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Dietitian Orla Walsh: What's the difference between an allergy and intolerance?

Our resident dietitian Orla Walsh explains the science behind food allergies and food intolerances


Food intolerance can make your day-to-day life miserable

Food intolerance can make your day-to-day life miserable

Dietitian Orla Walsh

Dietitian Orla Walsh


Food intolerance can make your day-to-day life miserable

'Food hypersensitivity' is when someone experiences any sort of negative reaction from eating a particular food. Approximately 1 to 10pc of adults and children have this condition.

It includes small, unpleasant reactions to a particular food, right up to a dangerous physical reaction. When these 'hypersensitivity' reactions involve the immune system they are called a 'food allergy'. A severe allergy can cause an anaphylactic reaction which can result in death, unless treated immediately by adrenaline injection and followed up by medical assistance. In contrast, 'Food intolerance' doesn't usually involve the immune system and is generally not life-threatening. That said, just because an intolerance doesn't kill you doesn't mean that it can't make your day-to-day life miserable.

Food allergy

The immune system is made up of a network of body cells, tissues and organs that work together to protect the body. Its job is to seek out and destroy disease-causing organisms or substances. A food allergy is when the immune system reacts to a specific protein found in food as it wrongly believes that the protein is a 'baddie'. Food allergies can be Immunoglobulin E (IgE) mediated, non-IgE mediated, or a mixture of the two.


Dietitian Orla Walsh

Dietitian Orla Walsh

Dietitian Orla Walsh

IgE mediated reactions

An immediate reaction to an allergy is when symptoms occur within two hours of eating the food. Sometimes they are very rapid and happen within 1-2 minutes. These reactions are considered to be IgE mediated reactions. After eating this food, the body creates IgE antibodies to fight against a specific protein within the food that it wrongly believes to be harmful.

Even very small amounts of food can bring out considerable reactions. The IgE antibodies cause the release of substances such as histamine, which result in allergic symptoms. Classic symptoms include the likes of an itchy rash, swelling and in some cases vomiting and diarrhoea. Although symptoms vary in severity, they can be life threatening.

Non-IgE mediated reactions

When symptoms from an allergy occur after two hours the reaction doesn't usually involve IgE antibodies but rather a different type of immune reaction. In some instances symptoms may not appear for more than 24 hours. In IgE mediated reactions, tiny amounts of the food can lead to huge responses. However, with non-IgE mediated reactions there may be a threshold level for a particular food. In these reactions it's important to figure out how much you can tolerate before you have reactions. More delayed reactions tend to result in symptoms like stomach upset and/or skin reactions such as atopic eczema.

How common are allergies?

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Allergies are common, affecting 5-6pc of young children in Ireland. Most children with food allergy have other conditions such as eczema. Occasionally they may also suffer from asthma and allergic rhinitis. Allergies to milk, egg, peanut, nuts and fish account for more than 90 per cent of cases.

Although most children will outgrow allergy to milk or egg, most children will not outgrow a peanut, fish, shellfish or tree nut allergy. Tree nuts include hazelnuts, almonds, walnuts, Brazil nuts, cashew and pistachio nuts. (For more see p9)

In adults, the most common type of food allergy is one involving symptoms linked to raw fruits and vegetables. This is called pollen food syndrome or oral allergy syndrome because this type of food allergy occurs in people who have an IgE mediated allergy to pollen.

Reactions occur to the proteins in fruits and vegetables as they are very similar to pollen allergens. The most common foods involved are apples, tree nuts and fruit with stones in them. Aside from this type of allergic reaction, adults are more likely to suffer from an allergic reaction to shellfish, peanuts, legumes and seeds.

Food Allergy Testing

It's important to ensure that you get the right test as some are non-specific and non-sensitive. Information about your symptoms as well as background on your and your family's health is always collected.

This background information is then backed up further by a blood test looking for specific IgE antibodies, or a Skin Prick Test. As delayed-type food allergies do not involve the production of IgE antibodies there are more challenging to diagnose.

Dietitian-supervised exclusion and reintroduction is considered to be the only supportable diagnostic and possibly therapeutic intervention.

Food Intolerance

As mentioned above, food intolerances often do not involve the immune system. Food intolerances come in many different shapes and sizes. For example they may be pharmacological or enzymatic.

Pharmacological intolerances involve reactions to particular naturally occurring substances in foods. They include:

• histamine, as found in the likes of red wine, strong and blue cheeses, tuna, mackerel and pork products;

• salicylates, as found in a wide variety of plant foods;

• caffeine or theobromine, as found in chocolate.

Reactions to food additives, histamine, salicylate and sulphites often show a wide variance in symptoms.

The most common enzymatic food intolerance is lactose intolerance which is a reaction to the sugar naturally occurring in milk. For some this leads to bloating, production of gas and diarrhoea. Often someone will have a threshold for the amount of milk and yoghurt they can consume before they experience negative symptoms. In other words, they can have some but not a lot.

Lactose intolerant implies that a person is unable to digest lactose, the naturally occurring sugar in milk and milk-containing products like yoghurt. Lactase is the enzyme or chemical that our body produces to break down lactose. Lactose intolerance is when there is little or no lactase being produced in the body. There are three types:

1. Hereditary lactose intolerance which is genetic. This is rare and means that no lactase enzyme is produced by the body. If someone has hereditary lactose intolerance, they will not be able to eat or drink any lactose without being symptomatic.

2. Primary lactose intolerance is more common in certain races. For example, people who are African, Asian and American Indians are more likely to have primary lactose intolerance. This usually means that the person is able to have very little lactose in their diet after the breastfeeding period.

3. Secondary lactose intolerance is when it is secondary to a condition and disease. Generally once this condition or disease is controlled, lactose intolerance no longer occurs. This is seen in coeliac disease after a strict gluten-free diet is in place and the bowel heals and lactase levels return to normal.

When someone is lactose intolerant, they generally start to feel unwell within two hours of eating or drinking lactose. The severity of symptoms differs but usually includes diarrhoea, bloating, and cramping. Some people feel nauseous and gaseous with it.

Lactose intolerance doesn't lead to more serious bowel disorders. Diagnosis is often confirmed by a hydrogen breath test or a lactose tolerance blood test.

Hereditary lactose intolerance requires complete avoidance, which includes 'hidden ingredients' in food and certain medications. However, with primary and secondary lactose intolerance it's important to find your tolerance level. Often people tolerate lactose better if it's taken with a meal, rather than as a snack. Smaller portions of milk and yoghurt may be required but often people can tolerate cheese. If the person is eating out, they may wish to take an enzyme supplement with them to ensure they can eat their meal out without suffering embarrassing symptoms.

An easy way to see if lactose intolerance is bothering you is switching to lacto-free milks and yoghurts.

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