Daniella:

With low D there will be poor calcium absorption.
(low calcium means the bones will be robbed by the body to run the heart and nervous system).

With normal levels, if you don't consume enough calcium there will be nothing for the D to facilitate.

There are hormone things with the parathyroid that can upset the balance when both Vit D and calcium are normal.

1)Too much thyroid (from thyroid supplementation by RX or by autoimmune Grave's disease or hyperthyroidism) and bone tear down will accelerate faster than bone building.

2)There are other factors the bones need:
proper sex hormones--estrogen and testosterone

3)and these nutrients from this link:
http://neurotalk.psychcentral.com/thread115231.html

RSD has been shown to rob bones of calcium

And low activity, will leach bones some. (weight bearing exercise is needed)

Some drugs--- steroids, and others... listed here:
http://www.revolutionhealth.com/cond...ications-cause

The SSRI link was fairly newly added to that list.