Characteristics of the gut microbiota and serum metabolites in postmenopausal women with reduced bone mineral density Provisionally Accepted

Litao Yan¹ Xianfeng Wang² Tiantian Yu³ Zhiming Qi⁴ Huan Li⁵ Hao Nan¹ Kun Wang¹ Di Luo⁶ Fei Hua^7* Wendong Wang^8*

• ¹Department of Articular Orthopaedics, The First People's Hospital of Changzhou, The Third Affiliated Hospital of Soochow University, China
• ²Department of Orthopedic Surgery, Beijing Jishuitan Hospital Guizhou Hospital, China
• ³Department of Gynaecology and Obstetrics, Dalian Municipal Woman and Children’s Medical Center, China
• ⁴Department of Articular Orthopaedics, The Second People's Hospital of Dalian, China
• ⁵Changzhou Medical Center, Nanjing Medical University, China
• ⁶Department of Clinical Laboratory, Second Affiliated Hospital of Dalian Medical University, China
• ⁷Department of Endocrinology and Metabolism, First People's Hospital of Changzhou, China
• ⁸Department of Articular Orthopaedics, The Second People's Hospital of Dalian, China

Emerging evidence suggests that the gut microbiota is closely associated with bone homeostasis. However, little is known about the relationships among the bone mineral density (BMD) index, bone turnover markers, and the gut microbiota and its metabolites in postmenopausal women. In this study, to understand gut microbiota signatures and serum metabolite changes in postmenopausal women with reduced BMD, postmenopausal individuals with normal or reduced BMD were recruited and divided into normal and OS groups. Feces and serum samples were collected for 16S rRNA gene sequencing, liquid chromatography coupled with mass spectrometry (LC-MS)-based metabolomics and integrated analysis. The results demonstrated that bacterial richness and diversity were greater in the OS group than in the normal group. Additionally, distinguishing bacteria were found among the two groups and were closely associated with the BMD index and bone turnover markers. Metabolomic analysis revealed that the expression of serum metabolites, such as etiocholanolone, testosterone sulfate, and indole-3-pyruvic acid, and the corresponding signaling pathways, especially those involved in tryptophan metabolism, fatty acid degradation and steroid hormone biosynthesis, also changed significantly. Correlation analysis revealed positive associations between normal group-enriched Bacteroides abundance and normal group-enriched etiocholanolone and testosterone sulfate abundances; in particular, Bacteroides correlated positively with BMD. Importantly, the tryptophan-indole metabolism pathway was uniquely metabolized by the gut bacteria-derived tnaA gene, the predicted abundance of which was significantly greater in the normal group than in the control group, and the abundance of Bacteroides was strongly correlated with the tnaA gene.Our results indicated a clear difference in the gut microbiota and serum metabolites of postmenopausal women. Specifically altered bacteria and derived metabolites were closely associated with the BMD index and bone turnover markers, indicating the potential of the gut microbiota and serum metabolites as modifiable factors and therapeutic targets for preventing osteoporosis.