People who have food intolerance react to chemicals which either occur naturally in food or are added to foods during processing. Different people will tolerate different amounts of chemicals and larger amounts cause stronger symptoms.

The amount of the chemical which causes symptoms is called the ‘dose threshold’. Some people have a high dose threshold to all food chemicals and may never have symptoms after eating foods.

Some people have a low dose threshold to food chemicals and can have unpleasant symptoms after eating foods containing a particular chemical. This type of problem is very similar to the way that some people have side effects to certain medications.

More than one type of chemical may cause symptoms so a person may react to many different types of foods. Some foods contain the same chemicals and a person can react after eating a variety of these. This is because the chemical slowly builds up in the body until the dose threshold is reached. It also explains why the same food does not cause symptoms every time it is eaten.

Food intolerances often run in families, suggesting a genetic predisposition. Sometimes food intolerance only affects a person after a sudden change in diet or after an illness.

Natural chemicals that may cause a reaction include:

• salicylates
• amines
• glutamates

Food additives that may cause a reaction include:

• Preservatives
• Artificial food colours
• Flavour enhancers e.g. MSG

An Accredited Practising Dietitian (APD) uses an elimination diet and challenge procedure to investigate food chemical intolerance and find the chemicals responsible for symptoms. If a chemical found in a food which contains nutrients essential to a healthy diet is causing your symptoms, an APD will also advise you on the best replacement foods.