I shall speak first about control of the stomach, the opposite to gluttony, and about how to fast and what and how much to eat. I shall say nothing on my own account, but only what I have received from the Holy Fathers. They have not given us only a single rule for fasting or a single standard and measure for eating, because not everyone has the same strength; age, illness or delicacy of body create differences. But they have given us all a single goal: to avoid over-eating and the filling of our bellies. They also found a day's fast to be more beneficial and a greater help toward purity than one extending over a period of three, four, or even seven days. Someone who fasts for too long, they say, often ends up by eating too much food. The result is that at times the body becomes enervated through undue lack of food and sluggish over its spiritual exercises, while at other times, weighed down by the mass of food it has eaten, it makes the soul listless and slack.
They also found that the eating of greens or pulse did not agree with everyone, and that not everyone could live on dry bread. One man, they said, could eat two pounds of dry bread and still be hungry, while another might eat a pound, or only six ounces, and be satisfied. As I said, the Fathers have handed down a single basic rule of self-control: 'do not be deceived by the filling of the belly' (Prov. 24:15. LXX), or be led astray by the pleasure of the palate. It is not only the variety of foodstuffs that kindles the fiery darts of unchastity, but also their quantity. Whatever the kind of food with which it is filled, the belly engenders the seed of profligacy. It is not only too much wine that besots our mind: too much water or too much of anything makes it drowsy and stupefied. The Sodomites were destroyed not because of too much wine or too much of other foods, but because of a surfeit of bread, as the Prophet tells us (cf. Ezek. 16:49).
Bodily illness is not an obstacle to purity of heart, provided we give the body what its illness requires, not what gratifies our desire for pleasure. Food is to be taken in so far as it supports our life, but not to the extent of enslaving us to the impulses of desire. To eat moderately and reasonably is to keep the body in health, not to deprive it of holiness.
A clear rule for self-control handed down by the Fathers is this: stop eating while still hungry and do not continue until you are satisfied. When the Apostle said, 'Make no provision to fulfill the desires of the flesh' (Rom. 13:14), he was not forbidding us to provide for the needs of life; he was warning us against self-indulgence. Moreover, by itself abstinence from food does not contribute to perfect purity of soul unless the other virtues are active as well. Humility, for example, practiced through obedience in our work and through bodily hardship, is a great help. If we avoid avarice not only by having no money, but also by not wanting to have any, this leads us towards purity of soul. Freedom from anger, from dejection, self-esteem and pride also contributes to purity of soul in general, while self-control and fasting are especially important for bringing about that specific purity of soul which comes through restraint and moderation. No one whose stomach is full can fight mentally against the demon of unchastity. Our initial struggle therefore must be to gain control of our stomach and to bring our body into subjection not only through fasting but also through vigils, labors and spiritual, reading, and through concentrating our heart on fear of Gehenna and on longing for the kingdom of heaven.
Taken from "The Philokalia"Vol 1 On the Eight Vices, Pages 73-74