## 1 Introduction  
Animal models provide valuable information about the processes occurring in living organisms, enabling their use, at least partially, in understanding processes in the human body where, for bioethical reasons, such studies are impossible to conduct [1; 2; 3; 4]. [PERSON] and [PERSON] [5] suggest that, after mice and rats, _Gallus domesticus_ is the third most common species applied in medical research. Taking into consideration the similarity between the gastrointestinal tracts of broiler chicken and humans, _G. domesticus_ is used in experiments to provide information on processes related to gastrointestinal tract development, especially in the case of the small intestines' response to different types of dietary fibre and when modulating the immunology and microbial status of animals. Moreover, in recent years, they have been used as animal models to investigate the effect of light pollution in a room on intestinal injury [6; 7; 8]. Of course, similar experiments with soluble and insoluble fibre were conducted with rats and mice, where the effect on body weight gains and adipose tissue were assessed in the small intestine [9; 10]. Therefore, animal models of _G. domesticus_ can be a useful alternative to those of rats or mice for describing the impact of soluble and insoluble dietary fibre on the stabilisation of microflora as well as metric changes in the small intestine depending on the inclusion and type of dietary fibre. These types of animal models can also be useful for understanding nutrition in younger people with intestinal problems, where rebuilding the microbiome and structures of the intestinal mucosa as a result of nutritional disorders, gastrointestinal diseases, and the limited absorption of nutrients in people with obesity is possible [11; 12]. From the nutritional point of view for broiler chickens, concentrate mixtures based on the use of post-extraction soybean meal and a high share of wheat grain may lead to a deficiency in dietary fibre (DF) necessary for the proper functioning of both the upper and lower digestive tracts [13; 14; 15; 16]. A high amount of wheat, triticale, or barley grain in the diet, especially in young animals, may have an antinutritional effect due to the presence of arabinoxylans, which are non-starch polysaccharides (NSPs), which lower the passage rate of digesta [17; 18]. These compounds are present in cereal grains in soluble and insoluble forms, but their proportion affects the physicochemical properties of the food content, such as its viscosity, solubility, water-holding capacity, fermentability, bulk, and ability to bind bile acids [19; 20]. One way to limit the action of soluble fibre contained in wheat grain in the small intestine is to add components rich in structural carbohydrates, not exceeding an amount of 3% in diets for broiler chickens depending on the age of the birds [21; 22; 23]. Taking into account experiments with monogastric animals, a better definition of dietary fibre was stated in an analysis conducted by [PERSON] [24], taking into account total (TDF), insoluble (IDF), and soluble (SDF) dietary fibre. By-products of the agro-food industry show different contents of soluble and insoluble fibre [25; 26]. The component most rich in insoluble fibre is out hull (OH), composed of lignocellulosic, the main component; insoluble cellulose (35-45%); hemicelluloses (32-35%); and acid-insoluble lignin (17-20%) [27; 28; 29]. On the other hand, sunflower hull (SH), soybean hull, sugar beet pulp (SBP), and wheat bran (WB) show higher amounts of SDF [30; 31]. The differences in the proportions of the components of SDF and IDF affect the production parameters of birds; the length and weight of select digestive sections, as well as their associated organs; and consequently, the health of the birds, particularly their 'gut health' [19; 32; 33]. Studies related to the effect of different fibre sources on the gastrointestinal tract of birds are not new, and comparisons have been performed earlier by many research teams [34; 35; 36; 37]. [PERSON] et al.