What is a food allergy?

A true food allergy can cause severe symptoms, and may have the potential to cause a life-threatening extreme allergic reaction (anaphylaxis). However, many people who have an unpleasant reaction to something they eat or drink, are more likely to have what's called a 'food intolerance' rather than a food allergy. Proven food allergies affect approximately 6% of children under three years old, but only 1.5–2% of adults.

Immune system reacts in food allergy

A food allergy involves a reaction of your immune system, whereas a food intolerance does not. If you have a food allergy your immune system makes antibodies against a particular (normally harmless) food. It's as if your immune system mistakes that type of food for a harmful foreign invader.

This means that when you eat the particular food again, the pre-formed antibodies swing into action and trigger a cascade of reactions in your tissues that result in some or all of the typical symptoms of food allergy:

• mouth itching and swelling
• rash
• hives
• runny nose
• vomiting
• abdominal pain
• diarrhoea
• swelling of the throat and tongue
• sometimes difficulty breathing.

These symptoms usually occur a few minutes to two hours after eating the food.

Anaphylaxis: a medical emergency

Food allergy symptoms are often severe, and sometimes life threatening. A severe, sudden, life-threatening reaction is called anaphylaxis. Anaphylaxis is a medical emergency that needs immediate treatment.

Symptoms of anaphylaxis include:

• swelling of the lips and tongue
• difficulty breathing
• widespread rash
• vomiting
• wheeze
• sudden drop in blood pressure
• collapse.

Common food allergies

The foods most commonly associated with food allergy are:

• milk
• eggs
• nuts, especially peanuts
• grains, such as wheat, oats and rye
• soy
• fish and shellfish
• sesame
• tomatoes, berries and other fruit.

The allergic reaction is usually to a protein within the food.

Most children who have an allergy to soy, wheat, milk or eggs grow out of this by the time they start school, whereas an allergy to seafood or nuts is likely to be severe and last for life.

See also: Children with allergies - tips for starting school

Finding out if you have a food allergy

See your doctor for diagnosis of food allergy. Combined with a description of your symptoms, your doctor may suggest blood tests or skin prick tests to confirm a food allergy. You may be referred to an allergy specialist for these tests. It is important to know exactly what food is causing your allergy so you can avoid it in the future.

Treatment of food allergy

The treatment for food allergy is to completely avoid the food that causes your allergy. People who have a severe allergy should be careful to avoid even touching the food or eating trace amounts of it, which may be present in processed foods. A dietitian can show you how to recognise the relevant ingredient on food labels. See also: Anaphylaxis - avoiding food allergens

If you are thought to be at risk of a severe allergic reaction to a food, your doctor or specialist may advise you to wear an identifying bracelet (eg, MedicAlert) and possibly to avoid certain medicines. He or she should also give you a written ‘Anaphylaxis Action Plan’ telling you what to do in the event of a severe allergic reaction.

Adrenaline self-injection devices

As part of your Anaphylaxis Action Plan, your doctor or specialist may advise that you carry with you at all times an adrenaline self-injection device (eg, EpiPen, Anapen). The medicine in the device (adrenaline) helps treat severe allergic reactions by shrinking blood vessels that have become too expanded, making your heart beat more strongly and helping you breathe.

Your doctor or specialist should explain when and how to use it (an action plan also gives guidance). You can practise with a training device that has no needle or medicine so you know what to do with the real device if needed. Children may be prescribed either a regular or reduced-dose device, depending on the severity of their allergic reactions and their body weight. Check the device regularly and keep an eye on the expiry date.

What is food intolerance?

Food intolerance is an abnormal response to a food (often to a ‘food chemical’ or additive), resulting in symptoms of illness. Food intolerance is not an immune system reaction and does not generally cause life-threatening symptoms. The suspect food chemical may occur naturally in the food or it may be added to foods to enhance their flavour or preserve them.

A food intolerance can mean you have symptoms to a range of foods because the suspect chemical may be present in many different foods. In contrast, a person with a food allergy will often react to only one or two foods.

More common than food allergy

Food intolerance is a more common condition than food allergy and may cause diarrhoea, nausea, cramping or headache soon after eating the food, hours later, or even days after eating the food. The severity of symptoms can vary because the suspect food chemical can accumulate in the body, depending on how much of the suspect food (or foods that contain the suspect chemical) you have eaten. Eating a small amount of the suspect food may not cause symptoms, but eating a lot of it can.

Sometimes a food intolerance occurs when your body is not able to process a food component. A good example of this is lactose intolerance where the person lacks the enzyme necessary to break down the milk sugar (lactose) for proper digestion.

Food intolerance won't show up in the blood tests or skin prick tests used to diagnose food allergy. The symptoms of food intolerance are often similar to the symptoms of food allergy and many other conditions.

Dietitian advice

To find out if you have a food intolerance, your doctor may refer you to a dietitian, who will supervise eliminating and reintroducing various foods in your diet, one at a time, to check the effect on your symptoms. You should not carry out an elimination diet without supervision by your doctor or specialist, or a dietitian.

The treatment for a food intolerance is usually to avoid eating large amounts of foods that contain the suspect food chemical, so you don't get an accumulation of the chemical in the body and can prevent symptoms from appearing. A dietitian can help you devise an eating plan that helps you achieve this, yet still allows you to enjoy a balanced diet that includes all the essential nutrients.

Products for people with a diagnosed food allergy

The New Zealand Manufactured Food Database produces commercial food lists (lists of "safe food") of New Zealand brand name foods, for people with specific food allergies. The NZ MFD can be visited on www.mfd.co.nz.

Original material provided by myDr, 2007. Edited by everybody, February 2012.