Nor let us suppose that the outward fast from visible food will suffice to secure perfection of heart and purity of body, unless it have conjoined with it a fast of the soul as well. For the soul too has kinds of diet that do it harm, sated werewith–even without indulgence in material food–it can run down the steep places of licentiousness.
Slander is a food of the soul, and very sweet to its taste. And anger, too, though no mild one, feeding the soul for a moment with evil meat, and laying it low at the same time with its deadly savor. Envy is a food of the mind, corrupting it with poisonous juices, and ceasing not to torment it miserably with the thought of a neighbor’s happy success.
…vainglory can be its food, soothing it for a time with sweet meats, then leaving it empty, bereft of all virtues and bare, abandoning it barren of all spiritual fruits and void…all unruly desires and wanderings of an unquiet heart can be a kind of food for the soul which feeds it on poisoned meats, and then abandons it, never more to taste the Bread of Heaven or true nutriment.
~St John Cassian
Prayerful Life 