The Institutes, Book 5
What a Monk Should Eat
For our food, then, we should choose not only what moderates the heat of burning lust and avoids kindling it, but what is easily prepared, what recommends itself by being cheap, and what suits the life of the brothers and their common use. For gluttony takes three forms. First, there is the kind that drives us to get ahead of the proper hour for a meal. Next, there is the kind that delights in stuffing the stomach and gorging on food of every sort. Third, there is the kind that takes pleasure in more refined and delicate feasting. So a monk should keep a threefold watch against it. First, he should wait for the proper time to break the fast. Second, he should not give way to gorging. Third, he should be content with any of the commoner kinds of food.
For anything taken beyond what is customary and in common use among all is branded by the ancient tradition of the fathers as stained with the sin of vanity, boasting and display. Among those we have seen to be rightly eminent for learning and discernment, or whom the grace of Christ has singled out as shining lights for everyone to imitate, we have known none who abstained from eating bread, which among them is counted cheap and easy to get. Nor have we seen that anyone who rejected this rule, gave up eating bread and took to a diet of beans or vegetables or fruit, was counted among the most respected, or even gained the grace of knowledge and discernment. For they not only lay it down that a monk ought not to ask for foods that others are not used to, so that his way of life will not be put on public display before everyone, made empty and idle, and so destroyed by the disease of vanity. They also insist that the common, chastening discipline of fasting ought not to be lightly revealed to anyone, but hidden and kept secret as far as possible. But when any of the brothers arrive, they rule that we ought to show the virtues of kindness and love rather than keep a severe abstinence and our strict daily rule. Nor should we consider what our own wishes and benefit or the fervor of our desires may require, but set before ourselves, and gladly carry out, whatever the refreshment of the guest or his weakness may ask of us.