Do you even lift, bro?
One of the most alarming things for me when this game first came out was how quickly all of my played Sims got rather ... large. I mean, no matter what they did all of my sims gained a lot of weight...
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Only some of my sims are the "sport type" and do sport regularly, while the others dedicate themselves to their own hobbies instead. In my game I'm also very lazy with cooking, and so I mostly feed my sims with fruits & vegs from their inventories. From what I see, usually this seems to help them maintain their original body type too.
