Calcium, in addition to being an integral part of your bones, is extremely important in cellular signaling as well. For example, when your muscles contract, calcium is the ‘messenger’ which relays that message from your neurons to the muscle fibers. Without enough calcium in the blood, many vital functions would begin to suffer! For this reason, our body needs to keep blood calcium at a certain level, and uses another regulator of calcium to do this: the parathyroid hormone (PTH).
Whereas Vitamin D works to increase calcium available from the diet, PTH is only concerned with whether there is enough calcium in the blood for the body to function properly. If there is not enough calcium, PTH helps our body take extra calcium from the bones, increasing bone density loss. For our immediate health, having adequate levels of blood calcium is more important than having dense bones, even though the ultimate result can be bones which are brittle and easily broken.
Vitamin D works antagonistically to PTH because vitamin D keeps our blood calcium levels high from food intake, meaning PTH doesn’t need to take calcium from the bones. A vitamin D deficiency can lead to a condition called secondary hyperparathyroidism, which means that too much PTH is being secreted, leading to even greater bone loss. Thus, vitamin D is crucial to the health of our bones for two closely related reasons:
1) Adequate levels of vitamin D increase the amount of calcium available from food, building bones up.
2) Inadequate levels of vitamin D increase the amount of PTH in our blood, breaking bones down.
There is also a third way in which vitamin D affects the health of our bones: VDRs are located in special bone cells called osteoblasts, and adequate vitamin D levels accelerate the formation of the bone mineral-matrix. While other factors also contribute to the formation of bones, without sufficient levels of vitamin D, the rate of bone formation is significantly decreased. This also makes vitamin D much more important in building strong bones than calcium, as no matter how much calcium you consume, it cannot accelerate the rate of bone formation like vitamin D does.
It is simplistic to reduce vitamin D to an either/or state, but that is the truth of it. All other factors aside, you can either be vitamin D sufficient, and be building bones, or you can be insufficient/deficient, and breaking bones down. There is no middle-ground.