Gluttony: a definition.

From the Catholic Encyclopedia (yeah, the old one) at New Advent.  The whole entry is worth reading for its analytical clarity and conciseness.  According to the encyclopedia, people commit the sin of gluttony when they

• use food or drink in such a way as to injure their health
• use food or drink in such a way as to impair the mental equipment needed to discharge their duties 
• eat or drink for the mere pleasure of the experience and for no other reason (though the reason may be implied rather than explicit)  

The sin is venial unless your gluttony interferes with your grave obligations to God, self, and others, in which case it becomes mortal.   

Here's a nice quote:

The moral deformity discernible in this vice lies in its defiance of the order postulated by reason, which prescribes necessity as the measure of indulgence in eating and drinking.  This deordination... may happen in five ways: ... too soon, too expensively, too much, too eagerly, or too daintily.

I am an instinctive list-maker, and I love the catalog of five ways to be a glutton.  It's even cooler in Latin.  

It strikes me that a common mistake is to assume that "necessity," the measure of indulgence according to the quoted material, refers only to the nutrients required to sustain physical and mental health.  Eating serves social purposes too, and so the "necessity" that measures indulgence also should include the necessities of charity, faith, and other virtues.  

In some cases "necessity" means less than what we need for physical health; for example, in times of scarcity "necessity" may mean going without so that someone else may have some.  And sometimes "necessity" means more than what we need for physical health:  graciously accepting  a portion of a homemade dessert, for example, is the proper and charitable response to the gift.