Well my current project was to see how hard it was to get an extremely overweight sim with the traits of slob, glutton and lazy, to be fit. When I played that household, she would get uncomfortable and fatigued easily when exercising. But she got her friend to mentor her a lot, and she has slimmed down and toned up a lot. She will never be skinny, but that's okay. She is healthy and curvy. She got up to level 6 fitness surprisingly quickly, too. The funny thing is, she's a sim in one of my "play sometimes" households. Now that I've switched back to my main household, I see her at the gym ALL the time, working out all day. I thought once I didn't control her anymore, she'd go back to her overeating, lazy ways, and that next time I saw her she'd have put the weight back on.