Food sensitivity

What's a food allergy?

Our bodies have many defence systems to protect us from substances that could harm us. The job of the immune system is to recognise these substances and destroy them, and it does this by producing toxic chemicals and inflammation.

A true food allergy provokes an exaggerated and abnormal immune response, and this is caused by certain proteins in food.

If a protein provokes a specific increase in immunoglobulin E (IgE) in the body, the reaction can be severe, rapid and even life-threatening (see 'Anaphylactic shock' below). Symptoms include swelling of the mouth and throat, and breathing difficulties.

Other allergic reactions that increase immunoglobulins others than IgE tend to develop more slowly and cause symptoms such as eczema, diarrhoea or vomiting.

Peanuts are the most common food to cause severe IgE-mediated reactions. A more typical example of a non-IgE allergic reaction is one caused by cow's milk protein.

What's food intolerance?

Reactions to food that don't cause the immune system to respond are known as food intolerance, and are caused by something in the food other than a protein. The symptoms of a food intolerance can be similar to a food allergy, but also include a whole range of other symptoms such as nausea or rashes. A food intolerance is rarely life-threatening in the way some food allergies can be. The symptoms tend to develop more slowly and require larger amounts of the food to cause the reaction.

Lactose intolerance is a typical example, where people who lack (or have low levels) of the enzyme lactase are unable to digest lactose properly. This results in symptoms such as bloating and diarrhoea.

Who's affected?

Food sensitivity is thought to affect 1.5 to 3.5 per cent of adults and 2 to 8 per cent of children. Up to 90 per cent of food sensitivities are caused by just a few foods. These are:

• Celery
• Cereals containing gluten
• Crustaceans (such as crab and prawns)
• Eggs
• Fish
• Milk
• Molluscs (such as mussels and oysters)
• Mustard
• Nuts
• Peanuts
• Sesame seeds
• Soya
• Sulphur dioxide or sulphites

The most common foods that children react to are milk, peanuts, nuts, eggs, soya and wheat.

Children under the age of three are more likely to develop food allergies, but often these disappear as the child grows up. They're more likely to develop a food sensitivity if there's a family history of allergy.

The number of people who develop food sensitivities seems to be on the increase, but the exact reason for this is unclear. Factors such as genes, exposure to certain foods and the development of the immune system at birth are involved in determining whether someone develops a food sensitivity.

Diagnosing sensitivity

It's important to avoid self-diagnosing food sensitivity. Removing foods from your diet without medical advice and the supervision of a dietitian may mean you reduce your intake of certain nutrients below recommended levels. Always seek medical advice if you suspect yourself or a family member might have a food sensitivity.

A health professional will take a detailed look at the kinds of foods you eat and may ask you to keep a food and symptoms diary. This will help track down and identify which foods you might be sensitive to.

Skin tests can also be used to measure the production of IgE. The size of a wheal (swollen circle) on your skin, caused by an allergy, is used to determine sensitivity. However, skin prick tests aren't useful in diagnosing delayed or non-IgE-mediated food allergy or food intolerance.

There are many commercially available tests that offer a diagnosis of food sensitivity, but using a test alone without consulting a health professional could result in the unnecessary removal of important foods or food groups from your diet.

For most people, a definite diagnosis of food sensitivity is made by manipulating the diet either by

• removing the suspected food or ingredient
• removing a few suspected foods
• including only a few foods for two to three weeks and then gradually reintroducing other foods, one at a time
• using an elemental diet - a special formula in which all the protein is broken down into amino acids (the 'building blocks' of protein)

removing the suspected food(s) should result in an improvement in symptoms after two to three weeks

The last two diets listed above involve the reintroduction of foods one by one every few days while symptoms are closely monitored. These types of diets require a lot of commitment by the individual and support from a health professional, family and friends. The reintroduction process can take months.

Food challenges

In most cases the reintroduction of foods can be carried out at home, but when the reaction is severe (see 'anaphylactic shock' below), food challenges need to be carried out in a specialist facility. These involve giving small amounts of food that previously caused an allergic reaction, while being monitored in a clinical or hospital setting.

Tips for managing food sensitivity

Avoiding foods you're sensitive to requires planning and knowledge of where a particular food or ingredient might be lurking in your diet. Avoiding shellfish, for example, will probably be more straightforward than avoiding nuts or soya, both of which can crop up in some of the most unlikely foods.

At first it can take time to get used to checking labels and becoming aware of foods likely to contain the culprit ingredient, so working together with a dietitian is important.

1 Reading the label

Learning to read food labels is vital. Checking the ingredient list needs to be done carefully, as the names of some ingredients may be listed in a form that isn't necessarily obvious. For example, soya can be called texturised vegetable protein (TVP).

Remember that manufacturers can change their recipes, so always check the label even if you buy the product regularly.

Most food manufacturers and supermarkets provide lists of foods that are free from certain ingredients, and these are often available on their websites.

Here's a list of common food sensitivities and some examples of where they might crop up in food and what they can be called on labels. These lists aren't exhaustive, but demonstrate how tricky food exclusion can be.

What about unpackaged foods?

Foods sold without packaging, for example in restaurants and sandwich bars, don't have to have ingredient labels. You can ask whether a food contains a certain ingredient, but bear in mind that it's possible some cross-contamination might have occurred during preparation. This could happen if the food you're avoiding is stored next to food you're buying, or it was cut with the same knife. If you have a severe allergic reaction to a certain food, it might be worth avoiding these unpackaged foods.

2 Good substitution

It's important to ensure that whatever is removed from your diet doesn't compromise your nutritional intake. If you remove a food that can be easily substituted, such as kiwi fruit, then it's relatively straightforward. But if you remove a major food group such as dairy products or wheat, this puts you at a greater risk of losing out on key nutrients in your diet.

It's important to discuss the implications of any dietary exclusion with a dietitian, who can help you choose substitutes to ensure your diet is nutritionally complete.

3 Planning is vital

A food sensitivity can leave you feeling left out when you visit a restaurant, a friend's home, attend a party or when on holiday. Planning what you can eat when out of your normal routine is vital to ensure enjoyment of food without feeling deprived.

The following advice may help:

• Ahead of your visit, check what's available to eat
• Explain beforehand what you can and can't eat, as it's not a good idea to just turn up and announce your food sensitivity
• Ask if the restaurant or holiday resort can provide alternatives for you
• Take a supply of suitable alternatives just in case

Medical review

It's important for a doctor to review cases of food sensitivity on a regular basis. Some types can change and improve over time, especially in the case of children, who often grow out of sensitivities such as cow's milk protein allergy. Checking will mean you don't keep imposing an unnecessary dietary restriction on yourself.

Anaphylactic shock

Anaphylaxis is a severe allergic reaction. The symptoms affect different areas of the body and include rashes, swelling of lips and throat, difficulty breathing, a fall in blood pressure and eventually loss of consciousness. The reaction usually happens very quickly after eating the culprit food and, if not treated swiftly (usually with an injection of adrenaline), can be fatal. People with severe allergic reactions will usually be given a supply of adrenaline injections to keep with them at all times.

It's important to tell family, friends, work colleagues or teachers at school about any severe allergies, so that everyone is aware of the potential problem.