In some cases they are triggered by physical or emotional stress or exposure to environmental toxins.

Food allergy should not be confused with 'food intolerance', of which the best-defined with an immunological component is the gluten intolerance syndrome coeliac disease.

It may be possible, however, to develop intolerances to foods you eat often - you may find you are less able to digest them than you used to be or that you react badly to some of them.