Metabolites mediate the causal associations between gut microbiota and NAFLD: a Mendelian randomization study

Although gut microbiota and serum metabolite composition have been observed to be altered in patients with non-alcoholic fatty liver disease (NAFLD), previous observational studies have demonstrated inconsistent results. As this may be influenced by factors such as confounders and reverse causality, we used Mendelian randomization to clarify the causal effect of gut microbiota and blood metabolites on NAFLD.

In this research, we performed a two-step Mendelian randomization analysis by utilizing genome-wide association study (GWAS) data obtained from MiBioGen and UK Biobank. To mitigate potential errors, we employed False Discovery Rate (FDR) correction and linkage unbalanced regression (LDSC) analysis. Sensitivity analyses including cML-MA and bidirectional Mendelian randomization were performed to ensure the robustness of the results.

In this study, a total of nine gut microbiota and seven metabolites were found to be significantly associated with NAFLD. MR analysis of the above findings revealed a causal relationship between Ruminococcus2 and cysteine-glutathione disulfide (OR = 1.17, 95%CI = 1.006-1.369, P = 0.041), as well as 3-indoleglyoxylic acid (OR = 1.18, 95%CI = 1.011-1.370, P = 0.036). For each incremental standard deviation in Ruminococcus2 abundance, there was a corresponding 26% reduction in NAFLD risk (OR = 0.74, 95%CI = 0.61-0.89, P = 0.0012), accompanied by a 17% increase in cysteine-glutathione disulfide levels (OR = 1.17, 95%CI = 1.01-1.37, P = 0.041) and an 18% increase in 3-indoleglyoxylic acid levels (OR = 1.18, 95%CI = 0.81-1.00, P = 0.036). The proportion mediated by cysteine-glutathione disulfide is 11.2%, while the proportion mediated by 3-indoleglyoxylic acid is 7.5%.

Our study suggests that increased abundance of specific gut microbiota may reduce the risk of developing NAFLD, and this relationship could potentially be mediated through blood metabolites.