That's not to say metabolic adaptation doesn't exist. It most certainly does. Metabolic adaptation is the catch-all term that refers to the very real fact that weight loss leads to a decrease in resting metabolism, a decrease in the thermic effect of meals (the cost of metabolizing what you eat), decreased energy cost of physical activity, and to changes to hunger hormones which in turn might well lead you to eat more. In general this also leads to the very real fact that weight loss is far from linear and that it usually stops sooner than expected or desired.
But it's the set point blamed regain back to starting weights that I struggle with conceptually.
Why?
Because generally speaking, it's presented as physiologically driven. My belief is that while metabolic adaptation definitely and somewhat depressingly affects how much weight might be lost with any given effort, it's really sociology that drive the bulk of most people's regains.
What I'm getting at is that people don't regain most or all of their lost weight because their bodies effectively tell them to, they regain most or all of their lost weights because when they quit whatever diets they were on, they revert back to the diets they were consuming beforehand, and by diets, I also mean lifestyles.
For instance, they might stop packing their lunches and head back to their cafeterias, food courts, or drive thrus. They might resume their regular nights out with friends and go back to drinking more alcohol and/or sugar-sweetened beverages. They might bring back some (or more likely all) of the snack foods and indulgences that they'd cut out while "being good". They might return to their older pre-established automated portion sizes and of course their older pre-established dietary staples.
In short, people regain their lost weights when they regain their lost lifestyles, as doing so brings them directly back up to their pre-weight loss average daily caloric intakes which in turn supported their pre-weight loss weights.
Which brings me back to another seminal confirmation bias of mine. The more weight you'd like to permanently lose, the more of your life you'll need to permanently change, which is why the world's best diet for you, is the one you actually enjoy enough to sustain. No, it might not lead you to lose as much as a magic wand would allow because metabolic adaptation does occur, but if you actually enjoy your new diet, and you don't head back to your old lifestyle when you quit the overly strict diet that's leaving you miserable, you need not worry that somehow, magically, due to a "set point", you're going to end up right back where you started.