Why does vitamin D dissolve BONE?

Not many people realize this action is true, but, you need to read this whole paragraph for the complete story. Of importance is the final overall result, not each action involved. But, an awareness of each stage can help determine the best way to evaluate and use supplements. Often this means both too little as well as too much might exhibit adverse effects. ref

When the calcium level in blood drops below a certain critical point, the parathyroid gland secretes a hormone, PTH, that starts a cascade of actions resulting in the activation of vitamin D into it's active hormone form. The hormone form of vitamin D has a short life span in the body. The goal for this hormone form of vitamin D along with PTH is to help return the blood calcium level back to normal. To accomplish this it triggers a number of processes. First, it signals the intestinal tract to absorb more calcium out of food. Absorption percent varies from 10% to 50% with average between 25% to 33%. Second, it signals the Kidneys and colon to stop the elimination of calcium. Third, along with PTH, it signals the start of a process that dissolves bone freeing calcium to raise the amount of calcium in blood back to normal.    

Thus, both PTH and the active form of vitamin D are usually present at the same time. So while PTH might stimulate more of the bone dissolving process, vitamin D also gets some of the responsibility.

The body swings back and forth between two calcium processes, one controlled by vitamin D to increase absorption and hold onto existing body calcium amounts (taking calcium out of bones does not eliminate it), and the other is to turn off the activation of the hormone form of vitamin D which shuts down absorption and speeds up calcium elimination out of the body. Plus, this process also pushes extra calcium in the blood back into bones.

The driver for what happens to calcium is not the density of bones, but the level of calcium in the blood. A certain level of calcium is required so nerves can fire, muscles contract, plus to stimulate numerous other metabolic functions. If calcium amount in blood is lower or higher than the set point, many controlling processes are enacted to quickly re-establish balance. Yes, large doses of highly concentrated calcium supplements, which can increase blood calcium levels, requires a reverse process mentioned below. Both regulations use up vital body energy and stress organs and glands into action. Organs like the kidneys and liver are involved which have other critical functions they should be performing. 

THE COUNTER PROCESS

Now, Mother Nature knows the importance of bone density and that dissolving too much would have a negative impact on bone strength. Thus, bones serve as a reservoir of calcium and have extra just for this purpose. Plus, nature had the insight to also have the hormone form of vitamin D trigger the start of the bone building process that will eventually not only replace the calcium taken out of bone to normalize blood levels, but to also add more bone building as well. When calcium intake is low, the body holds onto calcium already in the body. When intake of calcium is in excess, the body packs more into bone reservoirs which increases density for a short time, and it also turns off the absorption channels and speeds up the elimination of calcium. Balance and Harmony! The critical point here is the short time aspect and the amount of calcium that can be safely packed into bone. After some time, a point is reached that no longer helps bone, but starts to hinder bone dynamics. Bones can even become too rigid with higher intakes of daily calcium. Bones need to flex yet be strong. Density balanced with bone structure create strength.

THE CHALLENGE OF CALCIUM HARMONY IN THE BODY

Many people are told to take 1000 mg. of calcium every day to protect bones and build up bone density. This sets up two body responses that over time, start to present a problem for the body. First, large amounts of calcium overwhelm the checks built into the digestive system that wants to just trickle calcium into the system so the calcium elimination process is not abruptly stimulated. Second, large amounts of calcium all at once block off avenues of absorption for other vital minerals, like magnesium, phosphorus, and zinc. Yes, one elimination process packs more calcium into bones, and builds more bone, too. These appear to be a beneficial effect. But, wait, another aspect of the elimination process is that the body no longer needs to activate vitamin D into the hormone form, a form that is 1000 times more active than the longer lasting storage form of vitamin D. Doctors measure the storage form and not the hormone form levels. To test for hormone D form actions, Doctors measure PTH levels.

DOES HORMONE VITAMIN D HAVE OTHER FUNCTIONS?

Here is where the professionals appear to exhibit tunnel vision. Yes, the hormone form of vitamin D participates in a number of vital body functions. It not only helps balance blood calcium levels and build bone, it also has vital immune system actions, LL-37, as well as it plays a role in blood sugar control. What happens when the body is everyday bombarded with excess calcium? The body elimination processes work overtime to attempt to maintain a calcium equilibrium. And the body does a pretty remarkable job for a period of time, but then..

...The Countries with the highest per person intake of dairy products exhibit the weakest bone statistics...

While the active absorption of calcium is turned off when blood levels are adequate, the passive absorption for calcium is always working, and extra calcium can still get into the blood stream. This continues the shut off of the need for more calcium and also stops the activation of vitamin D into it's hormone form for a longer period of time than the normal dietary swings in calcium intakes would generate. This can be corrected by cycling calcium supplements on and off, but this takes some knowledge and testing to make sure body functions remain in balance. You will not find any information on this aspect anywhere. 

Can these processes be overwhelmed to the point of exhaustion?

This is perhaps what is happening, especially in dairy consuming Countries. The Countries with the highest per person intake of dairy products exhibit the weakest bone statistics. Countries that consume less dairy, generally have better bones. But the people there are also more active and eat a closer to the earth diet. Thus, total calcium intake may not be the most important aspect in bone integrity. A balance with perhaps some swings back and forth to let the body have its checks and balance actions would better serve the hormone vitamin D activation needs. article

WRAP UP

While one of the mechanisms of the hormone form of vitamin D is to dissolve bone to release calcium to balance blood calcium levels, once that level is achieved, this process stops. The other functions of hormone vitamin D that increase the absorption of bone building minerals plus stimulates or turns on the cells that build more bone, have a net result for the hormone form of vitamin D that builds more bone than it dissolves.

But, there are limits and nutrient ratios that should be maintained for balance and harmony for bone building and maintaining activities. Building healthy bone is more than just calcium with vitamin D. Calcium needs magnesium (ref) and phosphorus for the forming of bone hydroxyapatite crystals. Vitamin D needs magnesium to activate, plus vitamin D also needs vitamin K2 to finish the job vitamin D starts with osteocalcin production. And the actions of vitamin D need to combine with a form of vitamin A to accomplish many of the actions attributed to vitamin D. Strong bones need a support team of many nutrients and actions, like exercise to maintain and renew bone integrity.