Gluttony is a sin against the moral virtue of temperance.

Summa, IIa IIae, Q.148, a.1, respondeo.

Temperance is one of the four Cardinal Virtues which are the main pillars of the moral life.

Gluttony is one of the seven deadly sins. From these seven sins arise the other sins.

Summa, IIa IIae, Q.148, a.6.

Gluttony is consenting to the desire to eat or drink an unreasonable amount or in any circumstances which are unreasonable.

Here is how St. Thomas Aquinas teaches this important truth:

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.

Summa, IIa IIae, Q.148, a.1 respondeo.

Thus, e.g., if a person knows he eats too much and weighs too much, he is aware he has committed (is committing) the sin of gluttony. That is, he knows he eats more and weighs more than is reasonable. When our doctor tells us “you need to lose weight”, our doctor is telling us we are committing the sin of gluttony.

Gluttony is a mortal sin in a variety of circumstances. One example is when eating is the focus of our lives, i.e., when we “live to eat”. St. Paul refers to such gluttony when he laments people “whose god is their belly”. (Phil. 3:19.)

Another example of the mortal sin of gluttony is when a man foresees that his habit of gluttony will likely cause him serious future health problems. Such serious health problems can occur in many ways, e.g., because his excessive weight will foreseeably shorten his life, or will likely cause harm to his heart or his joints, will likely cause diabetes, etc.

Here is how St. Thomas Aquinas teaches this important truth:

The glutton intends, not the harm to his body, but the pleasure of eating: and if injury results to his body, this is accidental [i.e., aside from his intention]. Hence this does not directly affect the gravity of gluttony, the guilt of which is nevertheless aggravated, if a man incurs some bodily injury through taking too much food.

Summa, IIa IIae, Q.148, a.3, ad 3 (emphasis and bracketed words added).

There are five main ways that a person can commit the sin of gluttony:

By eating or drinking

“Drinking” here, refers to any excessive (i.e., unreasonable) drinking of a fluid, e.g., sugary drinks. Alcohol intoxication is a separate problem and sobriety is a special virtue. Here is how St. Thomas explains this truth:

Meat and drink are alike capable of hindering the good of reason, by embroiling the reason with immoderate pleasure: and in this respect abstinence is about both meat and drink alike. But intoxicating drink is a special kind of hindrance, ... wherefore it requires a special virtue ... which is sobriety.

Summa, IIa IIae, Q.149, a. 2, respondeo & ad 1, (emphasis added).

Intoxication is a special sin because intoxication is acting against a special virtue (viz., sobriety). Here is how St. Thomas explains this truth:

Drunkenness may result from inordinate concupiscence and use of wine: in this way it is accounted a sin, and is comprised under gluttony as a species under its genus. For gluttony is divided into “surfeiting and drunkenness,” which are forbidden by the Apostle (Romans 13:13).

Summa, IIa IIae, Q.150, a.1, respondeo.

too much;

By seeking food or drink which is excessively (unreasonably) expensive or exotic;

By seeking food or drink which is too daintily (unreasonably) prepared;

By eating or drinking too fast (“wolfing down” our food); and

By eating or drinking too greedily (“like a pig”).

St. Thomas Aquinas, greatest Doctor of the Church, follows St. Gregory the Great, also a Doctor of the Church, teaching there are these five types of gluttony:

The vice of gluttony tempts us in five ways. Sometimes it forestalls the hour of need; sometimes it seeks costly meats; sometimes it requires the food to be daintily cooked; sometimes it exceeds the measure of refreshment by taking too much; sometimes we sin by the very heat of an immoderate appetite —which are contained in the following verse: “Hastily, sumptuously, too much, greedily, daintily”.

Summa, IIa IIae, Q.148, a.4 sed contra (referring to a quote in objection 1), quoting St. Gregory’s word De Moral. xxx, 18.

St. Thomas further teaches:

[G]luttony denotes inordinate concupiscence in eating. Now two things are to be considered in eating, namely the food we eat, and the eating thereof. Accordingly, the inordinate concupiscence may be considered in two ways. First, with regard to the food consumed: and thus, as regards the substance or species of food a man seeks “sumptuous” — i.e. costly food; as regards its quality, he seeks food prepared too nicely—i.e. “daintily”; and as regards quantity, he exceeds by eating “too much.”

Secondly, the inordinate concupiscence is considered as to the consumption of food: either because one forestalls the proper time for eating, which is to eat “hastily,” or one fails to observe the due manner of eating, by eating “greedily.”

Summa, IIa IIae, Q.148, a.4, respondeo.