OSTEOPOROSIS, SEDENTARY LIFE-STYLE, AND INCREASING HIP-FRACTURES - PATHOGENIC RELATIONSHIP OR DIFFERENTIAL SURVIVAL BIAS

Osteoporosis, although a disorder of antiquity, has become more preval ent in developed countries and is a major risk factor for skeletal fra cture. 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 be en suggested as a significant contributing factor for the increased pr evalence of osteoporosis. However, differential survival, reflecting c hanging competing mortality risks, will alter the gene pool of a survi ving population cohort. Thus, the gene pool (and hence, disease suscep tibilities) of 70-year-old individuals in 1990, for example, should no t implicitly be assumed to be the same as 70-year-old individuals in 1 950. Consequently, differences in the prevalence of osteoporosis or in cidence of hip fracture between current and past elderly cohorts do no t 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 o f some aging-related disorders such as osteoporosis.