According to Dr. Joel Fuhrman from 'Fasting and Eating for Health" : "Due to the body's built-in survival mechanisms, the amount of water needed while fasting is minimal. The desire for fluid diminishes and may be nil in some fasters.To minimize side effects and assure the safety of all patients, however, fasters need to be encouraged to drink water to prevent dehydration. One quart of water a day is usually sufficient for most individuals, but some need to be encouraged to drink two or more quarts, depending on their laboratory parameters. Dehydration does not generally occur in the overweight patient. Rather, it is seen more frequently in the thin patient. If considerable dehydration ensues, the patient should be fed."