Just as in real life, your Sims will gain weight if they’re not exercising. Sims lose weight from doing exercise, and they get the best results from jogging and treadmill. If you want your Sim to stay fit, you need to convince them to exercise regularly. You can go the Hugh Hefner route and date multiple hot blondes or, if you prefer, you can even enter a group marriage – let your freak flag fly! How do I stop my Sims from getting fat? With the polygamy mod, your Sims are now able to have a relationship with -and marry – more than one person at a time.