What is a Food Allergy and a Food Intolerance?

Eating certain foods can sometimes lead to uncomfortable, painful, and severe physical reactions. A food allergy is where proteins within foods trigger an immune-mediated hypersensitivity, which can either be Immunoglobulin E (IgE) mediated, causing an immediate onset reaction, or non-IgE mediated, where there is a delayed onset reaction.  A response to food that does not involve the immune system is often a food intolerance, with the onset reaction being immediate or delayed.

The most common allergies are milk, egg, peanuts, tree nuts, cereals containing gluten, soya, celery, shellfish, fish, sesame, mustard and sulphites. However, it is not limited to this list.

The most common intolerances are lactose, histamine and salicylate substances. However, there are other foods that people can be sensitive to.

Food allergy or intolerance?

If you have an immediate food allergy (IgE mediated), your body cannot recognise the food protein as ‘safe’, which causes antibodies to combat the specific allergen. As a result, there is a release of inflammatory factors – histamine and other inflammatory substances, which causes symptoms to occur. It takes minimal amounts for the food to cause a reaction and it can take between two minutes to two hours after the food has been eaten. If you have a delayed food allergy (non-IgE), it still involves your immune system, but other cells are involved leading to a different reaction, which usually takes between two hours to two days to show after eating the food.

A food intolerance does not involve your immune system. The type of intolerance can be broken down into enzymatic or pharmacological and you often need to have had a lot of this food, and the reaction can be immediate or delayed and range in severity.

What are the symptoms that can occur?

Food allergy symptoms

If you have a food allergy, the IgE or non-IgE mediated symptoms can vary.

IgE mediated:


  • Itching (pruritis)
  • Skin redness (erythema)
  • Hives/ raised itchy rash (acute urticaria)
  • Swelling on the lips, face or around the eyes (acute angioedema)

GI System:

  • Swelling of the lips, tongue and palate (angioedema)
  • Oral itching (oral pruritis)
  • Nausea
  • Abdominal pain
  • Vomiting
  • Diarrhoea

The Respiratory System:

(Often in combination with one or more of the above symptoms)

  • Upper respiratory tract symptoms e.g., nasal itching, sneezing, congestion
  • Lower respiratory tract symptoms e.g., cough, chest tightness, wheezing, shortness of breath


  • A potentially life-threatening condition
  • Can occur with some or all the previous symptoms
  • Immediate IgE-mediated response
  • Typically;
    • Angioedema (swelling)
    • Blood vessels dilate (peripheral vasodilation)
    • Decrease in blood pressure and circulatory collapse

Non-IgE mediated:


  • Itching (pruritis)
  • Skin redness (erythema)
  • Atopic eczema

The GI system:

  • Loose or frequent stools
  • Blood and/or mucus in stools
  • Abdominal pain
  • Perianal redness
  • Pallor and tiredness
  • Gastro-oesophageal reflux disease

Food Intolerance symptoms

  • Abdominal pain
  • Diarrhoea
  • Flatulence and bloating
  • Headaches or migraines
  • Heartburn
  • Nausea

How to Test for Allergies and Intolerances

Some clinical conditions can masquerade as food allergies or food intolerances, such as IBS, IBD, dumping syndrome, gastric ulcers, gall bladder disease and pancreatic insufficiency. Therefore, it is important to rule these out. This is often done through a mixture of establishing clinical history, which will help indicate which investigations are most appropriate, and thorough medical examination, where nutritional status should be assessed.  Establishing a diagnosis can involve several stages. Firstly, gathering a clear medical history and having a clinical assessment, then clinical diagnostic tests or a food and symptom diary need to be completed. A diagnostic exclusion diary will then likely be recommended after these have taken place. Finally, an open food challenge with or without double blind placebo-controlled food challenge, which is needed to confirm an allergy or intolerance.

Food Allergy testing

If an IgE mediated food allergy is suspected, then a skin prick test and/or blood tests are often used to detect specific IgE antibodies and any likely co-allergens. The skin prick test will look at suspected triggers and uses commercial solutions or fresh foods to prick the skin. It is only able to measure IgE mediated reactions.  The blood tests can detect allergen specific IgE, and the Radioallergosorbent Test (RAST) is often best used. This method cannot identify non-IgE mediated food triggers and non-immediate adverse reactions. It’s important to note that if patch testing is needed or oral food challenges to diagnose an IgE mediated food allergy, they should only take place where there are resuscitation facilities available e.g., a hospital, as some symptoms can occur quickly and could be life-threatening.

Diagnosing a non-IgE mediated allergy involves establishing a detailed clinical history and assessment – this could also involve a symptom diary. A trial elimination of the suspected allergen may be implemented for around two to six weeks, with a reintroduction after the trial. Again, it’s important that this is completed under medical or dietetic supervision, with the knowledge of appropriate timings of elimination and reintroduction of foods. 

Food intolerance testing

When testing for an intolerance, a similar process is followed with a trial elimination of the suspected food intolerance. After two to six weeks, a reintroduction of the food will occur. Food challenges can also take place, where the suspected food is given orally in small amounts and gradually built up to larger amounts if tolerated – all whilst being monitored.

Alternative Testing

It’s important to note that there are many alternative diagnostic tests out there that should be avoided. This can include the pulse test, vega test, leucocytotoxic test, finger-prick blood tests and hair analysis tests as these tests, have no scientific basis and can lead to misdiagnosis. Furthermore, they could lead to inappropriate dietary restrictions, which could lead to malnutrition.

Treatment of Allergies and Intolerances

If you do find that you have a food allergy or food intolerance, it is important that you take it seriously. Ensure that you avoid eating the foods to the best of your ability, and if relevant, if you are cutting out complete food groups from your diet such as dairy or wheat, you are replacing them with enough of the nutrients you need. There are plenty of ‘free from’ products in many major supermarkets that may be suitable for your allergy or intolerance, which are clearly labelled. Furthermore, it is now a UK law where all prepacked and non-prepacked foods must have all the major allergens emphasised within the ingredients, which are often in bold so they are easy to see. When eating out, all restaurants or cafes must provide you with an allergen menu – if you are having any trouble with this, then a member of staff should be able to assist you. If applicable, please ensure you always carry emergency medication on you e.g., an EpiPen, and make sure your friends and family know how to use it. If there are foods you are unsure about, or you have concerns and need further dietary advice, please reach out to a dietitian, or ask your doctor for a referral.

By Jessica Thomerson

By Jessica Thomerson

Dietetic student, revised by Reema Patel, Registered Dietitian at Dietitian Fit & Co


BDA (2021) Food Allergy and Food Intolerance: Food Fact Sheet https://www.bda.uk.com/resource/food-allergy-food-intolerance.html

NICE (2011) CG116 Food allergy in under 19s: assessment and diagnosis. https://www.nice.org.uk/guidance/cg116 

NICE (2016) Food Allergy Quality Standard (QS118) https://www.nice.org.uk/guidance/qs118

NHS (2019) Food Allergy https://www.nhs.uk/conditions/food-allergy/

RCPCH (2011) Allergy Care Pathways for Children – Food Allergy  https://www.rcpch.ac.uk/resources/allergy-care-pathway-food-allergy