This mod solves the issue related to teenage sims' weight, that causes them to lose too much weight sometimes to the point of being severely underweight.
How does the game behave by default ?
In the Sims 4, teenagers have their own metabolism : it has been set so that they'll gain only half as much weight as any older sim. This was intended to give teenage sims a higher metabolism and to prevent them from being overweight. Unfortunately, the way teenagers' metabolism works can sometimes cause them to become skinny to the point of being underweight.
What does this mod change to the game's behavior ?
Teenage sims' metabolism has been "fixed" to function exactly the same way as adult sims' metabolism. The mod will prevent them from being skinnier than adults. In return, teenage sims have now the same risk of being overweight as adult sims. The issue can be avoided with having the two other parts of the Natural metabolism mod installed.
