## 5 Discussion  
We reviewed 17 studies, most of which focused on mice and rats, with very few on guinea pigs and pigs. The primary plastic studied was polystyrene. One indicated that plastic absorption was influenced by the age of the individual. Individuals younger than 3 weeks old showed higher tissue absorption, despite having shorter and thinner intestines, resulting in a smaller surface area for plastic absorption. However, 7-week-old individuals showed similar tissue absorption to 3-week-old individuals, even with a longer and thicker gut. The mentioned study found that 7-week-old individuals absorbed the most plastic compared to 3- and 15-week-old individuals. Increasing age was linked to thinner intestinal segments and a decrease in the absorption rate [120]. Research conducted on mice for 28 days revealed that PS accumulated in the liver, kidneys, and intestines, leading to inflammation, increased hepatic lipid peroxidation, hepatic metabolic disorder, oxidative stress, neurotoxicity, the infiltration of inflammatory cells in the colon, and the increased expression of pro-inflammatory cytokines. Additionally, it was found that there was increased intestinal permeability and decreased mucus secretion [121; 127; 135]. Furthermore, administering PS over a longer period of time (5-6 weeks) resulted in gut microbiota dysbiosis, decreased mucus secretion in the colon, changes in bile acid metabolism, and gut barrier dysfunction. This was accompanied by the increased expression of inflammatory factors, intestinal immune imbalances, and histological lesions in the intestinal mucosa [123; 124; 129]. In juvenile rats, research identified a decreased rate of growth in body weight and in the organ indices of the kidneys, heart, and ovaries. It was also observed that there were changes at the serum level, with an increase in blood urea nitrogen, creatinine, and proinflammatory mediators IL-1b, IL-6, and TNF-a [132]. Pregnant and lactating females were involved in research with PS to show the long-term harmful effects of MPs. This study focused on the intestine, which experienced changes in intestinal barrier function and imbalances in gut microbiota. These changes affected the health of the mothers and, as a result, the health of the offspring [85]. The presence of negatively charged carboxylated and positively charged amine PS influenced the changes that occurred after intestinal absorption. These changes included marked dysbiosis of the gut microbiota, decreased body weight, and increased levels of ALP, AST, T-Bil, CK, r-glutamine transferase, and serum creatinine. Exposure to PS-NH2 amine PS also resulted in damage to intestinal crypts, the disappearance of intestinal villi, the thinning of the walls of the intestine and stomach, and the presence of obvious layers of inflammatory exudates [128]. The MP polyethylene can be toxic when combined with other substances, such as organophosphorus flame retardants. These interactions can disrupt amino acid and energy metabolism, leading to oxidative stress and neurotoxicity [122]. Additionally, PE contaminated with phthalate esters can cause intestinal inflammation, increased intestinal permeability, and an altered gut microbiota [125]. Micoplastic modifications of polyethylene (PE-MP) appeared after repeated exposure to the toxicant, leading to granulomatous inflammation in the lungs and the presence of microplastics in the lungs, stomach, duodenum, ileum, and serum [130]. Polyethylene terephthalate (PET) was found to have no toxic effects on rats, as there were no clinical signs of toxicity or changes in specific markers of liver, cardiac, and renal function. However, there were increased levels of oxidative stress indicators. In pet pigs, the presence of microplastics led to changes in the enteric nervous system and in the histological structure of the duodenum. Notably, these changes were more significant in the group of animals receiving a dose of 1 g/day compared to those receiving 0.1 g/day [133; 134].