Objection 1. It would seem that gluttony is not a mortal sin. For every mortal sin is contrary to a precept of the Decalogue: and this, apparently, does not apply to gluttony. Therefore gluttony is not a mortal sin.
Objection 2. Further, every mortal sin is contrary to charity, as stated above (Question 132, Article 3). But gluttony is not opposed to charity, neither as regards the love of God, nor as regards the love of one's neighbor. Therefore gluttony is never a mortal sin.
Objection 3. Further, Augustine says in a sermon on Purgatory [Cf. Append. to St. Augustine's works: Serm. civ (xli, de sanctis)]: "Whenever a man takes more meat and drink than is necessary, he should know that this is one of the lesser sins." But this pertains to gluttony. Therefore gluttony is accounted among the lesser, that is to say venial, sins.
Objection 4.On the contrary, Gregory says (Moral. xxx, 18): "As long as the vice of gluttony has a hold on a man, all that he has done valiantly is forfeited by him: and as long as the belly is unrestrained, all virtue comes to naught." But virtue is not done away save by mortal sin. Therefore gluttony is a mortal sin.
I answer that, As stated above (Article 1), the vice of gluttony properly consists in inordinate concupiscence. Now the order of reason in regulating the concupiscence may be considered from two points of view. First, with regard to things directed to the end, inasmuch as they may be incommensurate and consequently improportionate to the end; secondly, with regard to the end itself, inasmuch as concupiscence turns man away from his due end. Accordingly, if the inordinate concupiscence in gluttony be found to turn man away from the last end, gluttony will be a mortal sin. This is the case when he adheres to the pleasure of gluttony as his end, for the sake of which he contemns God, being ready to disobey God's commandments, in order to obtain those pleasures. On the other hand, if the inordinate concupiscence in the vice of gluttony be found to affect only such things as are directed to the end, for instance when a man has too great a desire for the pleasures of the palate, yet would not for their sake do anything contrary to God's law, it is a venial sin.
Reply to Objection 1. The vice of gluttony becomes a mortal sin by turning man away from his last end: and accordingly, by a kind of reduction, it is opposed to the precept of hallowing the sabbath, which commands us to rest in our last end. For mortal sins are not all directly opposed to the precepts of the Decalogue, but only those which contain injustice: because the precepts of the Decalogue pertain specially to justice and its parts, as stated above (Question 122, Article 1).
Reply to Objection 2. In so far as it turns man away from his last end, gluttony is opposed to the love of God, who is to be loved, as our last end, above all things: and only in this respect is gluttony a mortal sin.
Reply to Objection 3. This saying of Augustine refers to gluttony as denoting inordinate concupiscence merely in regard of things directed to the end.
Reply to Objection 4. Gluttony is said to bring virtue to naught, not so much on its own account, as on account of the vices which arise from it. For Gregory says (Pastor. iii, 19): "When the belly is distended by gluttony, the virtues of the soul are destroyed by lust."
Source: Summa Theologica (New Advent)