John Cassian said this quote

“ Let us not believe that an external fast from visible food alone can possibly be sufficient for perfection of heart and purity of body unless with it there has also been united a fast of the soul. For the soul also has its foods that are harmful. Slander is its food and indeed one that is very dear to it. A burst of anger also supplies it with miserable food for an hour and destroys it as well with its deadly savor. Envy is food of the mind, corrupting it with its poisonous juices and never ceasing to make it wretched and miserable at the prosperity and success of another. Vanity is its food which gratifies the mind with a delicious meal for a time but afterward strips it clear and bare of all virtue. Then vanity dismisses it barren and void of all spiritual fruit. All lust and shift wanderings of heart are a sort of food for the soul, nourishing it on harmful meats but leaving it afterwards without a share of its heavenly bread and really solid food. If then, with all the powers we have, we abstain from these in a most holy fast our observance of the bodily fast will be both useful and profitable. ”