BONE MINERAL DENSITY AND HIP FRACTURE BY BODY MASS INDEX

Abstract

Research on the intersection of obesity and bone-related outcome such as osteoporosis and fracture risk is of significant public health concern as older adults will represent 20% of the US population by 2030; the majority of whom will have either or both conditions. The mechanical loading of extra weight is assumed to prevent osteoporosis and risk of fracture. However, half of all hip fractures occur among overweight or obese older adults. Many cross-sectional studies, including Paper 1 of this dissertation, find a protective effect of obesity on osteoporosis. Paper 1, using linear regression models and data from the National Health and Nutrition Examination Survey (NHANES 2005-2008) for adults ages > 50 (n=3,296) found every unit increase in body mass index (BMI) was associated with a 0.0082 g/cm2 increase in bone mineral density (BMD). However, Paper 2 of this dissertation using longitudinal data shows that obese older adults lose more bone density over time. Using multivariable generalized estimating equations and 10 years of data from 2,570 older adults in the Health, Aging, and Body Composition Study found that obese older adults lost 0.002 g/cm2 of femoral neck BMD per year more compared with normal weight older adults (p<0.001). Prior literature on obesity and risk of fracture is mixed, although the majority of studies, as well as Paper 3, find obesity to be protective against hip fracture risk. Cox proportional hazard models and data from 2,790 U.S. older adults in NHANES III linked to Medicare claims data (1991-2007) revealed obese older adults had a 15% lower risk of hip fracture (HR=0.85, 95% CI: 0.76, 0.96) while overweight older adults had the same risk level (HR=0.94, 95% CI: 0.85, 1.04) compared to normal weight older adults. The main strength of this dissertation was the comprehensive examination of obesity and bone-related outcomes using large diverse samples of older adults and multiple statistical methods. Future research should consider other measures of body composition and bone strength. Understanding the complex relationship between body mass, bone mass, and risk of fracture is pertinent, particularly as the majority of older adults are either overweight or obese.