Its interconnecting structure provides much of the strength of healthy bone, but is especially vulnerable to osteoporosis. Besides protecting internal organs and allowing us to move about, bone is also involved in the body's handling of minerals. Of the 2 to 4 pounds of calcium in the body, nearly 99 percent is in the teeth and skeleton. The body keeps the blood level of calcium within a narrow range. When needed, bones release calcium.
Osteoclasts break down bone, and osteoblasts build it.
In youth, bone building prevails. Bone mass peaks by about age 30, then bone breakdown outpaces formation, and density declines. After that, all we can do is try to postpone and minimize the steady withdrawals. Osteoporosis is the bankruptcy that occurs when too little bone is formed during youth, or too much is lost later, or both. But there's another reason.