cause allergy are milk, eggs, nuts (especially peanuts), sesame, shellfish and other seafood, wheat, soya and some fruits such as citrus and kiwi.

If you think you have, or your child has, a food allergy, you should seek medical advice. Usually your GP would be the first port of call.

You may be referred to a consultant in hospital (often an immunologist or an allergist). This would be for further tests to confirm the diagnosis. This would often happen if a severe allergic reaction or anaphylaxis has occurred, or if a child is not growing well and has gastrointestinal symptoms such as diarrhoea.

If you think you or your child have a food intolerance, it depends on the severity of symptoms as to how promptly you seek medical advice. In milder, less clear-cut situations, it is often worth spending some time yourself trying to work out which foodstuff is the culprit.

Food diary

Keeping a diary of what is eaten is a simple activity that can be helpful in some cases. You need to write down everything that is eaten and any reaction that you have. A food diary does not provide a diagnosis but it does suggest which food(s) could be the culprit(s), allowing further tests.

Specific tests

If it is thought that you have an IgE-mediated allergy, there are two main types of test that can be done:

Skin prick test
This is usually done in a specialist hospital allergy clinic. Because an anaphylactic reaction is possible, there needs to be access to resuscitation equipment including adrenaline (epinephrine). A small drop of liquid containing the food substance is placed on the skin of the forearm and a needle is used to prick the skin, through the droplet. This allows the food to enter the skin. After a period of time, the droplet is wiped off and the skin is read. This means looking to see if there has been a reaction, and how big this is. Usually there would be an itchy red area with a surrounding white raised swelling called a wheal. See separate leaflet called Skin Prick Allergy Test for more details.

Blood tests
Blood tests can detect if your blood contains specific IgE antibodies. So, if you had allergic reactions to peanuts, your blood might contain antibodies to peanuts.The main difficulty with these blood tests is that they are only available for certain specific foods. They are expensive and don't always help in diagnosis. They can indicate you are sensitised to a particular allergen, but this does not necessarily mean you are allergic to it. Results need to be interpreted carefully alongside the history of allergic reaction. The advantage of blood tests is that they are safe in people who have anaphylaxis or severe skin diseases that would make skin prick testing difficult. 

Elimination and challenge

If a non-IgE food allergy is suspected, elimination and challenge can be used to try to confirm the diagnosis of allergy. This is only really appropriate when it seems clear which foodstuff is causing the problem. The food should be excluded from the diet completely for 2-6 weeks, to see if symptoms improve. After this time, the food should be reintroduced, to see if symptoms return. Dietary advice from a dietician or doctor may be required during the elimination period. This is to make sure the diet is not deficient in any essential nutrients and to provide an alternative to the food excluded.

Formal double-blinded, placebo-controlled food challenge tests are the ideal test for diagnosis but are logistically difficult and expensive so not done very often in practice. In this test, you spend a day being monitored as you swallow a solution with increasing amounts of the food which you could be allergic to, and another day when you do the same but the solution doesn't have that food in it. Neither you or the person testing you knows which day is the test of the food and which is the 'placebo'. The test needs to be done somewhere with facilities to manage an anaphylactic reaction, should one occur.

Overall you can see why it can be quite difficult to definitely establish a food allergy diagnosis!

You may see many commercial companies offering allergy testing, particularly on the internet. Be very wary of these.

The following tests have no medical evidence behind them - that is, they cannot be relied upon to give any accurate results. At best, these tests are a waste of money and are of no help in diagnosis. At worst they are dangerous - either by failing to pick up a significant allergy or by over-diagnosing multiple allergies. The following tests are not recommended:

  • IgG blood tests. (It is IgE that is measured in reliable allergy tests performed in the NHS.)
  • Kinesiology.
  • Hair analysis.
  • White blood cell tests - eg, a leukocytotoxic test.
  • Pulse test.
  • Electrodermal (Vega) test.

These tests tend to suggest long lists of foods that could be avoided unnecessarily. This can lead to the exclusion of multiple foods from the diet. Whilst this might be unhealthy for an adult, it can be positively harmful in a child or an adult with complex medical health needs. An extremely restricted diet can lead to severe nutritional deficiencies and eventually to malnutrition.

Children should not follow a restricted diet, unless supervised by a dietician. They require a well-balanced diet, containing all the major food groups, vitamins and trace elements to ensure adequate growth and development.

Avoid the food that causes allergy - but this can be difficult. A dietician can help make sure you still have a healthy balanced diet. Antihistamines medicines can be helpful. You should have an adrenaline (epinephrine) injection with you at all times if you have severe allergy.

Allergen avoidance

The only real treatment for food allergy and intolerance is to avoid that food. This is very important if an extreme form of allergic reaction (anaphylaxis) has occurred.

It can be very difficult to avoid certain foods completely, particularly when eating out and buying food. Often foods are contaminated with traces of another food. An example would be that a factory may produce several types of breakfast cereal, including ones containing nuts. A brand that doesn't specifically contain nuts as an ingredient may contain tiny traces of nut. This is because it has picked them up from machines in the factory, perhaps used after a nutty cereal has been made. For this reason, it is crucial to check the labels on packaging. There are stringent rules for manufacturers regarding labelling.

