The reason we stop growing in a cycle comes down to a few factors...
1: Genetics...
2: Food... if you dont continually give your body more cals, protien, etc. as the cycle pregresses, you will hit a wall.
A/R down regulation is just not true. We create millions every hour. Everyone has a genetic limit, some just hit it faster, and some know how to memipulate(sp) there food intake to push past it.