Food Allergy & Intolerance

Is it a food allergy or a food intolerance?

It is becoming increasingly common to hear people say “I can’t eat that, I’m allergic.” As the frequency of this statement has increased, so too has the myths (allergies are a fad), misunderstandings (food allergy and food intolerance are the same) and dubious claims supporting some testing methods and cures.

As the consequences may be very different, it is important to understand the difference between food allergy and food intolerance as they involve different systems of the human body and may require different management strategies – even though symptoms may be similar.

Food allergy

A food allergy occurs when a person’s immune system over-reacts to the protein components of a specific food.

Our body contains antibodies which are supposed to protect us from viruses, parasites and infections. When a person is having an allergic reaction to a particular food, their immune system mistakenly identifies the food protein as an invader and begins to create antibodies against it. These antibodies, called IgE, attach themselves to mast cells, which are abundant under the surface of the skin and in the nose, eyes, lungs and gastrointestinal tract. When the allergen is encountered the IgE antibodies seize it, causing the mast cells to release powerful chemicals, including histamine. This causes the allergic reaction. A reaction may occur within minutes or up to a few hours after contact.

Reactions vary in severity from mild to moderate reactions (e.g. hives, stomach cramps, nausea) to life-threatening anaphylaxis with rapidly spreading hives, tissue swelling, breathing difficulty and/or collapse.

Sometimes the presence of a food allergy is less obvious and can be characterised by infantile colic, reflux of stomach contents, eczema, chronic diarrhoea, and a failure to thrive. Recent studies have found that up to 40 – 50% of eczema cases in young children are triggered by food allergy.


Common food allergensWhile any type of food can trigger an allergic reaction, the most common food allergens are egg, cow’s milk (dairy), peanuts, tree nuts (eg almonds, cashews, Brazil nuts), soy, wheat, seeds, fish and shellfish.

It is estimated that between 6 – 8% of children and 2 – 4% of adults have a food allergy. Reasons for the increase in prevalence of food allergy are not known. There is a known genetic component, with children of parents with allergies at higher risk, but environmental factors are widely acknowledged as driving this epidemic. Factors currently being investigated include ‘the hygiene hypothesis’ (are we now too clean?); sun avoidance leading to lack of Vitamin D; dietary changes; pollutants; and the effect of stress on the immune system.

Food allergies are serious

Food allergies are serious and need to be treated as such – they can kill! With no known cure (yet) food allergy prevention approaches are now a routine part of travel, school, sports, and the workplace. The days of lunchbox peanut butter sandwiches are over in many schools as complete bans on all peanut-containing products are in place. Some schools handle it on a class by class basis with requests to keep certain classes nut free zones.

As schools are charged with the responsibility of keeping the school environment safe for all children, some of whom have long lists of food allergies, the burden on teachers and the school is not insignificant. Parents of non-allergic children can play their part by ensuring they comply with requests from schools to keep their child’s lunchbox free of known trigger foods.

Food intolerance

Food intolerances don’t involve the immune system at all. A food intolerance will cause a reaction by irritating nerve endings in different parts of the body, rather like the way certain drugs can cause side-effects. As with drugs, people with a sensitive constitution may react to food chemicals more than others. A food intolerance is not life-threatening in the way that a food allergy may be but it can still cause significant health issues.

Food chemicals are found in natural foods (e.g. salicylates, amines, glutamate, and natural sugars) as well as one or more of the common food additives (e.g. artificial colours, preservatives, antioxidants, and flavour enhancers).

The speed of onset and severity of reactions can vary widely. Symptoms can began within an hour or two, but more often takes several hours to develop. Typical reactions last a few hours, but more severe ones can sometimes go on for several days.

Food intolerances will typically affect one or more of the following systems – the skin, airways, nervous system, and/or the gastrointestinal system. Food intolerance reactions are dose-dependent. A small amount of a chemical rich food may cause no symptoms, whereas a larger amount that exceeds a person’s tolerance threshold may provoke a reaction. However, even eating small amounts regularly can cause a gradual build up with symptoms developing after a few days. Consequently chronic or recurrent symptoms can develop without the cause being obvious.

