I think the slim potion is the only way to change their genetic body shape because my Sim spends hours on the treadmill and can't lose weight. Worst still was after lifting weights at the gym, he became super bulked up though still fat. I have him go to the gym everyday to run so I can see if he does lose weight, but it's not happening yet.
edit. As Juana explained below, once the Sims are created with that body shape it won't change - the insta lean potion had no effect for my Sim because he was already as lean as he's going to be :(