<p>Introduction: Environmental pollution by microplastics (MPs) is expanding, and there are concerns about the health effects of MPs and the chemicals contained in MPs. This study focuses on intestinal inflammation (Leaky Gut syndrome; LGS) caused by consumption of high-fat diets, and investigates the effects of MP on obesity and diabetes to clarify the toxicity of MP on the metabolic system.</p><p>Methods: C57BL6/J mice fed a high-fat diet (HFD) with or without polystyrene MPs were examined for effects on the metabolic disorder and LGS. Fluorescent-labeled MPs standard were orally administered to the particle-exposed groups.</p><p>Results: There was no significant weight difference between the HFD and HFD+MPs groups, but glucose intolerance was significantly exacerbated in the HFD+MPs group, and liver enzymes and serum lipids were elevated. Significant worsening of fatty liver was observed. Fluorescence microscopy showed that labeled MPs accumulated in the intestinal mucosa of the small intestine in the HFD+MPs group, and the concentration of short-chain fatty acids in the feces was significantly decreased. In addition, flow cytometry was used to evaluate the dynamics of inflammatory cells in the lamina propria of the small intestinal mucosa, which showed a significant increase in type 1 innate lymphoid cells and a significant decrease in type 3 innate lymphoid cells in the HFD+MPs group, and inflammation-related genes were significantly increased in the HFD+MPs group, while Il22 expression was decreased.</p><p>Conclusion: MPs induce intestinal inflammation and exacerbates various metabolic disorders, including impaired glucose tolerance.</p>