Understanding the difference between intolerance and other types of food reaction is an important starting point because the approach to dealing with them is quite different. Unlike allergies and coeliac disease, which are immune reactions to food proteins, intolerances don’t involve the immune system at all. They are triggered by food chemicals which cause reactions by irritating nerve endings in different parts of the body, rather in the way that certain drugs can cause side-effects in sensitive people.
The chemicals involved in food intolerances are found in many different foods, so the approach involves identifying them and reducing your intake of groups of foods, all of which contain the same offending substances. By contrast, protein allergens are unique to each food (for example, egg, milk and peanut), and dealing with a food allergy involves identifying and avoiding all traces of that particular food. Similarly, gluten, the protein involved in coeliac disease, is only found in certain grains (wheat, barley, rye) and their elimination is the basis of a gluten-free diet.