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Food allergy and cross-reactions

What you do if you are allergic to food

Reviewed by Dr Dan Rutherford, GP

What is food allergy?

In principle you can be allergic to all kinds of food, but reactions to shellfish, milk, fish, soya beans, wheat, eggs, nuts as well as fruit and vegetables are most common.

Food allergy is an allergic reaction – a hypersensitivity reaction, to something that has been eaten. However, it should not be confused with food intolerance, which is completely different. In both cases you get symptoms when you eat a certain kind of food even in small quantities, food allergy is very rare and therefore it is important if you suspect that you or your children are allergic to a type of food, to have an accurate diagnosis from your doctor or a specialist in allergic diseases.

A lot of people avoid eating things to which they mistakenly believe they are allergic.

Why do you become allergic to food?

In order to develop food allergies your immune defense must be predisposed to overreact to foreign proteins in the environment with the specific IgE-antibody. This is called a type 1 allergy. Antibodies are the body’s antidotes to alien substances.

When you are exposed to proteins in food, specific IgE may be developed in such great amounts that it will cause an allergic reaction the next time you eat that food.

As a rule, you cannot get an allergic reaction the first time you eat a new kind of food, but there are exceptions to any rule.

Examples of food allergens and cross-reacting allergens

In principle you can be allergic to all kinds of food, but reactions to shellfish, milk, fish, soy beans, wheat, eggs, nuts as well as fruit and vegetables are most common.

Birch pollen cross-reacts with hazelnut and other nuts, apple, pear, cherry, walnut, almond, plum, kiwi, potato peel, tomato and carrot – this is called ‘cross hypersensitivity’.

Grass cross-reacts with beans and green peas.

What does it feel like to have food allergy?

The allergic reactions vary a lot and may begin either immediately after the consumption of the food or several hours later. Typical symptoms are:

  • swollen lips, face or throat.
  • a prickly sensation in the mouth and on the lips.
  • vomiting, stomach cramp and diarrhoea.
  • headache.
  • fatigue and irritability.
  • hay fever.
  • asthma.
  • rash, nettle rash and pre-existing infantile eczema may get worse.
  • very rarely, anaphylactic shock, which is a circulatory collapse that needs acute treatment with adrenaline.

Symptoms that are mild one day, can be life threatening the next because of the danger of suffocation or anaphylactic shock. You should never expose yourself to a reaction by eating foods you know you are allergic to, just because you want to avoid putting people to extra trouble or to avoid causing them any embarrassment.

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

What can you do to avoid food allergy?

  • If you are allergic you should look out for the things you are allergic to, and carefully note any cross-reactions.
  • Your allergist – a specialist in allergic diseases – can provide guidance on which foods you can or cannot eat.
  • Be aware of the symptoms and consult your doctor whenever you are in doubt about something.

How does the doctor make the diagnosis?

The depth of the doctor’s examination depends on the severity of the symptoms.

There may be another cause for the symptoms. A lack of ability to digest certain kinds of food may point to coeliac disease or lactose intolerance or non-immunological reactions – histamine in shellfish – and minor poisonings. This is seen among 2 per cent of the population. An inflammatory condition in the intestinal canal (ulcerative colitis and Crohn’s disease) can also cause the symptoms.

If the symptoms are pronounced and the suspicions of food allergy are well founded, a diagnosis by a specialist in allergic diseases will be necessary.

Allergy testing with either a prick-test or a RAST (blood test) may be useful to eliminate allergies, but unfortunately, they often show reactions that have no practical significance. So it is important that all kinds of allergy testing is performed and evaluated by experts.

An allergist will suggest you keep to the following program. Two weeks on a normal varied diet while keeping a detailed diary of everything that is eaten and any reactions there have been. If there have been reactions, the food involved should be avoided over the following two weeks while continuing to keep the diary.

If the symptoms get considerably better, the next step is an ‘open’ provocation of your allergy where both you and your doctor know you are eating the food that may be causing your problems.

If the symptoms return, you will be exposed to a number of double-blinded placebo-controlled provocations where neither the allergist nor you know whether you are eating the suspected food or not. This is the usual procedure to obtain a diagnosis of food allergy.

Possible deterioration

  • The mucous membranes in your mouth and throat may swell and cause difficulty in breathing.
  • Asthma attacks and anaphylactic shock may occur.
  • It gets harder and harder to get a proper nutritious diet.

Possible development

Food allergy is a rare disease that primarily affects 3 per cent of all infants. Most of them grow out of their allergy before they reach the age of three.

Only 1 per cent of the adult population suffers from food allergy. And most of them are able to live a normal life with only a few restrictions in their diet.

How is food allergy treated?

If a thorough examination identifies the food you are allergic to, the most important treatment is of course, to stop eating it. Medical treatment will not cure the disease but will remedy the symptoms. Typically, antihistamines will alleviate the itching but they are not likely to have a preventive effect.

Based on a text by Dr Flemming Andersen




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