The Role of Calcium in Bone Development

Bones serve as a basic support system protecting vital organs and as a reservoir for calcium - the most abundant mineral in the body. In fact, 99% of the body's calcium is found in bones and teeth (the other 1% is found in cells, blood, and other body fluids).

Despite its static appearance, bone is constantly being formed and broken down. This process, called remodeling, is the resorption (breaking down) of existing bone and deposition of new bone to replace that which has been broken down. At any one time, about 5% of bone surfaces in adults are undergoing remodeling. A number of interrelated hormonal, nutritional, mechanical, and genetic factors influence remodeling.

Resorption of old bone and formation of new bone are processes that continuously overlap. The importance of these processes varies at different times throughout the life cycle. In general, from birth until about age 20, the bones are in a phase of active growth. This stage is characterized by an increase in bone length and bone width. Shaping of the growing bones, called modeling, also occurs at this time. Between the ages of 12 and 30, the rapid phase of bone dimensional growth tapers off and consolidation occurs with the attainment of peak bone mass. Although dimensional bone growth ceases at maturity, adult bone is constantly being remodeled. It is generally accepted that peak bone mass or maximum bone density and strength occurs by age 30. Studies indicate that peak bone mass at several skeletal sites (especially the proximal femur and vertebrae) may be reached as early as late adolescence. Peak rate of calcium accretion occurs at about age 12.5 years for girls and 14 years for boys.

Beginning in the 40s or later, resorption of existing bone starts to exceed formation of new bone, resulting in a net loss. Age-related bone loss is influenced by both genetic and environmental factors. This also occurs at different times in the two different types of bone - trabecular and cortical. Trabecular bone, which is spongy in appearance, forms the internal support network for the cortical shell, vertebrae, and other bones. Cortical or compact bone forms the outer shell of almost all bones and is predominant in shafts of long bones such as those in the arms, legs, hands, and feet.