Although the Bible does not give a direct command on this issue, examples of fasting appear in both the Old and the New Testaments. One of the most telling passages in which fasting is mentioned is Matthew 6:16, where Jesus is teaching His disciples basic principles of godly living. When speaking on fasting, He begins with, “When you fast,” not “If you fast.”
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The Word of God does not specifically command believers to spend time in prayer and fasting. At the same time, prayer and fasting is definitely something we should be doing. Far too often, though, the focus of prayer and fasting is on abstaining from food. Instead, the purpose of Christian fasting should be to take our eyes off the things of this world and focus our thoughts on God. Fasting should always be limited to a set time because not eating for extended periods can be damaging to the body. Fasting is not a method of punishing our bodies and it is not be used as a "dieting method" either. We are not to spend time in prayer and fasting in order to lose weight, but rather to gain a deeper fellowship with God.
Those desiring to receive Holy Communion keep a total fast from all food and drink from midnight the night before (see Eucharistic discipline). The sole exception is the Communion offered at the Easter Sunday midnight liturgy, when all are expressly invited and encouraged to receive the Eucharist, regardless of whether they have kept the prescribed fast.
In a 2011 study published by the American Association for Cancer Research, researchers at Genesis Prevention Center at University Hospital in South Manchester, England tested the effects of three kinds of diets on 115 women. One diet looked like the Fast Diet (five days of normal eating and two days following a calorie-restricted, low-carb diet each week), another restricted carbs two days a week but had no calorie restrictions and a final group followed a calorie-restricted Mediterranean diet for all seven days of the week. After four months, participants following the intermittent low-carbohydrate diets lost an average of 9 pounds, while those on the Mediterranean diet lost an average of 5 pounds.
César Chávez undertook a number of spiritual fasts, including a 25-day fast in 1968 promoting the principle of nonviolence, and a fast of 'thanksgiving and hope' to prepare for pre-arranged civil disobedience by farm workers. Chávez regarded a spiritual fast as "a personal spiritual transformation". Other progressive campaigns have adopted the tactic.