Allergies, sensitivities and intolerances: understanding why you react to foods

Types of Reactions

In the 1st century BC, Lucretius wrote “what is food for one man may be bitter poison to others” (quod ali cibus est aliis fuat acre venenum). Such is the case with food allergies. Individuals with food allergies or other types of sensitivities react adversely to eating foods and food ingredients that most people can eat with no problems.

Testing for food “allergies” can be controversial, partly because they are poorly understood. The term “allergy” is widely used to describe reactions to foods. However, Nutritionists make a distinction between the type of reactions and have classified food reactivities as:

  • Allergy
  • Intolerance
  • Sensitivity
  • Food sensitivities & IBS

There are many reasons why you might react to a particular food. For instance:

Basic Contents

E.G reactions to the proteins or lectins in a food

Natural processes

Aging, insects, worms, fungi, fermentation process (tea, malt)


Pesticides, fungicides etc


Bacterial, fungal, toxins (seafood)

Genetic Engineering

Our bodies don’t always recognise genetically modified foods as natural, and hence these can provoke a reaction.


Marinating, smoking, salting, freezing, microwaving


Preservatives, antibiotics, hormones, colourings

Food Allergies

In allergic reactions the immune response is mediated by the degranulation of mast cells and basophils as a result of antigen binding to IgE (and to a lesser extent, IgG) antibodies. As a result, immune chemicals which include Histamine are released. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.

Unfortunately, when released into the body inappropriately or in too high a quantity, histamine is a potentially devastating substance.
It is immediate in nature and can be severe. Symptoms can include headaches, rashes, flushing, itching, swelling, runny or blocked nose, irregular heartbeat, diarrhoea, nausea, vomiting or abdominal pain.

The immune system is not infallible, cross reactions are inevitable and arise when distinctive chemical features (epitopes) have look-a-likes. Cross reactions between inhaled allergens and foods are common.

Food Intolerances

  • These do not involve the immune system but may be enzymatic (e.g. lactose intolerance) or triggered by chemicals such as food additives e.g. salicylates, sulphites etc

  • Fructose intolerance happens when absorption of fructose is impaired due to insufficiency of fructose carrier molecules in the small intestinal lining. This results in fructose being retained inside the gut lumen, where it acts as a substrate for bacteria that ferment the sugar and can produce IBS-like symptoms

  • Lactose intolerance occurs when an individual has problems digesting milk sugar (lactose) as the lactase enzyme is missing. This results in the lactose being retained inside the gut, where like fructose, it acts as a substrate for bacteria, which ferment it. This also results in IBS-like symptoms.

  • It is possible to test, via hydrogen breath test for Lactose and Fructose intolerance

  • Lactose and fructose intolerance can be a sign of SIBO and may perpetuate the bacterial overgrowth.

Food Sensitivities

Food sensitivities are characterised as:

  • Immune system-mediated responses that involve various classes of antibody (including IgG and IgM) and the formation of immune complexes (antigen binding to antibody in the blood).

  • These reactions are more delayed with onset of symptoms between several hours to 7 days after ingestion

  • The gold standard for the identification of food sensitivities is via an elimination diet, whereby all known allergens are avoided for 2 weeks, then re-introduced one at a time at intervals to assess reactivity.

Food Sensitivities & IBS

There is good research to support the use of IgG testing in IBS, a study by Atkinson et al, showed that patients who avoided the IgG foods that they had tested positive to, were able to achieve on average a 26% reduction in symptoms.

At IBS clinics, we offer food sensitivity testing as standard, with the results of the sensitivity test being then used to form the basis of an elimination diets. Often re-introduction of these foods after a period of abstinence is possible, so lifetime avoidance of identified problem foods is often not necessary.


Mullin et al, Testing for Food Reactions: The Good, the Bad, and the Ugly Nutr Clin Pract 2010 25:192

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