On the manner of Fasting:

That one and the same rule of fasting cannot be observed by everybody.

AND so on the manner of fasting a uniform rule cannot easily be observed, because everybody has not the same strength; nor is it like the rest of the virtues, acquired by steadfastness of mind alone. And therefore, because it does not depend only on mental firmness, since it has to do with the possibilities of the body, we have received this explanation concerning it which has been handed down to us, viz.: that there is a difference of time, manner, and quality of the refreshment in proportion to the difference of condition of the body, the age, and sex: but that there is one and the same rule of restraint to everybody as regards continence of mind, and the virtue of the spirit. For it is impossible for every one to prolong his fast for a week, or to postpone taking refreshment during a two or three days' abstinence. By many people also who are worn out with sickness and especially with old age, a fast even up to sunset cannot be endured without suffering. The sickly food of moistened beans does not agree with everybody: nor does a sparing diet of fresh vegetables suit all, nor is a scanty meal of dry bread permitted to all alike. One man does not feel satisfied with two pounds, for another a meal of one pound, or six ounces, is too much; but there is one aim and object of continence in the case of all of these, viz.: that no one may be overburdened beyond the measure of his appetite, by gluttony. For it is not only the quality, but also the quantity of food taken which dulls the keenness of the mind, and when the soul as well as the flesh is surfeited, kindles the baneful and fiery incentive to vice.

THE TWELVE BOOKS OF SAINT JOHN CASSIAN ON THE INSTITUTES OF THE COENOBIA AND THE REMEDIES FOR THE EIGHT PRINCIPAL FAULTS, BOOK V, CHAPTER 5