Referral to a dietician

Dieticians are qualified health professionals. They can assess, diagnose and treat diet and nutrition problems. Nutritionists and therapists are voluntarily registered and anyone can use the title. For therapists in particular, this can mean unproven 'treatments' based on personal opinion and belief.

If you have a food allergy or intolerance, after seeking medical advice, a dietician can be a useful person to see for further help and advice. You can be referred to an NHS dietician. The advice can be particularly useful for children with food allergies, as it is vital they maintain a balanced diet for growth and development.

Medication

Antihistamines can be helpful where food allergies cause symptoms such as itchy rashes (urticaria).

Adrenaline (epinephrine) may be prescribed in an injectable form (such as Emerade®, EpiPen® or Jext®) if a person has a history of anaphylaxis. It is important that this should be carried on your person at all times, and that you are trained in its use or how to give it to your child.

Other treatments are available under specialist advice for very severe allergies. In some cases, people with severe allergies can be 'de-sensitised' by immunotherapy treatment. This involves developing a tolerance by gradually increasing from tiny doses of the protein of the food you are allergic to. There is a risk of anaphylactic reactions during the process, and the treatment takes quite a long time.

Other advice

Support and education are important for people with food allergies or for parents of children with food allergies. This may be provided by your GP, practice nurse or hospital specialist. Written information and advice on how to manage allergic reactions are extremely important. There are various support groups available for people with allergies and some people like to access help themselves this way.

It is a good idea to have a medical emergency identification bracelet or equivalent if you have a history of anaphylaxis.

As the immune system matures and a child gets older, many grow out of allergies to eggs, wheat, milk and soya. Adults with food allergy may also grow out of their food allergy if an elimination diet is followed carefully. About a third of adults and children lose their reactions after 1-2 years of elimination diets.

Allergic reactions to peanuts, seafood, fish and tree nuts rarely get better.

A note about coeliac disease

Coeliac disease is not a food allergy, nor is it a food intolerance. It is an autoimmune disease which means that the immune system (which normally protects the body from infections) mistakenly attacks itself. Other autoimmune diseases include type 1 diabetes, rheumatoid arthritis and some thyroid disorders. Coeliac disease is caused by a sensitivity to gluten. Gluten is a component of wheat, so it is found in foods such as bread and pasta. See separate leaflet called Coeliac Disease for more details. Some people can have an intolerance to gluten, which is a different condition, and is one of the food intolerances relevant to the above leaflet.

Egg allergy and immunisations

Egg allergy is not a reason to avoid the flu vaccination, nor is it a reason for a child to miss his or her immunisations (including measles, mumps and rubella (MMR)). If there is a confirmed anaphylactic reaction to egg-containing food, it may be safer for a child to be vaccinated in hospital. However, most anaphylactic reactions to MMR are related to other ingredients in the vaccine, and not the egg. Anaphylaxis to gelatin or neomycin (an antibiotic) is a contra-indication to MMR. Children with milder reactions should definitely receive their vaccinations in the normal way.

Peanut allergy, pregnancy, breastfeeding and weaning

The latest research shows that there is no clear evidence to say that eating, or not eating, peanuts during pregnancy affects the chances of your baby developing a peanut allergy. Pregnant women need not avoid eating peanuts.

If you are breastfeeding and enjoy eating peanuts or other nuts there is no reason to avoid them, unless you are allergic to them. They can be eaten as part of a healthy balanced diet.

Nuts are a choking hazard for children so great care must be taken. World Health Organization (WHO) advice is that babies should be fed only breast milk for their first six months. However, if weaning on to solid food occurs before 6 months of age, peanuts and nut-containing foods should be avoided in case a food allergy is triggered. The same advice is also true for seeds, cow's milk, eggs, wheat, fish and shellfish. In general terms, when weaning, it is best to introduce one new food at a time. That way, if there is a problem, such as an allergy, it is easier to tell which food(s) could be the culprit(s).

Further reading and references

  • ; NICE Clinical Guideline (February 2011)

  • ; Allergy UK

  • ; Allergy UK

  • ; European Academy of Allergy and Clinical Immunology (EAACI), 2014

  • ; British Society for Allergy & Clinical Immunology

  • ; Review article: the diagnosis and management of food allergy and food intolerances. Aliment Pharmacol Ther. 2015 Jan41(1):3-25. doi: 10.1111/apt.12984. Epub 2014 Oct 14.

  • ; Food allergy: epidemiology and natural history. Immunol Allergy Clin North Am. 2015 Feb35(1):45-59. doi: 10.1016/j.iac.2014.09.004. Epub 2014 Nov 21.

  • ; Allergen-specific oral immunotherapy for peanut allergy. Cochrane Database Syst Rev. 2012 Sep 12(9):CD009014. doi: 10.1002/14651858.CD009014.pub2.

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