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Fasting in the form of abstaining from all food and drink may not be practical for the elderly, those who are diabetic or ill. For some, refraining from food and drink is possible without any danger to their health, but if they choose to do so for more than a day, then eating once in the evening allows the fast to continue the next day. For someone unable to fast, but who can surrender some part of their diet–abstaining from all sweets, for example–it can serve the purpose. Underlying the idea of the fast are two things. First, submission to God. Second, aiding the poor. (Isa. 58: 6.) You can accomplish those purposes even if the "fast" you choose has nothing to do with food. However, our appetite for food is one of the most direct ways to discipline the will of the body. Remember though, it is your thoughts, not your belly, where the real battle is fought.