St. John Chrysostom (d. 407AD) says about Lenten fasting:
“Let our every care be for the salvation of souls, and for ways of curbing the motions of the flesh and demonstrating a real fast. Abstinence from food after all, is undertaken for this purpose, to curb the exuberance of the flesh and bring the beast under control.
The person fasting ought most of all keep anger in check, learn the lesson of mildness and kindness, have a contrite heart, banish the flood of unworthy passions, keep before one’s eyes that unsleeping eye and that incorruptible tribunal, avoid becoming enthralled by money, be lavish in almsgiving, drive all ill-will to one’s neighbor from the soul. This is real fasting, as Isaiah says when speaking as God’s mouthpiece:
‘I did not choose this fast, says the Lord – not to bend your neck like a dog collar, nor to make your bed of dust and ashes, not to call a fast of this kind acceptable, says the Lord.’ So what kind, pray? ‘Loose the bonds of crippling contracts,’ he says, ‘share your bread with the hungry, welcome the homeless poor into your home.’ And if you do these things, he says, ‘then your Light will burst forth like the dawn, and your healing with quickly emerge.’
Do you see, dearly beloved, what true fasting really is? Let us perform this kind, and not entertain the facile notion held by many that the essence of fasting lies in going without food till evening. This is not the end in view, but that we should demonstrate, along with abstinence from food, abstinence from whatever is harmful, and should give close attention to spiritual duties. The person fasting ought to be reserved, peaceful, meek, humble, indifferent to the esteem of this world. You see, just as one has neglected the soul, so it is necessary to neglect empty esteem as well, and to have regard only for him who examines our inmost being, and with great care to direct prayers and confessions to God, and provide for oneself according to one’s ability the help that comes from almsgiving.”
(Daily Readings from the Writings of St. John Chrysostom, edited by Anthony M. Coniaris, pp 55-56)