Reduced Lean Mass and Fat Mass Exacerbate Effects of Estrogen Deficiency on Bone

Abstract

Lean mass is a stronger predictor of bone geometry in loaded limbs than fat mass in exercising women; however, estrogen exposure likely modulates the relationship between lean/fat mass and bone. We recently demonstrated that energy and estrogen status interact to impact vBMD, bone geometry and estimated bone strength (eBS) but that energy deficiency was often only detrimental in the presence of estrogen deficiency. This suggests that estrogen deficiency increases the vulnerability of bone to metabolic disruptions that accompany energy deficiency. We hypothesize that the impact of lean/fat mass on bone is dependent on estrogen status. PURPOSE: To compare the predictive value of lean mass index (LMI, kg*m-2) and fat mass index (FMI, kg*m-2) on vBMD, geometry, and eBS in the tibia in estrogen replete and estrogen deficient women. METHODS: Exercising women (n=60, 18-30 yrs) were grouped by 1) Estrogen deficient (E2D, n=27): oligo/amenorrheic <6 cycles/12 mo, and 2) Estrogen replete (E2R, n=33): eu/oligomenorrheic ≥6 cycles/12 mo. Body composition was assessed via DXA. vBMD, bone geometry, and eBS were assessed at the 4% (distal) and 66% (proximal) tibia via pQCT. Multivariate stepwise regression determined predictors of bone outcomes. RESULTS: LMI was a positive predictor of distal tibia total, trabecular, and cortical vBMD in E2D women only, accounting for 28-36% of the variance (p<0.004). LMI was a positive predictor of distal tibia cortical area in E2R women (R2=0.137, p=0.037). FMI was not predictive of bone outcomes in E2R women but was a positive predictor of cortical area at the distal tibia in E2D women (R2=0.162, p=0.038). At the proximal tibia, FMI and LMI were positive predictors of total vBMD (R2=0.494) and cortical thickness (R2=0.571) in E2D women only (p<0.015). LMI was a positive predictor of total area (R2=0.353) and negative predictor of cortical vBMD (R2=0.141) in E2R women only (p<0.044). FMI and LMI were positive predictors of distal tibia BSI in E2D women (R2=0.435, p<0.029). LMI positively predicted BSI in E2R women but explained less variance (R2=0.152, p=0.027). CONCLUSIONS: In the absence of adequate estrogen exposure, reduced fat or lean mass results in significant detriments to bone health in exercising women. It appears that in the face of reduced fat or lean mass, estrogen may be protective to bone.