I noticed that with my fatter Sims, even when they hit the treadmill, they were still fat again by the next day. Looking at the code I realised that this is because a sim gains 40% fat per meal! So here is a Mod to slow down the process.
I think I cut all mention of it out... but I'll say it here just in case. This Mod makes your Sims gain 7% of fat per meal not 14%. I got confused because it works out to 14% a day if you eat 2 meals. The Mod doesn't need to be changed, I coded that correctly, I just posted the wrong figures on here.
What this Mod Does
Slows the weight gain from 40% per meal (on the basis that eating takes 1 hour) to just over 7% per meal. This works out to roughly a week before they regain their original figure, plenty of time to hit the treadmill again.
What this Mod does not do
This Mod will not stop your sims getting fat. It just slows the process down to a more realistic level
The Technical Bit
The Game calculates Weight gain on an "amount of time spent eating" ratio. By default, a Sim will become "completely fat" (or their Target weight as determined by genetics) in just 2.4 hours. At roughly an hour per meal, and 2 meals a day, this means a sim will be back to their target weight by breakfast the next day! I have tweaked it so that it takes 14.2 hours worth of food to regain their fat. (I know, its a silly number, but I couldn't directly influence the time, it had to be a percentage of 24 hours, to one decimal place so I set it to "0.6 Sim days") Again at 2 meals a day, it will take 1 week for them to regain their target weight (much more sensible).
If your Sims have gone without Cardio Excersise (Treadmill) for 12 days then by that time they would have regained their original weight, regardless of whether they have eaten for 14.2 hours or not.
Muscle loss works exactly the same way except it isn't influenced by how you eat. After 12 days of no Strength Workouts your Sim goes back to normal (Note you won't actually see it happen, its gradual over the 12 days)
Oh and for those of you wondering it doesn't matter what you eat the weight gain is the same. Autumn Salad is no better for your Sims than Lobster Thermidore.
This Mod is compatible with all EP's/SP's and all levels
This Mod is incompatible with all mods that edit the "Sim Descriptions" XML.
Lytefoot for pointing me in the direction of the right XML
Peter Jones for S3PE
MTS for Hosting
Shimrod for giving me a baseline to work from