Slowly reduce your intake of coffee, tea and other caffeine-containing drinks over several days. This will reduce the potential for headache, lethargy and irritability during the fast. Follow the same gradual reduction of high-sugar foods. The body will "crash-land" if excessive caffeine and sugar are stopped overnight. A fast of various fruit and vegetable juices is the nutritionist's recommendation. The traditional, purest form of fast consists of drinking only water. An adult needs three quarts of water daily. We normally get this from a mix of foods and beverages. During a fast, it's crucial to drink at least 12 cups of water a day. When you begin a one-day fast, your body is still getting energy from foods you ate the day before so there will be minimal discomfort. Choose meaningful quiet activities--such as prayer, Bible study, conversations with friends and spiritual correspondence--instead of high-energy ones.