Bones are under continual remodeling. They are broken down, materials released to circulation, and rebuilt. If materials released to the circulation are removed by other parts of the body, the bone is rebuilt differently, and chances are this means less well, less strong.
When nutrients are deficient, this allows more critical sites within the body to in effect borrow the nutrients from bone that are released into the circulation. Which nutrients? The ones that make up bone – calcium, phosphate, sodium (there is an entire mole of sodium within bones!), magnesium – all of them, whatever is needed for something more critical than having stronger bones.
When nutrients are plentiful or sufficient, this allows the bone to be rebuilt stronger or to specs, respectively.
Nice system – but we need to follow its inner workings periodically to see how we are doing with our diets. Urine samples what is in plasma so urine tests would be the least invasive and would provide a wealth of information about what is going on within bones and within the rest of the body.