Scattered throughout our bones are two important types of bone cells called osteoclasts and osteoblasts. These cells perform the critical function of bone remodeling, in which old or damaged bone is taken away, or resorbed, and new, healthy bone is laid down in its place. The job of osteoclasts is to eat away the old bone, then osteoblasts come in to lay down the new bone. Throughout childhood and into our twenties, bone formation exceeds bone resorption, so that we are truly building stronger and healthier bones. After about age thirty, however, for reasons that are still unclear, this process starts to reverse, and bone resorption slightly outstrips bone formation. By age thirty, most of us have achieved our peak bone mass, and then we start to lose bone very slowly.
Understanding Peak Bone Mass
Every person has a biologically determined peak bone mass. By this I mean that gender, race, and other hereditary factors predetermine the maximum amount of bone that each of us can develop by the time we reach our thirties. Women tend to have lower peak bone masses than men. African Americans tend to have higher peak bone masses than Caucasians. About 80% of this peak bone mass is hereditary while the other 20% is affected by lifestyle and other factors.
A woman may be genetically "programmed" to have a normal peak bone mass at maturity. If she has an unhealthy lifestyle, however (smoking, poor diet, and so on), she may only achieve 80% of her potential peak bone mass, and end up with bones that are less dense than she might have had otherwise. In contrast, another woman may be destined to have a rather thin skeleton due to hereditary factors so that even with an ideal lifestyle, her peak bone mass may measure significantly below that of her peer group.
