intestine indirectly by stimulating the production of 1,25(OH)2D. PTH also induces bone resorption, thereby releasing calcium into the blood. Thus, although PTH maintains a normal circulating calcium concentration during calcium deprivation, it does so at the expense of skeletal mass.
Osteoporosis is characterized by reduced bone mass, increased bone fragility, and increased risk of fracture (WHO, 1994). According to the World Health Organization (WHO), individuals with BMD more than 2.5 standard deviations (SD) below the mean for young adult women are osteoporotic (Kanis et al., 1994; WHO, 1994). By this definition, the prevalence of osteoporosis among postmenopausal women in the United States is 21 percent in Caucasian and Asian, 16 percent in Hispanic, and 10 percent in African American women (Looker et al., 1995). An additional 38 percent of American women aged 50 and older meet the WHO definition of osteopenic (for example, have BMD values 1.0 to 2.5 SD below the young adult reference mean) (Looker et al., 1995).
In the United States each year, approximately 1.5 million fractures are associated with osteoporosis, including 300,000 hip fractures, 700,000 vertebral fractures, 250,000 distal forearm fractures, and 250,000 fractures at other sites (Riggs and Melton, 1995). In Canada in 1993, approximately 76,000 fractures were associated with osteoporosis, including 21,000 hip fractures, 27,000 vertebral fractures, and 27,000 wrist fractures (Goeree et al., 1996). Incidence rates for most fractures rise exponentially with age (Cooper and Melton, 1992). For individuals at age 50, their risk of having a hip fracture at some point in the future is estimated at 17 percent for Caucasian women, 6 percent for African American women, 6 percent for Caucasian men, and 3 percent for African American men (Cummings et al., 1993; Melton et al., 1992). It has been estimated that a Caucasian woman's risk of a hip fracture is equivalent to her combined risk of developing breast, uterine, and ovarian cancer (Riggs and Melton, 1995). Health care costs associated with osteoporotic fractures in 1995 were estimated at $13.8 billion (Ray et al., 1997). Because of the expected increase in the number of individuals in the age range of highest risk, the incidence of hip fractures in the United States may triple by the year 2040 (Schneider and Guralnik, 1990).