Objection 1. It would seem that gluttony is not a capital vice. For capital vices denote those whence, under the aspect of final cause, other vices originate. Now food, which is the matter of gluttony, has not the aspect of end, since it is sought, not for its own sake, but for the body's nourishment. Therefore gluttony is not a capital vice.
Objection 2. Further, a capital vice would seem to have a certain pre-eminence in sinfulness. But this does not apply to gluttony, which, in respect of its genus, is apparently the least of sins, seeing that it is most akin to what is in respect of its genus, is apparently the least gluttony is not a capital vice.
Objection 3. Further, sin results from a man forsaking the food of virtue on account of something useful to the present life, or pleasing to the senses. Now as regards goods having the aspect of utility, there is but one capital vice, namely covetousness. Therefore, seemingly, there would be but one capital vice in respect of pleasures: and this is lust, which is a greater vice than gluttony, and is about greater pleasures. Therefore gluttony is not a capital vice.
On the contrary, Gregory (Moral. xxxi, 45) reckons gluttony among the capital vices.
I answer that, As stated above (I-II, 84, 3), a capital vice denotes one from which, considered as final cause, i.e. as having a most desirable end, other vices originate: wherefore through desiring that end men are incited to sin in many ways. Now an end is rendered most desirable through having one of the conditions of happiness which is desirable by its very nature: and pleasure is essential to happiness, according to Ethic. i, 8; x, 3,7,8. Therefore the vice of gluttony, being about pleasures of touch which stand foremost among other pleasures, is fittingly reckoned among the capital vices.
Reply to Objection 1. It is true that food itself is directed to something as its end: but since that end, namely the sustaining of life, is most desirable and whereas life cannot be sustained without food, it follows that food too is most desirable: indeed, nearly all the toil of man's life is directed thereto, according to Ecclesiastes 6:7, "All the labor of man is for his mouth." Yet gluttony seems to be about pleasures of food rather than about food itself; wherefore, as Augustine says (De Vera Relig. liii), "with such food as is good for the worthless body, men desire to be fed," wherein namely the pleasure consists, "rather than to be filled: since the whole end of that desire is this--not to thirst and not to hunger."
Reply to Objection 2. In sin the end is ascertained with respect to the conversion, while the gravity of sin is determined with regard to the aversion. Wherefore it does not follow that the capital sin which has the most desirable end surpasses the others in gravity.
Reply to Objection 3. That which gives pleasure is desirable in itself: and consequently corresponding to its diversity there are two capital vices, namely gluttony and lust. On the other hand, that which is useful is desirable, not in itself, but as directed to something else: wherefore seemingly in all useful things there is one aspect of desirability. Hence there is but one capital vice, in respect of such things.
Source: Summa Theologica (New Advent)