A similar problem also occurs in the weightlessness of space, when even the youngest, fittest astronauts suffer some bone loss.
Space research is helping scientists to understand what happens and to find a way to combat the problem.
One method is to use volunteers who stay in bed for many weeks. These bed rest studies show how bones change when no weight is being put on them.
ESA is also developing new exercise machines and medical equipment for use on Earth and in space.
One device tested on ESA microgravity flights is a step-on, step-off machine. Another mimics normal walking and running by testing the effects of vibrations on bone.
Last update: 19 October 2011