From: Luke 5:33-39
A Discussion on Fasting
 And they (the scribes and the Pharisees) said to Him (Jesus), "The disciples of John fast often and offer prayers, and so do the disciples of the Pharisees, but yours eat and drink."  And Jesus said to them, "Can you make the wedding guests fast while the bridegroom is with them?  The days will come, when the bridegroom is taken away from them, and then they will fast in those days."  He told them a parable also: "No one tears a piece from a new garment and puts it upon an old garment; if he does, he will tear the new, and the piece from the new will not match the old.  And no one puts new wine into old wineskins; if he does, the new wine will burst the skins and it will be spilled, and the skins will be destroyed.  But new wine must be put into fresh wineskins.  And no one after drinking old wine desires new; for he says, `The old is good.'"
33-35. In the Old Testament God established certain days as days of fasting--the main one being the "day of atonement" (Numbers 29:7; Acts 27:9). Fasting implied total or partial abstinence from food or drink. Moses and Elijah fasted (Exodus 34:28; 1 Kings 19:8) and our Lord Himself fasted in the desert for forty days before beginning His public ministry. In the present passage Jesus gives a deeper meaning to the word "fasting"--the deprivation of His physical presence which His Apostles would experience after His death. All through His public life Jesus is trying to prepare His disciples for the final parting. At first the Apostles were not very robust and Christ's physical presence did them more good than the practice of fasting.
Christians should sometimes abstain from food. "Fast and abstain from flesh meat when Holy Mother Church so ordains" ("St. Pius X Catechism", 495). That is the purpose of the fourth commandment of the Church, but it has a deeper meaning, as St. Leo the Great tells us: "The merit of our fasts does not consist only in abstinence from food; there is no use depriving the body of nourishment if the soul does not cut itself off from iniquity and if the tongue does not cease to speak evil" ("Sermo IV in Quadragesima").
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.