Quantifying the Balance Between Total Bone Formation and Total Bone Resorption: An Index of Net Bone Formation

Context: Bone gain vs loss across the skeleton loss depends on the balance between total bone formation and total bone resorption. Objective: The objective of the study was to determine whether resorption and formation markers can be combined to gauge net bone formation across the skeleton. Design: The study included a cohort followed up across menopause transition (Study of Women's Health Across the Nation). Setting and Participants: Community-dwelling women, 42-52 years old, premenopausal or early perimenopausal at baseline, participated in the study. Outcome: The study included the following measures: 1) bone balance index (BBI) created by estimating the relationship between resorption (urinary N-telopeptide) and formation (osteocalcin) markers when the total formation equals the total resorption in 685 women with stable bone mineral density (BMD) (>5 y beforethe final menstrual period [FMP]) and applying this relationship to measured bone turnover markers in 216 women beginning to lose bone y from FMP); and 2) annualized percentage declines over the following 3-4 years in the lumbar spine (LS) and femoral neck (FN) BMD. Results: Adjusted for covariates, the BBI was greater (more favorable) in women with a greater body mass index (P =.03) and lower (less favorable) in women closer to the FMP (P = .007). Each SD decrement in BBI was associated with 0.27%/y faster LS BMD decline (P = 0.0 4) and a 38% higher odds of faster-than-average loss of LS bone mass (P = .008, c-statistic 0.76). BBI was not associated with decline in FN BMD. Urinary N-telopeptide alone was not associated with either LS or FN BMD decline. Conclusions: An index that quantifies net bone formation vs resorption can be created from bone turnover markers and may help identify individuals at high risk for LS bone loss.