Osteoporosis, sedentary lifestyle, and increasing hip fractures: pathogenic relationship or differential survival bias.

Osteoporosis, although a disorder of antiquity, has become more prevalent in developed countries and is a major risk factor for skeletal fracture. Accordingly, the increasing incidence of hip fracture among the elderly within developed nations has been attributed to an increased prevalence of osteoporosis. An increasingly sedentary lifestyle has been suggested as a significant contributing factor for the increased prevalence of osteoporosis. However, differential survival, reflecting changing competing mortality risks, will alter the gene pool of a surviving population cohort. Thus, the gene pool (and hence, disease susceptibilities) of 70-year-old individuals in 1990, for example, should not implicitly be assumed to be the same as 70-year-old individuals in 1950. Consequently, differences in the prevalence of osteoporosis or incidence of hip fracture between current and past elderly cohorts do not necessarily imply differences in environmental risk factors such as levels of physical activity. Instead, variation in competing mortality risks over time may produce differential survival with selection bias and "naturally" lead to increases in the incidence and prevalence of some aging-related disorders such as osteoporosis.