Association between body mass index, bone bending strength, and BMD in young sedentary women.

Abstract

The rationale was to determine whether body mass index (BMI) is a predictor of bone bending strength and bone mineral density (BMD) in young sedentary women. Results show that BMI is not a predictor of bone bending strength and that young women with low BMI also have low BMD. The purpose of this study was to determine whether body mass index (BMI) is a predictor of tibial or ulnar bending strength and bone mineral density (BMD) in sedentary women. Sedentary women (n = 34), age 19-27 years, with low BMI (LBMI < 18.5 kg/m², n = 16), and normal or high BMI (NHBMI between 18.5 and 29.9 kg/m², n = 18) participated as study subjects. Study outcomes included tibial and ulnar bending strength (EI in Nm²) using a non-invasive mechanical response tissue analyzer (MRTA); BMD and bone mineral content (BMC) of the whole body (WB), femoral neck (FN), total hip (TH), lumbar spine 1-4 (LS1-4), and ulna; and bone turnover biomarkers. The LBMI group have lower (p < 0.01) body weight [group difference (Δ) = 32.0%], lean mass (LM) (Δ = 23.1%), fat mass (FM) (Δ = 77.2%), and tibial bending strength (Δ = 22.0%), compared to the NHBMI. The LBMI group also have lower (all p < 0.025) BMC in WB (Δ = 19.9%), FN (Δ = 20.1%) and TH (Δ = 19.0%), compared to the NHMBI, not in BMD results. Multivariate regression analysis shows that significant predictors of tibial bending strength are tibia length (adjusted R² = .341), age (adjusted R² = .489), ulna BMD (adjusted R² = .536), and LM (adjusted R² = .580). BMI was positively correlated with tibial EI (p < 0.05), height, weight, FM, LM, body fat% (all p < 0.01), and BMD of WB, FN, TH, and LS 1-4 (p < 0.05 or < 0.01). Our results show that BMI is not a significant predictor of tibial or ulnar bending strength in young sedentary women.