ISSUE FOCUS FEED & ADDITIVE MAGAZINE October 2023 27 EFFECTIVE DIGESTION AND ABSORPTION Improved gastrointestinal functionality is often accompanied by improved digestion and absorption, however, one might pose the question whether this is a direct cause or consequence of the observed improvement of GIT functionality (Khadem et al., 2016). Also, one needs to consider that intestinal inflammation negatively impacts GIT function. Indeed, a proliferation in pathogens in the GIT can lead to an increase of defensive responses which is accompanied by a decrease in digestive efficiency and reduced absorption on both macro and micronutrients. Malabsorption is most noticeable for micronutrients, especially iron and zinc (Davin et al., 2012, 2013), while amongst macronutrients, fat is the one that is impacted the most (Koutsos et al., 2003, 2006). Malabsorption can be ascribed to increase in passage rate of digesta along the GIT, hence reducing the time available for nutrients to be digested and absorbed. Malabsorption can also be the consequence of the loss of integrity and function of the GIT barrier, and of a direct response coordinated towards the pathogen by the immune system (Klasing, 2007). GIT MICROBIOTA The intestinal microbiota contributes to several physiological functions (Salzman et al., 2007; Lee and Hase, 2014; Marchesi et al., 2016; Round and Mazmanian, 2009) such as protective, structural and metabolic functions. The intestinal microbiota contributes to the regulation of the host homeostasis by contributing to optimal digestion and absorption, regulation of energy metabolism, prevention of mucosal infections and modulation of the immune system. Consequently, dietary manipulation of the GIT microbiota composition represents an attractive tool to prevent intestinal issues and to promote animal performance. Nutritional interventions should be designed to promote conditions in the GIT that would create and maintain a balance between the host and the GIT microbiota, and to prevent disturbance of the structure and function of the GIT. What are the factors that influence the development of the GIT microbiota in farm animals? What is the role of the GIT microbiota in the relationships between animal nutrition, GIT physiology, animal health and welfare? While these questions are not easy to answer in a short time, for them to be addressed in their entirety, a multidisciplinary system-based approach must be adopted. It will be imperative to address these questions in order to provide some needed help in the development of alternatives to AGPs. This approach will be crucial to make dietary intervention strategies with new and existing products possible. GUT MUCOSA The GIT barrier is composed of the mucus layer overlying a monolayer of intestinal epithelial cells and an underlying set of cells composed of mesenchymal cells, dendritic cells, lymphocytes and macrophages constituting the gut associated lymphoid tissue (GALT). Intestinal epithelial cells (IEC) are central in this system as they secrete and regulate the composition of the mucus layer as well as they interact with the underlying cells. The GIT mucosal epithelium and the associated GALT are the first sites in the host that are exposed to variations in nutrient intake. While, relativeFigure 1. Main components contributing to gastrointestinal functionality.
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