Quantifying the balance between bone formation and resorption: An index of net bone formation

Context and Objective: Bone resorption and formation markers increase when turnover increases, whether there is net bone gain or net loss. The objective was to determine if resorption and formation markers can be combined to gauge net bone balance. Design: Cohort followed 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.Outcome Measures: 1) Bone balance index (BBI) created by estimating the in-balance relationship between resorption and formation markers in 685 women in stable bone balance (>5 years before the final menstrual period, FMP), and applying this relationship to measured turnover markers in 216 women beginning to lose bone (<2 years from FMP). 2) Annualized percent declines over the following 3-4 years in lumbar spine (LS) and femoral neck (FN) bone mineral density (BMD). Results: Adjusted for covariates, BBI was greater (more favorable) in women with greater body mass index (p=0.03), and lower (less favorable) in women closer to the FMP (p=0.007). Each standard deviation decrement in BBI was associated with 0.27%/year faster LS BMD decline (p=0.04) and 38% higher odds of faster-than-average loss of LS bone mass (p=0.008, c-statistic=0.76). BBI was not associated with decline in FN BMD. Urinary N-telopeptide (U-NTX) alone was not associated with either LS or FN BMD decline.Conclusions: An index that quantifies net bone formation versus resorption can be created from turnover markers, and may help identify individuals at high risk for bone loss