Body size and hip fracture risk in older women: a prospective study. Study of Osteoporotic Fractures Research Group.

PURPOSE: To determine the relationship between measures of body size and the risk of hip fracture in elderly women. PARTICIPANTS AND METHODS: The association between measures of body size and hip fracture risk was assessed in 8,011 ambulatory, nonblack women 65 years of age or older enrolled in the Study of Osteoporotic Fractures with measurements of total body weight, percent weight change since age 25, hip girth, lean mass, fat mass, percent body fat, body mass index, modified body mass index, and femoral neck bone mineral density (BMD) at the second examination. These 8,011 women were followed prospectively for incident hip fractures occurring after the second examination, which were confirmed by review of x-ray films.
RESULTS: During an average of 5.2 years after the second examination, 236 (2.9%) women experienced hip fractures. Similar associations were observed between hip fracture risk and all measures of body size including total body weight, percent weight change since age 25, hip girth, lean mass, fat mass, percent body fat, body mass index, and modified body mass index. Women with smaller body size had a higher risk of subsequent hip fracture compared with those with larger body size, while women with average and larger body sizes shared similarly lower risks of subsequent hip fracture. For example, the incidence rate of hip fracture was 9.35 per 1000 woman-years in women in the lowest quartile of total weight compared with 4.63 per 1000 woman-years in women in the highest quartile of total weight (age-adjusted relative risk 1.93, 95% confidence interval (CI) 1.34 to 2.80), while rates of hip fracture among women in the second and third quartiles of total weight (5.22 and 4.32 per 1000 woman-years, respectively) were not significantly different from the rate among women in the highest quartile (P > 0.64). The increased risk of hip fracture among women of smaller body size remained after further adjustment for additional potential confounding factors including height at age 25, smoking status, physical activity, health status, estrogen use, and diuretic use. After further adjustment for femoral neck BMD, women with smaller body size were no longer at significantly increased risk of hip fracture compared with those with larger body size. For example, after adjustment for height at age 25, smoking status, physical activity, health status, estrogen use, and diuretic use, thin women had a 2.5-fold increase in the risk of hip fracture (multivariate relative risk 2.51, 95% CI 1.69 to 3.73) compared with the referent group composed of the heaviest women. After further adjustment for femoral neck BMD, the multivariate relative risk of hip fracture among thin women compared to heaviest women was 0.98 (95% CI, 0.64 to 1.50).
CONCLUSION: Older women with smaller body size are at increased risk of hip fracture. This effect is because of lower hip BMD in women with smaller body size. Assessment of body size for prediction of hip fracture risk can be accomplished by measuring total body weight.