Association Between Parity and Bone Mineral Density in the National Health and Nutrition Examination Survey

Objective: Bone remodeling relies on a dynamic process of concurrent deposition and resorption of bone material, which regulates bone mineral density (BMD), a critical component of overall bone health. Chronic dysregulation of the remodeling process during an individual's life can result in low BMD, osteoporosis, reduced mineral reserves and/or increased fracture risk. Prior studies have investigated the link between parity and BMD, positing that one cost of reproduction is increased bone resorption above deposition, resulting in net BMD loss. Further, bone remodeling is sensitive to repetitive mechanical loading, suggesting that differences in bone loading could modify associations between parity and BMD. We seek to understand how reproductive investment (using parity as a proxy) challenges bone remodeling. Methods: We examined associations between parity and regional BMD using anthropometric, dual-energy x-ray absorptiometry, and questionnaire data from the National Health and Nutrition Examination Survey (2007-2018 cohorts; n = 5144). Results: In unadjusted linear regressions, higher parity was associated with lower BMD in all regions except the thoracic spine, arms, and total BMD (p < 0.004). In regressions adjusting for BMI and age, parity was positively associated with BMD in the pelvis, arms, and total BMD (p < 0.004). The maximally controlled models, which adjust for race/ethnicity, sedentary time, poverty income ratio, and lifetime estrogen exposure, among other health and lifestyle variables, yielded similar results. Discussion: Our results suggest that more rigorous statistical modeling and selection of reproductive cost variables may help explicate the biological mechanisms underlying conflicting parity-BMD associations and their impact on bone health and aging.