If you consume something out of the ordinary, your body is not used to digesting it. For example, the necessary enzymes and bacteria are not likely to be in large supply. Also, you may be hyper- or hypo- sensitive to neurotransmitters (like dopamine) that are released in reponse to the chemicals that are unusually present in the foreign food.

Cut out vegetables from your diet, for example, and you will experience serious intestinal distress when you reintroduce them.