Fasting means reducing one’s intake of food and drink below what is suitable, and doing so for uniquely spiritual reasons. It is an act of penitence and physical mortification for exercise in temperance and the health of souls.
Several caveats are necessary for fasting:
1) it must be a temporary measure;
2) it must be done with prudence, so that it causes some discomfort, but never sickness or weakness;
3) it must not interfere with one’s duties;
4) it must not draw attention to itself.
Mt. 6:16-18 – 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 received their reward. 17 But when you fast, anoint your head and wash your face, 18 that your fasting may not be seen by men but by your Father who is in secret; and your Father who sees in secret will reward you.
The prophets of the Old Testament, especially Isaiah 58, are scornful of fasting if it is not backed up with love of God and care of one’s neighbor. So they anticipate the teaching of Jesus in Matt. 6:16-18.
But many texts teach that fasting is a worthy companion of prayer: Psalms 35:13, 69:11; Joel 1:14, 2:15; Daniel 9:13. Especially does fasting seem appropriate in seasons of penance, like Lent, or when one wants to feel greater sorrow for sin and greater delicacy of conscience about one’s shortcomings and one’s need of God.
In all things, Jesus Christ is our model. He fasted 40 days and 40 nights with prayer before taking up His work teaching and miracles that was to lead Him to the Cross. When the Christian is faced with important decisions, he follows Christ when he prays and fasts to obtain God’s light and guidance for the next steps in his life.
1 Tim. 4:4-5 – For everything created by God is good, and nothing is to be rejected if it is received with thanksgiving.
Dan. 10:2-3 – In those days I, Daniel, was mourning for three weeks. 3 I ate no delicacies, no meat or wine entered my mouth, nor did I anoint myself at all, for the full three weeks.
Mt. 9:15 – And Jesus said to them, “Can the wedding guests mourn as long as the bridegroom is with them? The days will come, when the bridegroom is taken away from them, and then they will fast.