I found one thing that may be working for me but will require a lot more testing.
I created a few Sims with Athletic Aspirations to get the High Metabolism Creation Trait. I then later switch it to the Aspirations I want for them once my Sim enters the world. I usually always give them a knowledge aspiration so they will be a Quick Learner, but I've just done this with a few so I can compare. I'm doing this with some that will or won't be exercising a lot and just watching how things progress as I see them around. Because it's really affecting Sims in previous households the most. I noticed that Sims so far with the creation trait are staying the same size for now, but I'll see how it goes. I've used the trait before over the years but I usually prioritise other Creation Traits more.
