## 4 Discussion  
### Animal Models in Human Studies  
Animal models help explain the nature of phenomena occurring in living organisms. In the case of _Gallus domesticus_, research has been carried out on adults, but some experiments have also been performed on embryos; as a result, research has been conducted on developmental biology, virology, immunology, oncology, the epigenetic regulation of gene expression, and human diseases using this model organism [4; 48; 49; 50]. [PERSON] and [PERSON] [51] studied this animal model and linked dysbiosis in the cecum with the occurrence of aggressive behaviour in poultry, the mechanism of which can also be compared with that underlying dysbiosis of the large intestine in humans, as observed in neuropsychiatric studies. The hindgut seems to be the most promising part of the digestive tract of broiler chickens where studies can be conducted on the use of dietary fibre and its effect on the development of individual sections, as well as its relationship with microflora and the stress behaviours related to fibre balance and microbial dysbiosis caused by stressors [52]. [PERSON] et al. [53], in their meta-analysis and systematic review, emphasised that low concentrations of dietary fibre in a high-fat-and-sugar diet often result in type 2 diabetes. [PERSON] and [PERSON] [54], on the other hand, noted an association among microbial dysbiosis in the colon, metabolite production, and an increase in the occurrence of colorectal cancer. Of course, due to the specificity of animals' nutrition and the structure of their digestive tracts, not all processes contributing to human nutrition can be replicated in animal models. However, processes that occur in some sections take similar courses, especially when taking into account the structure of the epithelium and its relationship with the symbiotic microorganisms in a given section, which affect the maintenance of intestinal health and cooperation with the body's immune system [55]. However, as highlighted by [PERSON] et al. [56], from the point of view of gut health, in the case of poultry, Firmicutes and Bacteroidetes are the dominant groups of microorganisms, while in the case of humans, their ratios constitute a health/metabolism-associated marker. These similarities allow for the results from model organisms to be applied to human studies and, subsequently, for microflora and epithelium regeneration methods to be created. Damage to the microflora and epithelia can also be induced by stress, as observed in studies by [PERSON] et al. [57], where an AA broiler model was used to determine improvements in the intestinal barrier using _Bacillus subtilis_ M6; the results of that study can also be applied to human intestinal barriers. However, in the case of _Gallus domesticus_, the physiological structure of the upper digestive tract has practically no similarities to the structure of the human digestive tract: Their gizzards compensate for their lack of teeth, the crop helps with storing and preparing food for further digestive processes, and only the glandular stomach performs similar functions to those of the human stomach [58]. The situation is different in the intestines, where processes are comparable with those of other monogastric animals, in this case, with the specificity of digestion in humans. In the case of the cecum, their processes are similar to those occurring in the human colon and are used in research on reducing dysbiosis in this region, with a simultaneous effect on the regeneration of the epithelium during the reconstruction of the mucin layer [56]. Additionally, the digestibility and absorption of nutrients can be studied up to the end of the ileum; the results of such comparisons can be used to create models of processes occurring in the human digestive tract that result in digestive disorders, as well as methods to repair the interaction between digestive microflora and the immune system.