PROJECT SUMMARY Osteoporosis is the most common metabolic bone disease related to aging, with significant sex/ethnic/population differences in its risk. It is mainly characterized by low bone mineral density (BMD), deteriorated bone quality/strength, and subsequent increased risk to low-trauma osteoporotic fractures (OF). Our current pathophysiological knowledge and diagnostic tools for osteoporosis are limited, and available medication is not satisfactory. New prediction/diagnosis/treatment methods developed from novel biomarkers/pathways/targets are much needed. Metabolomics offers systematic profiling of metabolites, revealing molecular variation closest to the disease risk and phenotype. Biomarker discoveries and the corresponding biological insights gained from metabolomics studies have improved our understanding of many disorders. However, metabolomics studies for osteoporosis, especially in humans remain scarce, leaving many knowledge gaps to be filled. Current gaps include: 1) no metabolomics studies for osteoporosis in elderly US populations; 2) no sex/ethnicity-difference examinations; 3) no metabolomics studies for OF per se; 4) no chemical structural determination and validation for unknown metabolites detected in the studies which used untargeted metabolomics approaches; and 5) no functional validation of identified osteoporosis-related metabolites. The Overall Goal of Project 3 is to systematically discover metabolomic profiles associated with BMD and/or OF in US populations, including both Caucasians and African Americans. We will use state-of-the-art untargeted/global and targeted metabolomics (e.g., lipidolomics and short chain fatty acids) approaches, maximizing the coverage of detectable metabolites. The osteoporosis/OF-related metabolites that are detected by the untargeted approach will go through structural validation for known metabolites using standard compounds, and structural determination for unknown metabolites as well as absolute quantification for evaluating their true effects. The Specific Aims are: Aim 1) To identify metabolites associated with BMD (including areal BMD and volumetric BMD) and bone quality traits (finite element analyses based on QCT and TBS based on DXA) in > 5,300 subjects from our current U19 project and the Louisiana Osteoporosis Study; Aim 2) To identify metabolites associated with OF in 150 subjects with OF and 150 controls who will be recruited through Core B; Aim 3) To identify metabolomic profiles that are specific to—or shared by—BMD and OF by comparing the results from Aims 1 and 2, and identify causal metabolites for BMD and OF using Mendelian Randomization approach; Aim 4) To conduct in vitro (e.g., effects on differentiation of osteoblasts and osteoclasts) and in vivo (e.g., effects on bone traits in aged mice) functional validation of causal metabolites associated with both osteoposoris and OF. This project promises to yield insights into the significance of the human metabolome to os...