Citation: Szabó, C.; Kachungwa Lugata, J.; Ortega, A.D.S.V. Gut Health and Influencing Factors in Pigs. Animals 2023, 13, 1350. https://doi.org/10.3390/ ani13081350 Academic Editor: Hyeun Bum Kim Received: 3 March 2023 Revised: 7 April 2023 Accepted: 11 April 2023 Published: 14 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). animals Review Gut Health and Influencing Factors in Pigs Csaba Szabó 1, * , James Kachungwa Lugata 1,2 and Arth David Sol Valmoria Ortega 1,2 1 Department of Animal Nutrition and Physiology, Faculty of Agriculture and Food Sciences and Environmental Management, University of Debrecen, Böszörményi Street 138, 4032 Debrecen, Hungary 2 Doctoral School of Animal Science, Faculty of Agriculture and Food Sciences and Environmental Management, University of Debrecen, Böszörményi Street 138, 4032 Debrecen, Hungary * Correspondence: szabo.csaba@agr.unideb.hu Simple Summary: The gastrointestinal tract of any organism is not only important from a digestion and nutrient absorption point of view, but it interacts in many ways with metabolism. Our knowledge regarding these connections is increasing continuously. Animals capable of intensive production to supply sufficient food are the most vulnerable to disruptions in the gut. Therefore, the knowledge of effective feed additives which can be used to avoid or cure these problems and their consequences is important in modern animal production systems. Abstract: The gastrointestinal tract (GIT) is a complex, dynamic, and critical part of the body, which plays an important role in the digestion and absorption of ingested nutrients and excreting waste products of digestion. In addition, GIT also plays a vital role in preventing the entry of harmful substances and potential pathogens into the bloodstream. The gastrointestinal tract hosts a significant number of microbes, which throughout their metabolites, directly interact with the hosts. In modern intensive animal farming, many factors can disrupt GIT functions. As dietary nutrients and biologically active substances play important roles in maintaining homeostasis and eubiosis in the GIT, this review aims to summarize the current status of our knowledge on the most important areas. Keywords: pig; gastrointestinal tract; feed additives 1. Introduction The pigs’ gastrointestinal tract (GIT) or gut is vital in processing food materials into absorbable nutrients that their body needs for maintenance and production. Aside from its digestive and absorptive function, the pigs’ GIT is vital in maintaining immune homeostasis. Their intestine is considered the largest immune organ in their body, accounting for more than 70% of the immune cells in the body [1–3]. Moreover, the porcine intestine has many microorganisms contributing to intestinal mucosal immunity. There is an interplay between the gut microbiome and the gut immune system, as the latter can regulate intestinal microorganisms’ distribution and composition and intestinal microbiota homeostasis by secreting various immune effector factors. Conversely, intestinal microorganisms can also promote the differentiation of immune cells, including regulatory T cells, through their specific components or metabolites [4]. Therefore, a fully functional and healthy GIT or gut is critical for the pigs’ welfare and production efficiency at every stage of life [5]. A healthy GIT can be achieved when the pig has effective digestion and absorption of food, the absence of gastrointestinal illness, normal and stable intestinal microbiota, an effective immune system, a state of well being, and is less susceptible to pathogenic bacteria and viruses [6,7]. Although the pigs’ GIT serves immune functions, it is still vulnerable to disruptions in any pig’s life. Persistent challenges are evident in modern pig production, which is relatively due to diet, management, and the living environment of the pigs [4]. Pigs at different stages of life can face gut-associated problems often exacerbated by the Animals 2023, 13, 1350. https://doi.org/10.3390/ani13081350 https://www.mdpi.com/journal/animals