"Do you call this a fast, a day acceptable to the Lord?" —Isaiah 58:5
Fasting is extremely powerful. Through it, the Lord releases those bound unjustly, unties the thongs of the yoke, sets free the oppressed, and breaks every yoke (Is 58:6). Fasting combined with prayer is the only way to be freed from certain demons (Mt 17:21).
Fasting, if not done in the power of the Holy Spirit, is powerful and destructive. It can result in an explosion and a major disruption of one's life (see Lk 5:37; see also 1 Kgs 21:12ff). Fasting done selfishly "ends in quarreling and fighting, striking with wicked claw" (Is 58:4).
Because fasting can make us like Jesus or like Jesus' enemies (e.g. the Pharisees) (see Lk 18:12), fasting must be done carefully. The Church is "the pillar and bulwark of truth" (1 Tm 3:15) and guides us in our fasting. For many centuries, the Church has directed us to imitate Jesus' fasting by fasting each of the forty days of Lent (excluding Sundays and major feast days). The Church recommends that we fast by eating only one full meal on each day of Lent. The Church mandates this on Ash Wednesday and Good Friday, and obliges us to abstain from meat on the Fridays of Lent. However, these obligations are the letter of the law. We must go beyond these and fast zealously and carefully according to the Holy Spirit (2 Cor 3:6).