Background Fact: Older individuals are sometimes less physically active than younger individuals, which makes older individuals more at risk for osteoporosis. Interestingly, this is the same phenomenon that makes astronauts at risk for osteoporosis if spending long periods in space.
Brainteaser: Try to reason out a feedback loop that could explain how a decrease in physical activity could worsen osteoporosis.
Perhaps the main thing to consider here is how the bones know that they are experiencing less physical activity and should therefore change their strength. In other words, what is the stimulus and the receptor in this feedback loop? It turns out, this comes down to one of our main bone cells: the osteocyte! Below is a brief excerpt from a scientific article published in 2012, titled Osteocyte Signaling in Bone
Osteocytes, the cells residing within the bone matrix and comprising 90% to 95% of the all bone cells, have long been considered quiescent bystander cells compared to the osteoblasts and osteoclasts whose activities cause bone gain and loss, and whose dysfunction lead to growth defects and osteoporosis. However, recent studies show that osteocytes play a crucial, central role in regulating the dynamic nature of bone in all its diverse functions. Osteocytes are now known to be the principal sensors for mechanical loading of bone. They produce chemicals that regulate bone formation and resorption. Osteocytes appear to be the major local orchestrator of many of bone’s functions."
Note in particular that osteocytes sense "mechanical loading," or the amount of pressure on our bones. This sort of makes sense if you look at the picture of osteocytes above. Those cells are squeezed in small gaps within compact bone. You could imagine, then, that pressure placed on the bones would cause osteocytes to experience more "squeezing" in their small spaces.
lifting weightsAs the quote above points out, the osteocytes are then able to signal osteoclasts and osteoblasts to adjust bone thickness appropriately. If you start lifting weights and putting extra pressure on your bones, your osteocytes will sense that and tell your osteoblasts to add more bone to make the bones stronger. With stronger bones, the pressure would then be relieved.
reduced physical activityOn the other hand, if you stop performing as much physical activity (or go into orbit around Earth where you experience less gravity!), your bones would experience less pressure. Your osteocytes would sense that and signal your osteoclasts to tell them less bone is required to meet the body's needs. Your osteoclasts might then remove bone, which would create weaker bones and bring the pressure on the bones back up to normal levels. Of course, that could also contribute to osteoporosis!
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