What is a Food Intolerance?

The terms ‘food allergy’, ‘food intolerance’ and ‘food sensitivity/ hypersensitivity’ are often used interchangeably and confused, but essentially they all mean an abnormal reaction to certain foods, which can manifest themselves in a certain number of ways. They can result from activation of the immune system (immune-mediated reactions) and the subsequent production of antibodies, or reactions that are not immune-mediated (non immune-mediated reactions).

Common symptoms include:

  • Bloating

  • Fatigue

  • Headaches or migrane

  • Diarrhoea and/or constipation

  • Abdominal pains

  • Skin problems

  • Weight issues

  • Anxiety

Immune-mediated Reactions

Reactions that trigger an immune response within the body are most often referred to as allergies and occur when the body over reacts to foods that do not usually produce such a response in the majority of people. This over-reaction triggers the immune system to produce antibodies to attack the ‘foreign’ food, which the immune system recognizes as a threat to the body.

These immune-mediated responses are grouped into four types: I, II, III and IV. These classifications are based on which part of the immune system is activated and how long it takes for a reaction to occur.

The two types of allergy associated with adverse reactions to food are:

  • Type I Allergy - IgE mediated allergy / Type I hypersensitivities / true allergy. These reactions are characterised by the production of IgE antibodies, the release of histamine and other chemical mediators upon exposure to the allergen (e.g. peanuts and shellfish). They are responsible for the immediate onset of symptoms that can occur within seconds or minutes following ingestion of certain foods. Symptoms often associated with a classical allergic response include: rashes, sneezing, difficulty breathing and anaphylactic shock. It is usually obvious which foods are responsible for a food allergy and these have to be avoided for life.

  • Type III Allergy - IgG mediated allergy / food intolerance / food hypersensitivity. These reactions are characterised by the production of IgG antibodies and the gradual formation of antigen/antibody complexes, which are deposited in tissues causing chronic inflammation. They are responsible for the ‘delayed-onset’ of symptoms, which can occur several hours or days after foods are ingested. Symptoms include: anxiety, depression, IBS, headaches/migraines, fatigue, hypertension, eczema, asthma, joint pain, chronic rhinitis, arthritis, weight problems and fibromyalgia. It is possible to eliminate the offending food(s) from the diet for a short period of time and then gradually re-introduce them when the symptoms have improved.

Non-Immune Mediated Response

Reactions that do not produce an immune response are often referred to as ‘food intolerances’. They can be caused by sensitivities to certain chemicals/ additives found in food or due to enzyme deficiencies:

Enzyme Insufficiency/Deficiency:

A) Lactose Intolerance

  • Caused by a deficiency of lactase (an enzyme that breaks down lactose).

  • Foods that contain lactose include: dairy products (milk, cheese, yoghurts, etc).

  • Symptoms include: bloating, diarrhoea and flatulence.

B) Histamine Intolerance

  • Caused by an elevated histamine level due to a deficiency or inhibition of diamine oxidase (DAO) – an enzyme that breaks down histamine (a chemical that triggers an inflammatory response).

  • Aggravated by foods high in histamine, including: red wine, cheese and tuna fish.

  • Some foods are low in histamine but can trigger the release of histamine in the body including: citrus foods, bananas, tomatoes and chocolate.

  • Symptoms include: migraines, dizziness, bowel/stomach problems, rhinitis, depression, irritation and reddening of the skin.


There are two types of abnormal responses to ingested food -

1 - Immune mediated*

Classified as 4 types, most relevant are Type 1 (IgE antibody reaction - ALLERGY) & Type 3 (IgG antibody reaction – FOOD INTOLERANCE)

2 - Non-immune mediated

Enzyme Insufficiency/deficiency & Histamine Intolerance

*An IgG reaction, commonly known as a food intolerance/sensitivity, is actually a Type III Allergy - in full medical classification terms. A Type I allergy (IgE reaction) is what is typically referred to as 'an allergy' and can be life threatening.

References – Cambridge Nutritional Sciences, 2014.

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