Natural food chemicals

Natural chemicals present in many healthy foods can cause just as many problems for sensitive people as the artificial ones used as food additives.  The chemical composition of foods varies greatly. The food chemicals most likely to upset sensitive individuals – salicylates, amines and glutamate – are found in many different foods and are therefore consumed in the greatest quantity in the daily diet.


Salicylates are chemicals that occur naturally in many plants – they’re a kind of natural pesticide – to protect the plants against insects and diseases. Salicylates are found in:

  • most fruits and fruit juices
  • some vegetables
  • herbs and spices
  • nuts
  • tea and coffee
  • honey and jams
  • flavour additives
  • beer and wine.

Salicylates are also found medications, fragrances, industrial chemicals, plastics and some pesticides, and can cause adverse effects when inhaled as well as eaten. The concentration of salicylate can vary according to plant variety, ripeness and season.


Amines are formed by the breakdown of proteins in foods. Levels increase in protein foods (meats, fish, cheese) as they age or “mature”, and in fruits as they ripen (e.g. bananas, tomato, avocado, pawpaw, olives).


Glutamate is an amino acid building block of all proteins and is found naturally in most foods. Glutamate enhances the flavour of food which is why foods rich in natural glutamate (e.g. cheese, tomato, mushrooms, stock cubes, soy sauce, meat extract, yeast extract) are used to add flavour to meals. MSG (pure monosodium glutamate) is used as an additive in savoury snack foods, soups, sources and Asian cooking for the same reason.

Food additives

Additives are used to enhance the flavour, appearance, freshness and shelf-life of foods. People who are sensitive to natural food chemicals are usually sensitive to one or more of the common food additives as well. Reactions to these may be easier to recognise than reactions to natural chemicals because of the higher concentrations added to processed foods. As with the natural chemicals, individuals vary in their sensitivity to particular additives. The ones most likely to be a problem for people with food intolerances are:

The code numbers of additives most likely to cause adverse reactions are:

Food Chemicals Known to Cause Reactions

Additive enriched foods


Common reactions to natural food chemicals and food additives include:

Common Food Intolerance symptoms

Food reaction diagnosis: is it an allergy or intolerance?

The starting point for diagnosing both a food allergy or a food intolerance is a noted history of reactions. A food diary recording:
foods eaten
the symptoms noted, and
the timing between the ingestion of food and the symptoms presenting themselves
may assist you to see a pattern and help identify possible suspect food(s).

Allergy testing

Skin prick tests or blood tests (RAST and Cap RAST) detect IgE antibodies to specific allergens which may help to diagnose an allergy. (IgG blood tests are not reliable). However, negative reactions do not necessarily mean that a tested food is not causing symptoms as that may indicate a different type of allergic response or intolerance. You should also be aware that a positive result does not necessarily mean you will react to that food. That is why a well recorded history is so important in diagnosing allergy.

Food challenges which are double-blind and placebo-controlled (i.e. where neither the patient nor the doctor know the content of the challenge) are considered the gold standard for diagnosing food allergy. The procedure takes time. Suspected food(s) are avoided for at least two weeks, antihistamines are discontinued, and doses of asthma medications are reduced as much as possible. Graded doses of either a challenge food or a placebo food are given. The food is hidden either in another food or in opaque capsules.

Medical supervision and immediate access to emergency medications and equipment are required because reactions can be severe. Challenges are terminated when a reaction becomes apparent, and emergency medications are given as needed. Patients are also observed for delayed reactions. If allergy to only a few foods is suspected, single-blind or open challenges may be used to screen for reactivity. Negative challenges are always confirmed with a larger, meal-sized portion of the food. Oral challenges should not be performed in patients with a clear history of reactivity or a severe reaction.

Food intolerance testing

Unlike allergies, there are no skin tests or blood tests that can help diagnose food intolerances. Even where a well-documented history exists which suggests symptoms are diet-related, often the wrong food components are suspected. To complicate matters, it is not possible to predict from a person’s symptoms which particular food chemical(s) they might be sensitive to.

The only reliable way of finding out which food chemicals may be contributing to your child’s symptoms is to eliminate all possible trigger substances at the same time, wait for symptoms to subside, and then reintroduce them one by one according to a systematic challenge protocol. This “elimination and challenge” procedure is best done under the supervision of a Dietitian or an appropriately qualified and experienced nutritionist.

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