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.
Over a period of decades, calcium is removed from bones to maintain blood levels if dietary or absorption levels are not adequate. The process of aging normally causes calcium to be lost from the bone, but osteoporosis is an accelerated loss of bone. Women generally have lighter, thinner skeletal frames than men and thus have less bone reserve of calcium. Women also lose bone rapidly after menopause. Men lose calcium from bone, but not at the high rates seen in women.