Does obesity really make the femur stronger? BMD, geometry, and fracture incidence in the women's health initiative-observational study

Abstract

Heavier individuals have higher hip BMD and more robust femur geometry, but it is unclear whether values vary in proportion with body weight in obesity. We studied the variation of hip BMD and geometry across categories of body mass index (BMI) in a subset of postmenopausal non-Hispanic whites (NHWs) from the Women's Health Initiative Observational Cohort (WHI-OS). The implications on fracture incidence were studied among NHWs in the entire WHI-OS. Baseline DXA scans of hip and total body from 4642 NHW women were divided into BMI (kg/m(2)) categories: underweight (<18.5), healthy weight (18.5-24.9), overweight (25-29.9), and mild (30-34.9), moderate (35-39.9), and extreme obesity (>40). Femur BMD and indices of bone axial (cross-sectional area [CSA]) and bending strength (section modulus [SM]) were extracted from DXA scans using the hip structure analysis (HSA) method and compared among BMI categories after adjustment for height, age, hormone use, diabetes, activity level, femur neck-shaft angle, and neck length. The association between BMI and incident fracture was studied in 78,013 NHWs from the entire WHI-OS over 8.5 +/- 2.6 (SD) yr of follow-up. Fracture incidence (cases/1000 person-years) was compared among BMI categories for hip alone, central body (hip, pelvis, spine, ribs, and shoulder girdle), upper extremity (humerus and distal), and lower extremity (femur shaft and distal but not hip). Femur BMD, CSA, and SM were larger in women with higher BMI, but values scaled in proportion to lean and not to fat or total body mass. Women with highest BMI reported more falls in the 12 mo before enrollment, more prevalent fractures, and had lower measures of physical activity and function. Incidence of hip fractures and all central body fractures declined with BMI. Lower extremity fractures distal to the hip trended upward, and upper extremity incidence was independent of BMI. BMD, CSA, and SM vary in proportion to total body lean mass, supporting the view that bones adapt to prevalent muscle loads. Because lean mass is a progressively smaller fraction of total mass in obesity, femur BMD, CSA, and SM decline relative to body weight in higher BMI categories. Traumatic forces increase with body weight, but fracture rates at the hip and central body were less frequent with increasing BMI, possibly because of greater soft tissue padding. There was no evident protective effect in fracture rates at less padded distal extremity sites. Upper extremity fractures showed no variation with BMI, and lower extremity fracture rates were higher only in the overweight (BMI = 25-29.9 kg/m(2)).

FIG. 1

A typical DXA scan of the adult hip showing locations of the five mineral mass profiles that are averaged in each region for geometry measurements with the HSA software.

FIG. 2

The relative scaling of femur BMD with BMI for the HSA subset of NHW women from WHI-OS after adjustment for significant covariates and normalization to mean of healthy weight (18.5–24.9 kg/m²). All means are significant vs. healthy weight.

FIG. 3

The relative scaling of narrow-neck BMD, bone CSA, and SM after adjustment for significant covariates and normalization to mean of healthy weight in an HSA subset of NHW women from WHI-OS: (A) adjusted means, (B) adjusted means expressed relative to body weight, and (C) adjusted means expressed relative to total body bone–free lean mass (note expended vertical scale for clarity). All parameters are significantly different from healthy weight in A and B. In C, *p < 0.05, **p < 0.01, and ***p < 0.001 vs. healthy weight.

FIG. 4

The relative scaling of (A) femur OD (note expended vertical scale for clarity) and (B) estimated BR at the narrow-neck, intertrochanter, and shaft regions of the proximal femur. BMI category mean values are normalized to mean value of healthy weight and adjusted for significant covariates in the HSA subset of NHW women from WHI-OS. Except for OD in underweight, all parameters are significant vs. healthy weight.

FIG. 5

Relative fracture incidence in cases per 1000 person-years in NHW women in WHI-OS by BMI category after adjustment for age, hormone use, and diabetes. Relative incidence (95% CI) is plotted for hip alone, central body including hip + spine, pelvis, ribs, sternum, clavicles and scapula, lower extremity (femur shaft and all distal sites), and upper extremity (humerus and all distal sites). *p < 0.05 vs. healthy weight.