What is fasting, and should Christians do it?
Fasting is the foregoing of a meal or consumption of food for a predetermined period of time. A fast may be warranted before blood tests or a medical procedure. Sometimes, very obese people need to go on a liquid diet, which would considered a fast as well.
A religious fast is something else, entirely. In a religious fast, a Christian will sacrifice his natural desire for food in order to spend time seeking God regarding a specific issue. The intention is that all the time that would be spent acquiring, preparing and consuming food will, instead, be spent in the presence of God through prayer, worship or meditation on the Word.
Also, the heightened sense of awareness obtained through hunger serves as a reminder of the purpose of the fast. The goal is to replace the feeding of the needs of physical hunger with the feeding of one's spiritual hunger by the means of intimacy with God.
Jesus fasted from all food and drink, except water (Luke 4:2). The Bible also describes some extreme times that people fasted from everything, including water. See Exodus 34:28, Deuteronomy 9:9, Ezra 10:6, Esther 4:16 and Acts 9:9. This type of fast is usually reserved for extreme circumstances.
There are also "partial fasts" that restrict only certain foods or meals. A person may choose to fast through breakfast to allow extra time for prayer and worship. While biblical fasts do refer to abstaining from food, I have heard, many times, of people "fasting" from the television. I am sure that if there had been televisions in Bible days, Jesus would have called everyone to "fast" from them!
Please note, however, that a fast, whether partial or absolute, is quite different than giving up something for "Lent." The motive in a fast cannot be that God will find favor with you because you gave up your favorite candy bar. The motive must be the realization that the world has crowded in around you and you need some clarity and some deep intimacy with God. It all comes down to the intent of your heart.