ISSUE FOCUS 40 FEED & ADDITIVE MAGAZINE February 2023 sites in the body. Lo et al. (2021) advocated, first, a direct mechanism via translocation of gut microbes, their components and/or their metabolites into the circulation and, second, an indirect mechanism in which stimulation of epithelial, stromal, or immune cells within the gut results in downstream responses that are relayed systemically. Thus, any substantial change in the composition of the intestinal microbiota, whether due to infections by pathogenic microorganisms, nutritional imbalances, or the presence of antinutritional factors in the feed, can lead to immune responses both locally and systemically affecting other vital organs. Furthermore, the gut includes many neurons, intestinal hormones and secondary transmitters that participate in the regulation of numerous physiological functions of the animal. The association between the enteric nervous system and the higher centers via the parasympathetic nervous system and/or endocrine system also plays a key role in animal health (Moeser et al., 2017). Thus, the brain and gut are connected and participate in bidirectional communication, this network collectively known as the gut-brain axis. The gut-brain axis is therefore another way in which gut health can influence the general health and well-being of food-producing animals, which is raising a growing interest in animal production (Cao et al., 2021; Fan et al., 2021) and will certainly generate a great wealth of literature in the future. WHY GUT HEALTH IS PARTICULARLY IMPORTANT IN YOUNG ANIMALS During the first days or weeks of life, depending on the animal species, the immature gut of young animals goes through growth and development rates much higher than those of the rest of the body, to become a so-called “supply” organ that provides nutrients to the body for the rest of the animal’s life. The immaturity of the young gut as well as its hectic changes make this organ much more susceptible to suffering imbalances between its components and, consequently, compromises its health. This situation is exacerbated by the high nutritional requirements at these early ages in modern intensive animal production. A young and immature gut, in frenzy of growth and development, is being asked for high performance as a “supply” organ… what a time bomb! The two main weapons to safeguard gastrointestinal health in young animals are the mucosal immune system and GALT as well as the microbiota. Young animals need time to develop both a complex bacterial community and their immature gastrointestinal immune system, and until such developments have taken place, they are vulnerable to the presence of potential pathogens, toxins or dietary antinutritional factors in their gut. The fact of the matter is to provide our young animals with the required nutrients without straining the machinery of their guts. DIETARY PROTEIN, PART OF THE BASIS OF GUT HEALTH IN YOUNG ANIMALS Dietary protein, and its constituent amino acids, are the nutrients with the highest requirements in young animals since they are crucial for intense muscle and organs’ growth at an early age. On the other hand, the gastrointestinal tract of young animals is not able to digest dietary protein as well as that of adult animals. The gut maturing process can take days (chicks), weeks (piglets) or even months (calves), to reach a totally competent gut.
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