In my game sims do gain and lose weight from their set point. I am far beyond having any living premades in my game in my legacy, but I do remember that Bob Pancakes slimmed down big time when he was alive. For some reason he was always at the gym when my sims were there. Isn't he lazy? Anyway, I've never used them because I don't play rotationally, but I have seen mods out there for this. I don't know if they're kept updated though. I think one was by Robin (Roburky), but I could be wrong. It might be a place to start anyway.
EDIT: Okay I was curious, so I googled LOL. Here you go. https://roburky.itch.io/sims4-fitness
