Objection 1. It would seem that gluttony is not a sin. For our Lord said (Matthew 15:11): "Not that which goeth into the mouth defileth a man." Now gluttony regards food which goes into a man. Therefore, since every sin defiles a man, it seems that gluttony is not a sin.
Objection 2. Further, "No man sins in what he cannot avoid" [Ep. lxxi, ad Lucin.]. Now gluttony is immoderation in food; and man cannot avoid this, for Gregory says (Moral. xxx, 18): "Since in eating pleasure and necessity go together, we fail to discern between the call of necessity and the seduction of pleasure," and Augustine says (Confess. x, 31): "Who is it, Lord, that does not eat a little more than necessary?" Therefore gluttony is not a sin.
Objection 3. Further, in every kind of sin the first movement is a sin. But the first movement in taking food is not a sin, else hunger and thirst would be sinful. Therefore gluttony is not a sin.
On the contrary, Gregory says (Moral. xxx, 18) that "unless we first tame the enemy dwelling within us, namely our gluttonous appetite, we have not even stood up to engage in the spiritual combat." But man's inward enemy is sin. Therefore gluttony is a sin.
I answer that, Gluttony denotes, not any desire of eating and drinking, but an inordinate desire. Now desire is said to be inordinate through leaving the order of reason, wherein the good of moral virtue consists: and a thing is said to be a sin through being contrary to virtue. Wherefore it is evident that gluttony is a sin.
Reply to Objection 1. That which goes into man by way of food, by reason of its substance and nature, does not defile a man spiritually. But the Jews, against whom our Lord is speaking, and the Manichees deemed certain foods to make a man unclean, not on account of their signification, but by reason of their nature [Cf. I-II, 102, 6, ad 1]. It is the inordinate desire of food that defiles a man spiritually.
Reply to Objection 2. As stated above, the vice of gluttony does not regard the substance of food, but in the desire thereof not being regulated by reason. Wherefore if a man exceed in quantity of food, not from desire of food, but through deeming it necessary to him, this pertains, not to gluttony, but to some kind of inexperience. It is a case of gluttony only when a man knowingly exceeds the measure in eating, from a desire for the pleasures of the palate.
Reply to Objection 3. The appetite is twofold. There is the natural appetite, which belongs to the powers of the vegetal soul. On these powers virtue and vice are impossible, since they cannot be subject to reason; wherefore the appetitive power is differentiated from the powers of secretion, digestion, and excretion, and to it hunger and thirst are to be referred. Besides this there is another, the sensitive appetite, and it is in the concupiscence of this appetite that the vice of gluttony consists. Hence the first movement of gluttony denotes inordinateness in the sensitive appetite, and this is not without sin.
Source: Summa Theologica (New Advent)