Aquinas says:
As stated above vice of gluttony properly consists in inordinate concupiscence (plain English: disordered desi). 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.
Aquinas sees desires as not leading to their natural good ends. If we simply blindly follow our passions, our fallen nature emerges, sin emerges. As such, to reorient our desires to their naturally good end, we must use our reason to look at the purpose of our desires and intentionally choose the path that seeks the greatest good.
His conclusions are wrong though because his understanding of man is limited by the age he lived in. As Pope Saint John Paul II taught us, we need a proper anthropology of man. Any flaws in our understanding of human nature will flaw our theology. This is why we need a theology of the body. The natural law
is a theology of the body. The two are not so radically different. For in looking at our body and understanding our body, we can see the natural law…or at least gain a greater glimpse of that law. But when we persists in clinging to old myths about the nature of our body, the conclusions of the natural law will be flawed or incomplete due to our ignorance.
If we understand what Aquinas observed on a deeper level, he’s simply looking at the behavior of obese and likely diabetic people who live in an age where you can’t monitor your blood sugar levels or inject insulin. He is only able to compare how these people eat with how healthier people eat, but his knowledge goes no further than this. This makes his conclusions incomplete.
The reality is that an obese diabetic person has rapidly rising and falling glucose levels from all the sugar they eat. They get an excess of sugar. If sugar levels are too high, it can kill them. Their body rapidly produces glucose to store the energy of the sugar (It stores it as fat). But if their diet doesn’t involve a lot of complex carbohydrates and proteins (which take longer to digest), their glucose levels drop too drastically yet again to dangerous levels which can kill them yet again. Their cravings for sugar and for eating NOW are rooted in their body trying to save itself from death.
I mean, it does get more complex than this, especially if you’re dealing with a diabetic. And our understanding of nutrition is
still one of the less developed sciences. There are still various theories. But overall, Aquinas’ rules are incomplete and his understanding of gluttony incomplete. It’s not his theology. It’s his anthropology.