CXCL8 and chronic obstructive pulmonary disease: Causal inference regarding the relationship between the gut microbiota metabolite Phenylacetylglutamine, the upstream gut microbiota Lachnospiraceae, and downstream related genes CXCL8, EGFR, and IL6 with COPD revealed a positive causal relationship between Phenylacetylglutamine and COPD.