If we "lose bone," where does it go? Can it be replaced once it's lost?
We usually don't begin to lose bone until we are in our mid-30s, which is after our peak bone mass has been reached. Around this age, the process of remodeling takes over and the balancing act of replacing lost bone with new bone begins.
If you do not continually take in enough of the nutrient building blocks for bones, such as protein, calcium, and vitamin D, and if you do not get appropriate exercise, the process of remodeling becomes unbalanced. If the osteoclasts outpace the osteoblasts during remodeling, holes will be made faster than they can be filled by the osteoblasts. The osteoclasts break down collagen, which is excreted in urine, and in this way bone is lost. When calcium and phosphorus are taken from the bones in your body to replace low levels of calcium and phosphorus in the blood, bone loss also occurs.
Once bone is lost, it can be replaced. However, increasing bone mass once it has been lost requires the right combination of exercise, intake of essential nutrients, and, often, prescription medications (see Part Three).
What roles do calcium and vitamin D play in developing bones?
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.
-  An instruction from a health care professional who is licensed to provide written authorization for medications or devices to be issued by a pharmacy.
-  An amino acid that strengthens collagen in bone formation.
-  A mineral necessary for the body to thrive; bone is made up of 40% calcium. When calcium in the blood is low, it is taken from bone. The body loses calcium through urine, sweat, and stool, and it must be replaced through intake of certain foods and exercise.
-  A type of vitamin that is necessary for the absorption of calcium and phosphorus from the intestinal tract.
-  A hormone resulting from the conversion of vitamin D by liver and kidney enzymes to aid in balancing the activity of the osteoclasts and osteoblasts.
-  A disease characterized by a gradual softening and bending of the bones with varying severity of pain; often comes from a vitamin D deficiency; may also be called "adult rickets."