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Water fasting is a type of fasting in which the practitioner consumes only water. One may water fast for a variety of reasons, including medical and religious requirements. Such fasting for prolonged periods poses significant health risks if done improperly.
Some engage in water fasting as a detox diet. While there is no scientific evidence that any detoxification occurs because of water fasting, there is evidence of harmful side effects from such diets. 
Jains maintain a strict water-only fast for 8 to 10 (digambar) days, during the days of Paryushan. The warm water consumed should be only between sunrise and sunset and not during the night, since night is a highly-susceptible time for micro-organism activity. For Jains fasting is a way of penance.
Roman Catholics must engage in the eucharistic fast, which is a water fast before receiving the Eucharist during the Mass. While no nutritional or caloric sustenance is permitted, practitioners may take medicine if required, and those whose health problems impede them from taking part in the fast are dispensed of the obligation.
Up until the reforms of the Second Vatican Council, this fast was required from the previous midnight, as it is in various Orthodox Churches. However, under Pope Paul VI, the obligatory fast was reduced to only one hour before receiving the Eucharist.
The Catholic Church has also promoted a Black Fast, in which in addition to water, bread is consumed. Typically, this form of fasting was only used by monks and other religious individuals who practice mortifications and asceticism, but all Catholics are invited to take part in it with the advice and consent of their spiritual director.