Background: Adolescence and young adulthood is a critical period for the development of peak bone mass. In addition, low bone mineral density (BMD) is a frequently overlooked consequence of eating disorders in this population. Thus, bone loss that leads to fracture in the young women represents an area of active research and clinical investigation. The purpose of this study is to investigate whether low BMI is associated with low bone mineral density (BMD) and bone bending strength in young women, and whether any such association could be explained by low levels of habitual physical activity, bone turnover markers or low percent body fat. The secondary purpose of this study is to examine the difference in mean BMD and bone bending strength between underweight body mass index (BMI) ≤ 18.9 kg/m2 and normal and overweight BMI 19.0 to 29.9 kg/m2 in young females. We hypothesised that young women with low BMI is associated with low BMD and low tibial bending strength, such association may be explained by low levels of habitual physical activity, low bone turnover marker activity or low percent body fat.
Method: Thirty females, age 18-30 years, with a BMI ≤ 18.9 kg/m2 (n=15) and a BMI between 1929.9 kg/m2 (n=15) served as study subjects. The dependent variables are BMD and bone bending strength. BMD values were expressed as Z-score units and in absolute values for femoral neck, lumbar spine, forearm, and leg obtained with a dual-energy X-ray absorptiometry (DXA, Hologic Discovery-W scanner, Bedford, MA, USA). Bone bending stiffness of the tibia and ulna were determined using a Mechanical Response Tissue analyzer called MRTA (NASA, Mountain View, CA, USA) and bone turnover markers by ELISA using bone biomarker kits (Quidel Corporation, San Diego, CA, USA).
Results: Weight, height, FM, LBM, Body Fat % and tibia length were significantly lower (all p<0.05) in the low BMI group compared with the normal+high BMI group. Relative to the normal+high BMI group, the low BMI group also exhibited significantly lower tibial bending strength (TEI) (148 vs. 195 Nm2, p<0.05), Femoral neck (FN) BMD (z-score -2.44 vs. 0.15), lumbar spine (LS) 1-4 BMD (z-score -2.08 vs. 0.09), whole body (WB) BMD (z-score =-0.50 vs. 0.53), forearm BMD (z-score -0.5 vs. 6.2), total hip (z-score -0.89 vs. 0.2) (all p<0.05). Multiple regression results show that significant independent predictors of TEI are total hip BMD (β=2.51) and LS1-4 BMD (β=-1.40) (R2 =0.367, p<0.05). Simple correlation coefficient between ulnar bending strength (UEI) and ulna BMD (r=0.25, p=0.27) and between TEI and hip BMD (r=0.29, p=0.12) were low.
Conclusion: We concluded that young women with BMI <18.9 kg/m2 have significantly low tibial bending strength, and low BMD for whole body, distal arm, hip, FN, and LS1-4 with z-scores ranging from -0.5 to -2.44. Total hip BMD and LS1-4 BMD were significant determinants of tibial bending strength in young females. (Funded by RSCA Mini Grant Award, Cal Poly Pomona, CA).
Disclosure: The authors declared no competing interests. This research was funded by a RSCA Mini-Grant Award of California State Polytechnic University, Pomona, California.