Bones
I read an interesting article on how the human body senses stresses and loads on all of the bones in the body. It then reinforces the stress areas in a process called "remodeling."
To build bone you need to work it. Lift weights, pound the pavement, walk- anything to stress the bone.
Astronauts loose bone in part because no gravity=no loads.
Ever notice you don't see any overweight elderly people? Many weigh a lot less than they did most of their lives. I wonder if the lower weight, reducing the loads on their bodies, is partly to blame for loss of bone density?
Considering their weight, it's a wonder dinosaurs weren't all bone.