John Cassian, Conferences I–X II.22: What is the usual limit both of abstinence and of partaking food

But this is the usual limit of abstinence; viz., for everyone to allow himself food according to the requirements of his strength or bodily frame or age, in such quantity as is required for the support of the flesh, and not for the satisfactory feeling of repletion. For on both sides a man will suffer the greatest injury, if having no fixed rule at one time he pinches his stomach with meagre food and fasts, and at another stuffs it by over-eating himself; for as the mind which is enfeebled for lack of food loses vigour in praying, while it is worn out with excessive weakness of the flesh and forced to doze, so again when weighed down with over-eating it cannot pour forth to God pure and free prayers: nor will it succeed in preserving uninterruptedly the purity of its chastity, while even on those days on which it seems to chastise the flesh with severer abstinence, it feeds the fire of carnal desire with the fuel of the food that it has already taken.