Jesus Himself fasted and taught His disciples to fast.
And when you fast, do not look dismal like the hypocrites, for they disfigure their faces that their fasting may be seen by men. Truly I say to you, they have their reward. But when you fast, anoint your head and wash your face, that your fasting may not be seen by men, but your Father who is in secret; and your Father who sees in secret will reward you. (Matthew 6:16-18)
The purpose of fasting is to gain mastery over oneself and to conquer the passions of the flesh. It is to liberate oneself from dependence on the things of this world in order to concentrate on the things of the Kingdom of God. It is to give power to the soul so that it would not yield to temptation and sin. According to St. Seraphim, fasting is an "indispensable means" of gaining the fruit of the Holy Spirit in one's life (cf. Conversation with Motovilov), and Jesus Himself taught that some forms of evil cannot be conquered without it (Matthew 17:21, Mark 9:29)
Man does not fast because it pleases God if His servants do not eat, for, as the lenten hymns of the Church remind us, "the devil also never eats." (Lenten Triodion) Neither do men fast in order to afflict themselves with suffering and pain, for God has no pleasure in the discomfort of His people. Neither do men fast with the idea that their hunger and thirst can somehow serve as a "reparation" for their sins. Such an understanding is never given in the scriptures or the writings of the saints which claim that there is no "reparation" for man's sin but the crucifixion of Christ. Salvation is a "free gift of God" which no "works" of man can accomplish of merit. (cf. Romans 5:15-17, Ephesians 2:8-9)
Men fast, therefore, and must fast, only to be delivered from carnal passions so that the free gift of salvation in Christ might produce great fruit in their lives. Men fast so that they might more effectively serve God who loves them and has saved them in Christ and the Spirit. Fasting without effort in virtue is wholly in vain.
Why have we fasted, and Thou seest it not? Why have we humbled ourselves, and Thou takest no knowledge of it?
Behold, in the day of your fast, you seek your own pleasure and oppress all your workers. Behold, you fast only to quarrel and fight...Fasting like yours ... will not make your voice to be heard on high.
Is not this the fast that I choose: to loose the bonds of wickedness...to let the oppressed go free…is it not to share your bread with the hungry, and bring the homeless poor into your house; when you see the naked, to cover them...
Then shall your light break forth like the dawn, and your healing shall spring up speedily; your righteousness shall go before you, the glory of the Lord shall protect you. Then you shall call, and the Lord will answer; then you shall cry, and He will say: Here I am. (Isaiah 58:3-9)
"Fasting in the body, 0 brethren, let us also fast from sin." This is the Church's song in the lenten season of fasting. It is also the teaching of the saints.
...in fasting one must not only obey the rule against gluttony in regard to food, but refrain from every sin so that, while fasting, the tongue may also fast, refraining from slander, lies, evil talking, degrading one's brother, anger and every sin committed by the tongue. One should also fast with the eyes, that is, not look at vain things...not look shamefully or fearlessly at anyone. The hands and feet should also be kept from every evil action.
When one fasts through vanity or thinking that he is achieving something especially virtuous, he fasts foolishly and soon begins to criticize others and to consider himself something great.
A man who fasts wisely...wins purity and comes to humility...and proves himself a skillful builder. (St. Abba Dorotheus, 7th c., Directions on Spiritual Training)
Saint Paul himself fasted, and in his teaching on food insists that men fast and do so in secret, without mutual inspection and judgment.
Brethren, join in imitating me, and mark those who so live as you have an example in us. For many of whom I have often told you and now tell you with tears, live as enemies of the cross of Christ. Their end is destruction, their god is their belly, and they glory in their shame, with minds set on earthly things. (Philippians 3:17-19)
All things are lawful for me, but not all things are helpful. All things are lawful for me, but I will not be enslaved by anything. Food is meant for the stomach, and the stomach for food - and God will destroy both one and the other. The body is not meant for immorality but for the Lord, and the Lord for the body. (I Corinthians 6:12-13)
Let not him who eats despise him who abstains, and let not him who abstains, pass judgment on him who eats, for God has welcomed him. Who are you to pass judgment on the servant of another?
He who observes the day, observes it in honor of the Lord. He also who eats, eats in honor of the Lord, since he gives thanks to God; while he who abstains, abstains in honor of the Lord and gives thanks to God.
Do not let what you eat cause the ruin of him for whom Christ has died ... for the Kingdom of God does not mean food and drink, but righteousness and peace and joy in the Holy Spirit, he who thus serves Christ is acceptable to God and approved by men.
Do not for the sake of food destroy the work of God…the faith that you have keep between yourself and God...whatever does not proceed from faith (whether eating or abstaining) is sin. (cf Romans 14)
The spiritual fathers, as strictly ascetic as they were, are very clear in their teaching about fasting. They insist with the Lord and the scriptures that men must fast in order to be free from passions and lust. But they insist as well that the most critical thing is to be free from all sin, including the pride, vanity and hypocrisy which comes through foolish and sinful fasting.
...eating beyond the point of being satisfied is the door of madness through which lust enters, for the belly is the queen of passions which man serves as a slave.
But you, firm in this knowledge, choose what is best for you, according to your own powers...for the perfect person, according to Saint Paul ought both "to be full and be hungry...and do all things through Christ who strengthens (Philippians 4:12-13)
Thus a man who strives for salvation...must not allow himself to eat to fullness...but should still eat all kinds of food so that on the one hand he avoid boastful pride and on the other not show disdain for God's creation which is most excellent...Such is the reasoning of those who are wise! (St. Gregory of Sinai, Instruction to Hesychasts)
St. Isaac of Syria says, "Meager food at the table of the pure cleanses the soul of those who partake from all passion...for the work of fasting and vigil is the beginning of every effort against sin and lust...almost all passionate drives decrease through fasting."
For the holy fathers taught us to be killers of passions and not killers of the body. Partake of everything that is permissible with thanksgiving, to the glory of God and to avoid boastful arrogance; but refrain from every excess. (The Monks Callistus and Ignatius, 14th c., Directions to Hesychasts)
If such is the teaching to hesychast monks, it is certainly applicable to all Christians as well. The whole essence of the matter is put simply and clearly in these two short stories from the fathers of the desert.
A certain brother brought fresh loaves of bread and invited his elders. When they had eaten much, the brother, knowing their travail of abstinence, began humbly to beg them to eat more. "For God's sake, eat this day and be filled." And they ate another ten. Behold how these that were true monks and sincere in abstinence did eat more than they needed, for the sake of God.
Epiphanius, bishop of Cyprus, called the abbot Hilarion to see him. A portion of fowl was set be- fore them and the bishop invited the abbot to eat. The old man said, "Forgive me, Father, but since the time I took this habit I have never eaten anything that has been killed."
And Epiphanius said to him, "And from the time I took this habit I have let no man sleep who has anything against me, and neither have I slept holding anything against anyone."
And the old man said to him, "Forgive me, Father, for your way of life is greater than mine." (The Sayings of the Fathers)