11 I do not, however, lay on you as an obligation any extreme fasting or abnormal abstinence from food. Such practices soon break down weak constitutions and cause bodily sickness before they lay the foundations of a holy life. It is a maxim of the philosophers that virtues are means, and that all extremes are of the nature of vice; and it is in this sense that one of the seven wise men propounds the famous saw quoted in the comedy, “In nothing too much.” You must not go on fasting until your heart begins to throb and your breath to fail and you have to be supported or carried by others.
No; while curbing the desires of the flesh, you must keep sufficient strength to read scripture, to sing psalms, and to observe vigils. For fasting is not a complete virtue in itself but only a foundation on which other virtues may be built. The same may be said of sanctification and of that chastity without which no man shall see the Lord. Each of these is a step on the upward way, yet none of them by itself will avail to win the virgin's crown. The gospel teaches us this in the parable of the wise and foolish virgins; the former of whom enter into the bridechamber of the bridegroom, while the latter are shut out from it because not having the oil of good works they allow their lamps to fail. This subject of fasting opens up a wide field in which I have often wandered myself, and many writers have devoted treatises to the subject.
I must refer you to these if you wish to learn the advantages of self-restraint and on the other hand the evils of over-feeding.
Source: Letters (New Advent)