Food allergy

Health and Nutrition > Diseases > A Reviewed by Dr Dan Rutherford, GP

What is food allergy?

Food allergy is when your immune system reacts to a particular food. This causes inflammation of the body's tissues, which in some cases can be life-threatening.

It should not be confused with food intolerance, which is completely different.

In both cases you get symptoms when you eat even small quantities of certain foods.

But food intolerance is not caused by the immune system. This means symptoms are usually limited to the digestive system (vomiting, diarrhoea) and are rarely life-threatening.

Who's at risk?

A lot of people avoid eating certain foods because they mistakenly believe they are allergic to them.

Actual food allergy is very rare and tends to run in families. Only 3 in 100 infants and 1 in 100 adults have a food allergy.

You are more at risk if you or any family members have other allergic diseases such as eczema, asthma and hay fever.

If you suspect that you are allergic to a type of food, you should get a diagnosis from your doctor or a qualified specialist in allergic diseases before making substantial changes to your diet.

Why do you become allergic to food?

You develop an allergy when your immune system overreacts to foreign proteins and produces antibodies that attack the body's own tissues.

The foreign protein that causes the reaction is known as an allergen. In food allergy, a particular type of food is the allergen.

A food allergy is a type 1 allergy. In this type of allergy, the immune system produces antibodies called IgE-antibody in response to the allergen.

The body only produces IgE after exposure to the allergen. This means your body will produce more IgE the next time you eat that food.

If enough of this antibody is produced, it will cause symptoms such as rash and swelling.

You cannot get allergic symptoms the first time you eat a new kind of food. But because traces of foods are found within other foods, eg nuts in chocolate, you may not be aware of the first time you came into contact with a particular allergen.

Food allergens and cross-reacting allergens In principle you can be allergic to all kinds of food, but reactions to the following are most common:

- shellfish
- milk
- fish
- soya beans
- wheat
- eggs
- peanuts
- fruit and vegetables.

Sometimes a food can be exposed to other allergens and they can cause your body to react to the food. This is known as a 'cross- reaction'.

Birch pollen cross-reacts with:

- hazelnut, walnut, almond and other nuts
- apple, pear, cherry, plum and kiwi
- potato peel, tomato and carrot.

Grass cross-reacts with beans and green peas.

What are the symptoms?

Allergic reactions vary a lot and can begin immediately after you eat the food or start several hours later. The symptoms below are typical of food allergy.

- Swollen lips, face or throat.
- Itchy sensation in the mouth and lips.
- Vomiting, stomach cramp and diarrhoea.
- Headache.
- Fatigue and irritability.
- Hay fever.
- Asthma.
- Rash, nettle rash and pre-existing eczema may get worse.

Once you have become sensitised to a food, your immune system produces more antibodies every time you eat that food. This means symptoms that are mild one day can be serious the next.

If you have a food allergy, you should never put yourself at risk of a reaction to make life easier when eating out or at other people's houses.

In rare cases, eating even a tiny amount of the food can cause anaphylactic shock. This is a life-threatening reaction by the immune system that causes the airways to narrow and heart to malfunction. It needs immediate treatment with adrenaline.

If you suddenly begin to have difficulty breathing, get medical treatment right away.

How is food allergy diagnosed?

. The extent of the doctor's examination depends on the severity of the symptoms.

1) Blood and skin tests A prick-test of the allergen on your skin or a RAST blood test may be useful to eliminate allergies.

Unfortunately, they often show reactions that have no practical significance. For this reason, allergy testing should be performed and evaluated by experts.

Test kits that are available in health food shops and over the internet are not reliable indicators of allergy.

2) Food elimination programme

To confirm diagnosis, an allergist (a specialist in allergic diseases), will suggest the following programme.

- Two weeks on a normal varied diet while keeping a detailed diary of everything eaten and any reactions.
- If you have a reaction, that food should be avoided over the following two weeks while you continue to keep the diary.
- If your symptoms get considerably better, the next step is to try an open provocation of your allergy by deliberately eating the food that may be causing your problems.
- If the symptoms return, you will be given more provocations where neither you nor the allergist know whether you are eating the suspected food. This is to eliminate any psychological cause for a reaction.

This is the usual procedure to obtain a diagnosis of food allergy.

What else could it be?

An inability to digest certain kinds of food may point to:

- coeliac disease
- lactose intolerance
- non-immunological reactions (reactions not caused by the immune system), eg to histamine in shellfish
- minor food poisoning.

Bowel symptoms can be due to an inflammatory condition in the intestinal canal such as ulcerative colitis and Crohn's disease.

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