Fasting is one of the principle means of making satisfaction for sins. By abstaining from eating all that is available to eat, one can do penance for sins, making reparation for them. Also one can gain strength against future sins by this self-imposed penance. Fasting is a way to chastise the body for the sins the body has committed. Additionally denying oneself the pleasure of eating helps to bring the body into control and subject to the soul.
St. John of the Cross tells us to keep in mind the value of our good works, fasts, alms and penances. Firstly the value of these is “not based on quantity and quality so much as on the love of God practiced in them”. (Ascent Book 3 Chap. 27) So our good works, fasts, almsgiving and penances should be done for the love of God, and we “should not set (our) heart on the pleasure, comfort, savor and other elements of self-interest (like trying to lose weight) these good works and practices usually entail, but recollect (our) joy in God and desire to serve him through these means.”
As always regarding fasting we have Christ for our example. He is perfect, so we do not see any extremes to the virtue he models for us. He did practice fasting and abstinence, though he did not need to do this. He had perfect control over his desires and the appetites. In gaining control over the sense appetites prudent discretion is in order. We are not to “kill (ourselves) with penances” and “weaken (ourselves) by fasts”. (Dark Night, Book 1, Chap 6) The idea is virtue, which is the mean between extremes. We should be doing our fasting with the proper motivation which is to repair the damage done by our sinfulness and to gain strength for future temptations to sin.
Fasting is also good because the mind is dulled when the body overeats. When one indulges in too much, the mind becomes sleepy and unable to meditate or pray with attentiveness.
The Church has us fast on Ash Wednesday and Good Friday. These are the days Catholics usually associate with fasting. Fasting during these penitential days of Lent is defined as one normal meal and two smaller meals that together do not equal a full meal. Snacking in between meals would also be excluded as part of the fast. The Church additionally ask us to fast before receiving Holy Communion. We are to fast from food and drink, with the exception of water, for one hour before receiving the Eucharist. It is good to be reminded as to why we are obliged to do this. We do the Eucharistic fast out of respect for the sacrament. We are about to received Jesus, and therefore it is fitting that we should not eat or drink other substances prior to receiving the Lord’s Body and Blood.