Certain features such as the pattern and type of symptoms can help to distinguish food intolerance reactions from those that might be a result of food allergy or some quite different cause unrelated to food.
The time relationship between eating the food and getting symptoms depends on many factors. If the food is only eaten very occasionally, symptom onset after digestion ranges from almost immediate to several hours. However this is different when the food is being consumed regularly, when each 'reaction' will run in to the next, leading to the development of chronic, almost continual symptoms with no clear immediate reaction each time the food is eaten.
In food allergy the range of symptoms is quite limited and predictable. In food intolerance, a very much wider range of symptoms may occur and multiple symptoms are usual. The conditions listed above have been shown by properly conducted scientific trials to be either caused or made worse by food intolerance. These conditions are shown linked to particular organ systems (gastrointestinal, central nervous, cardiovascular, etc.); however in practice, when food intolerance is involved, such conditions rarely exist alone, the typical patient having symptoms relating to a number of different organ systems. For example, a typical food intolerance person may suffer migraine, unexplained fatigue (central nervous system symptoms) abdominal pain, bloating and frequent diarrhoea (gastrointestinal system symptoms) and unexplained muscle and joint pains (musculoskeletal system symptoms).