Here's an excerpt, "Bone as a Responsive Material", taken from the University of Cambridge:(http://www.doitpoms.ac.uk/tlplib/bones/formation.php)
“Bone is considered to be a responsive material. The formation and resorption of bone occur continuously: the body responds to stress levels in different areas of bone to ensure the right amount of healthy bone tissue is maintained and the bone can be continually reshaped.
A stress of 25–40 MPa is sufficient to maintain the correct levels of bone. If the bone is under-stressed for prolonged periods of time, bone wastage will set in, and the bones will become thinner. This can be an issue if a patient is bed-ridden for a long time, and is also observed in astronauts after long periods in space. A similar effect occurs during osteoporosis, in which the activity of osteoblasts (bone-building cells) decreases with age. This results in an imbalance of resorption and formation, causing bones to become thinner and weaker.
The opposite effect can be seen when bones are suddenly subjected to higher levels of stress than normal. Studies have been conducted that show an increase in bone mass in new recruits to the army as they begin intensive training.”
Combating bone thinning is NOT a passive issue. The more you get up, move, and load your bones, then the more responsive and dense they will become!