This is the second, and final, new chapter for version 2.3.0. It came out a bit more food-oriented that originally intended, mostly because that's what the best source material was harping on. But it really applies to anything we might lust after or dread. Taking the edge off of emotions like these through the use of reason is a key aim of Stoicism.
The man who eats more than he ought does wrong, and the man who eats in undue haste no less, and also the man who wallows in the pickles and sauces, and the man who prefers the sweeter foods to the more healthful ones, and the man who does not serve food of the same kind or amount to his guests as to himself. (Musonius Rufus - On Food)
To the ancient Stoics, good health was nothing more than an "indifferent", albeit a preferred one. Being healthy was certainly better than being sickly, but was not a virtue in and of itself. The quest for six-pack abs and buns-of-steel was nothing but vanity, and did nothing for true fulfillment.
Mastering one's appetite is the very foundation of training in self-control. (Musonius Rufus - On Food)
However, temperance was unquestionably a virtue, and it's opposite, gluttony, a vice. Those two qualities were instrumental in determining the manner in which an individual reacted to his impressions. A glutton would accept all impressions on presentation, without pausing to impose reason upon his reaction. If a slice of bacon appeared delicious, it would be eaten. If a portion lima beans looked bland, it would be ignored.
How shameful it is to behave toward food in this way we may learn from the fact that we liken them to unreasoning animals rather than to intelligent human beings. (Musonius Rufus - On Food)
It is the reasoning faculty that sets us above the animals, and when we set it aside, its lack that makes us no better. Our reason allows us to analyze our initial impressions, and then assent or reject them. Because of our reason, we can objectively view the things we might initially desire, and decide whether taking them would actually be in our best interests. And we are at our best when we prevent our desires and aversions from overpowering our good sense.
It follows, then, that mastery of one's appetites is an essential step along the path toward mastery of one's entire life. If one is unable to cease from overeating, how can he learn to hold his tongue? If another will not eat her vegetables, will she be able to perform her duty?
And, so, daily we must prepare to battle our appetites. And some days, we will lose. No matter - the battle itself makes us stronger, so long as we never quit trying. Self-mastery is not a state one achieves. It is a skill one hones.