Calcium and vitamin D play vital roles in bone development. Once the osteoblasts have secreted the collagen to fill in the holes made by the osteoclasts, the collagen is strengthened by lysine (an amino acid) and hardened with calcium, giving bone its hardness and strength. The strengthening of the new collagen takes place over a 1- to 2-week period. Most calcium in the body is stored in bone. In fact, bone is 40% calcium.
When blood levels of calcium dip below normal, calcium is taken from the bone to restore normal blood levels. The body loses calcium through urine, sweat, and stool, and because it is constantly excreted, you must always take in enough calcium to maintain normal blood levels. Keeping your blood calcium at normal levels prevents the bone from releasing its stores of calcium, thus protecting your bone strength. Vitamin D is necessary for the absorption of calcium and phosphorus from the intestinal tract. If your body has inadequate vitamin D, calcium and phosphorus will not be absorbed from your intestines but instead will be taken from your bones, regardless of how much calcium you are taking in.
Vitamin D is also important for bone strength because it is transformed into a hormone called calcitriol by liver and kidney enzymes to aid in balancing the activity of the bone-forming cells (osteoblasts) and bone breakdown cells (osteoclasts). If you are severely deficient in vitamin D, you may develop osteomalacia (“softened” bones), which can result in bone pain, leg deformities, and fractures. So it is important to consistently get adequate vitamin D, either from supplements or from foods.