Wikipedia says that Diarrhea can result in dehydration.
Some religions demand that one fast on a regular basis. What are the possible consequences of fasting for most of the day if one has diarrhoea?
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Fasting, at least in Islam, means no water intake during the fast. The Islamic purpose of not allowing even water is to remind one that one should not take for granted the gifts of life. So, the purpose of the injunction is to remind you, and not to imperil your health. In the same way that the pillars of Islam have modifiers ( like not going to Mecca if your health is not strong enough ), then it seems clear that one should not fast when one is ill. And in the specific case of diarrhoea, then fluid loss can lead to hypovolaemia and renal failure.