Background: Adiposity is closely related to osteoarthritis, but the causal effects of different types of adiposity on osteoarthritis are indistinct. This study conducted a Mendelian Randomization (MR) analysis for the causal effects of general adiposity and abdominal adiposity on knee osteoarthritis (KOA) and hip osteoarthritis (HOA). Methods: The general adiposity was assessed by body mass index (BMI), while the abdominal adiposity was evaluated with waist circumference (WC), hip circumference (HC), and waist-to-hip ratio (WHR). The data used in this two-sample MR analysis originated from genome-wide association studies (GWAS). Significant (p < 5 × 10⁻⁸) and independent (r² < 0.01) single-nucleotide polymorphisms were selected as instrumental variables for the MR analysis. Subsequently, this study used the inverse variance weighted, weighted median, and other methods for the causal inference, and the results were presented as odds ratios (OR). Moreover, sensitivity analyses were conducted to assess the stability and reliability of the results. Results: The MR results revealed positive causal effects of BMI on KOA (OR: 1.694; 95% CI: from 1.492 to 1.923; p = 3.96 × 10⁻¹⁶) and HOA (OR: 1.412; 95% CI: from 1.196 to 1.666; p = 4.58 × 10⁻⁵). Additionally, WC and HC both positively and causally related to KOA (WC: OR: 1.827; 95% CI: from 1.564 to 2.134; p = 2.68 × 10⁻¹⁴; HC: OR: 1.610; 95% CI: from 1.357 to 1.912; p = 5.03 × 10⁻⁸) and HOA (WC: OR: 1.491; 95% CI: from 1.254 to 1.772; p = 5.85 × 10⁻⁶; HC: OR: 1.439; 95% CI: from 1.205 to 1.719; p = 5.82 × 10⁻⁵). However, no causal relationship existed between WHR and obesity. These results were robust according to the sensitivity analyses. Conclusions: This study indicated that both general and abdominal obesity had positive causal effects on knee osteoarthritis and hip osteoarthritis.

Background: The role of the intestinal microbiota in the pathogenesis of inflammatory bowel disease combined with axial spondyloarthritis (axSpA) is gaining widespread interest. Aims: This study was conducted to investigate the clinical and fecal microbiota characteristics of patients with both ulcerative colitis (UC) and axSpA. Methods: Clinical data were collected from patients with UC. Patients were divided into the axSpA and non-axSpA groups according to human leukocyte antigen-B27 serology and sacroiliac joint imaging results. We obtained fecal specimens from 14 axSpA and 26 non-axSpA patients. All samples underwent 16S ribosomal DNA sequencing. Results: Seventy-three patients with UC were included in this study, and the axSpA incidence was 19.2%. This incidence was significantly higher in patients with C-reactive protein > 10 mg/L. Firmicutes and Faecalibacterium abundances were decreased, and Proteobacteria and Escherichia_Shigella abundances were increased in the axSpA group compared with those of the non-axSpA group. Indicator analysis showed that Escherichia_Shigella was more likely to be an indicator species of axSpA. Additionally, many biosynthetic and metabolic pathways, including glutathione metabolism, fatty acid degradation, geraniol degradation, and biosynthesis of siderophore group nonribosomal peptides, were upregulated in the axSpA group. Conclusion: Patients with UC have a high axSpA incidence, which may be related to the relative abundances of Escherichia_Shigella in these patients. The abundances of various biosynthetic and metabolic pathways of the fecal flora were upregulated in patients with axSpA.