The pool of calcium in bone is flexible and changes to some extent throughout life. The concentration of calcium in blood cannot vary much without causing severe illness and even death. Some of the normal functions dependent on calcium are blood clotting, hormone release, antibody formation, muscle contraction, and protein synthesis. Vitamin D is a fat-soluble vitamin added to many dairy products. This vitamin is also activated in the body when the skin is exposed to sunlight.
Vitamin D increases the intestinal absorption of calcium.
As the calcium level in the blood increases, a feedback mechanism slows absorption and osteoclast activity. Increased blood calcium will shut off parathyroid hormone secretion and trigger release of calcitonin. This hormone has the opposite effect of parathyroid hormone. It inhibits vitamin D and decreases the calcium level in the blood by slowing its resorption from the bone. Osteoporosis is the result of long-term calcium imbalance.