Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter IX. Of the measure of the chastisement to be undertaken, and the remedy of fasting.

Chapter IX.

Of the measure of the chastisement to be undertaken, and the remedy of fasting.

The perfection then of abstinence is not to be gathered from calculations of time alone, nor only from the quality of the food; but beyond everything from the judgment of conscience. For each one should impose such a sparing diet on himself as the battle of his bodily struggle may require. The canonical observance of fasts is indeed valuable and by all means to be kept. But unless this is followed by a temperate partaking of food, one will not be able to arrive at the goal of perfection. For the abstinence of prolonged fasts—where repletion of body follows—produces weariness for a time rather than purity and chastity. Perfection of mind indeed depends upon the abstinence of the belly. He has no lasting purity and chastity, who is not contented always to keep to a well-balanced and temperate diet. Fasting, although severe, yet if unnecessary relaxation follows, is rendered useless, and presently leads to the vice of gluttony. A reasonable supply of food partaken of daily with moderation, is better than a severe and long fast at intervals. Excessive fasting has been known not only to undermine the constancy of the mind, but also to weaken the power of prayers through sheer weariness of body.