Struggle with the eight main passions: Fasting
V.1. ... a uniform rule concerning the manner of fasting cannot easily be kept because not all bodies have the same strength, nor is it, like the other virtues, achieved by firmness of mind alone...
2. Nonetheless there is one end of abstinence in all these instances -- that no one, according to the measure of his own capacity, be burdened by voracious satiety. [Or as Thomas Merton wrote: "...they should not eat so much that they feel that their stomach is stuffed: in other words, not to eat to their full capacity."] For it is not only the quality of food but also its quantity that dulls the heart's keenness, and when both the mind and the flesh have been sated the glowing kindling of the wicked vices are set ablaze.
IX. . . . The canonical rule for fasting is useful indeed and by all means to be observed, but unless this is followed by temperate eating habits it will be unable to attain to the goal of integrity.
(St.) John Cassian
The Institutes (trans. Boniface Ramsey, OP)