As environmental plastic waste degrades, it creates an abundance of diverse microplastic particles. Consequently, microplastics contaminate drinking water and many staple food products, meaning the oral ingestion of microplastics is an important exposure route for the human population. Microplastics have long been considered inert, however their ability to promote microbial dysbiosis as well as gut inflammation and dysfunction suggests they are more noxious than first thought. More alarmingly, there is evidence for microplastics permeating from the gut throughout the body, with adverse effects on the immune and nervous systems. Coupled with the now-accepted role of the gut-brain axis in neurodegeneration, these findings support the hypothesis that this ubiquitous environmental pollutant is contributing to the rising incidence of neurodegenerative diseases, like Alzheimer’s disease and Parkinson’s disease. This comprehensive narrative review explores the consequences of oral microplastic exposure on the gut-brain-axis by considering current evidence for gastrointestinal uptake and disruption, immune activation, translocation throughout the body, and neurological effects. As microplastics are now a permanent feature of the global environment, understanding their effects on the gut, brain, and whole body will facilitate critical further research and inform policy changes aimed at reducing any adverse consequences.

Humans are chronically exposed to microplastics (plastic particles < 1 mm) via oral ingestion. Yet, intestinal absorption of microplastics requires robust quantification and in vitro studies have largely lacked digestive conditions in experimental design. This study aimed to quantify how simulated digestion of microplastics affected the rate of internalisation by an intestinal epithelial cell line (IEC-6). Spherical 0.56 µm polystyrene microplastics were simulatively digested using a static digestion protocol. IEC-6 cells were exposed to 1 µg/mL of pristine or digested microplastics for 24 h. Microplastic internalisation was visualised with confocal microscopy and novelly quantified by imaging flow cytometry. Confocal microscopy confirmed internalisation of pristine and digested microplastics by IEC-6 cells, where pristine microplastics were internalised by 62.2 ± 11.8 % of cells which was lessened to 24.6 ± 5.3 % for digested microplastics. Most IEC-6 cells internalised 1-2 microplastics, and internalisation of > 2 microplastics per cell was increased (p < 0.01) after exposure to pristine microplastics (34.3 ± 10.3 %) compared to digested microplastics (8.9 ± 0.74 %). This study demonstrated that simulated digestion of polystyrene microplastics reduced internalisation by IEC-6 cells, providing unprecedented insights into the gastrointestinal effect of microplastic exposure and raising important considerations for future research.

Microplastics are an environmental and dietary contaminant of significant concern for human health due to chronic exposure and emerging evidence of cellular impact. However, understanding of this impact is limited by challenges in their detection and identification in biological samples. This study aimed to use Optical Photothermal Infrared Spectroscopy (O-PTIR) to detect and characterize real-world microplastics internalized by intestinal cells, and to investigate the cellular impact of such exposure. An intestinal epithelial cell line (IEC-6) was exposed to physically degraded fluorescent microplastics (25-100 μg/mL) made of four polymers for 24 h. O-PTIR reference spectra were acquired on single dyed primary plastics before fluorescent guided O-PTIR was used to identify and characterize microplastics in IEC-6 cells. We determined that the limit of resolution under these conditions was particles of ∼ 1 μm. Microplastics from all polymer types were successfully detected and discriminated using O-PTIR data alone. Importantly, principal component analysis of cytoplasmic O-PTIR spectra was used to identify microplastic-induced metabolic shifts. This chemometric analysis suggested that microplastic exposure resulted in oxidative stress and metabolic disruption, indicated by conformational protein changes-including early signs of misfolding and aggregation-which are hallmarks of a stress-adaptive cellular response. Our findings demonstrate that O-PTIR is an efficient, non-destructive technique for label-free microplastic studies in biological samples, enabling the identification, localization and characterization of heterogeneous microplastics, while simultaneously revealing their impact on cellular function and composition.

Microplastics are pervasive environmental pollutants that pose a growing concern for human health. Oral ingestion is a common route of human microplastic exposure, yet the proteomic response of the gut epithelium to microplastics remains unclear. This study aimed to investigate the cellular effects of pristine and artificially digested microplastic exposure in primary rat duodenal epithelial cells using untargeted proteomics. Cells were exposed to pristine or digested 0.5 µm polystyrene microplastics at 10 or 100 µg/mL for 72 hours and were then analyzed by tandem liquid chromatography and mass spectrometry (LC-MS). Proteins that were both significantly different in intensity compared to controls, with a threshold change of 1.3 or greater, were considered to be differentially expressed. This criterion identified 41 differentially expressed proteins after 100 µg/mL pristine MP exposure, with 19 downregulated and 21 upregulated. Following exposure to 100 µg/mL digested MP, only 3 differentially expressed proteins were upregulated and 7 were down regulated, demonstrating the impact of microplastic physicochemistry. FGSEA pathway analysis revealed that 270 Reactome pathways were significantly altered following microplastic exposure in either condition at both concentrations. These pathways contributed to functional domains including protein synthesis, DNA replication, cell cycle control and aerobic respiration. Overall, microplastic exposure was associated with upregulated mitochondrial respiration, and downregulation of nuclear-related processes including DNA synthesis, transcription and cell proliferation. This study provides targets for future investigation (mitochondria and nucleus) and emphasizes the need to consider biological and environmental conditions for in vitro models of microplastic exposure.