sending my sims to the gym

It depends on the workout I think. Whenever I wanted a sim to lose weight in a realistic way (meaning no diet ice cream :D ), I made them go the gym every few days doing cardio workout at the treadmill. Jogging seems much less effective from my experience. I had a sim whose new years resolution was to lose weight. I sent her jogging everyday and made her eat healthy, but it didn't seem to make a difference and she couldn't fulfill her resolution. So jogging seems less effective. I also think pushing the limits is a good way to make them lose weight. I had a sim lose a minimum amount of weight (judging by their animations, I didn't really notice a change) after making them exercise on the treadmill two times (the second time while pushing the limits). Also, which food the sim eats also seems to play a big part. All the dishes in the game have a calorie amount and any amount over 0 seems to increase the sim's weight over time. You can find a list of all the food and their respective calorie amount online, but from what I remember, salads (including fruit salad) and anything with fish has 0 calories, thus the preferable choice when you want your sim to lose weight.

As I've seen it, sims in this game don't lose weight that fast and it's a much more natural process than in TS3 from what I remember, so going onto the treadmill while pushing the limits every day should work. I hope this helped a bit.