It's much much more subtle than that. Take sucralose for example, a beautiful and fantastic zero-calorie sweetener that can help you lose weight.

Amirite?

Except its consumption will change the gut microbiome, eventually resulting in disbiosis, leaky gut, gluten intolerance, IBS, and SIBO.

Each of these conditions results in the CREATION of toxins from the very food that one eats - even the purest of organic/natural/free range/pasture-raised/grassfed/you name it - foods.

These toxins eventually overwhelm the liver, who can't process them. Then the body has no choice: either 1) let the poisons destroy the vital organs, or 2) accumulate said poisons for later processing. There's only one place to store toxins where they won't damage vital organs: fatty tissue.

Obviously #2 is preferrable, so the priority assigned to the maintenance of fatty tissue goes from "excess calories once every little maintenance and repair detail is taken care of" to "X calories immediately after beating the heart, exchanging oxygen and running the most critical processes of the vital organs".

Say you need 200 calories for beating the heart, exchanging oxygen and running the most critical processes of the vital organs. Then in that second equation, the 201th calorie goes to fat. Checkmate healthy weight.