I was making a sim and I saw that you could make your sim extremely fat, and I couldn't help but wonder if a person makes a sim fat, can the sim work out alot and get thin? I haven't tried it, and I don't want a fat sim, so that led me to the question... can your sim GAIN weight?
Just a question...
Hey, guess what!? I'm a girl gamer, and proud of it!!
You can both gain and loose weight relatively fast in this game. Faster if you have the Fast Metabolism reward, and basically instantly if you have the Body Sculptor reward (Which can basically alter your sim's body shape to your whim with an occasional backfire).
Under normal conditions though you can slim down or plump up a sim to their limit easily within a single day, it's not something that requires a steady regiment, a new life style or anything. It should be noted that sims naturally gravitate to their original weight and build however, so a sim who's designed to originally be fat will have to work out regularly to keep it up while a slim and trim sim wouldn't have to do anything at all to loose weight should they gain any.
Only Cardio workouts reduce weight, basically anything that involves treadmills or jogging. Strength based workouts however primary increase muscle mass so it's entirely possible to have a sim who's built like a sumo wrestler (Fat but all muscle).
In The Sims 3, all food is equally unhealthy, so if my memory serves me correctly, the most efficient way to get fat is to go the grocery store and buy a ton of apples and just eat them all in rapid succession. Sims basically only gain weight when they already have the "Stuffed" moodlet that appears if your sim eats when their Hunger bar is near or completely full. Not sure if the apple method still works still, but if it does, it would probably be a much faster alternative to raiding the fridge.