The Role of Yeast Saccharomyces cerevisiae in Supporting Gut Health in Horses: An Updated Review on Its Effects on Digestibility and Intestinal and Fecal Microbiota

SIMPLE SUMMARY: In horses, a healthy state of the gastrointestinal tract (GIT) is of utmost importance to support animals’ health and performance. Several factors combine to influence the equilibrium of the GIT, including the composition of the diet, feeding management, the use of therapeutic antimicrobials, as well as biotic and abiotic stressors. Such factors can lead to an imbalance of the intestinal microbial populations and their functions and, subsequently, to reduced nutrient digestion, finally impacting the overall health of the animals. Several feed additives are used to support gut health; in horse nutrition, yeast is one of the additives most used as a supplement in the diet. Different studies showed that the addition of yeast to horse diets is able to enhance the nutrient digestibility of feeds by modulating intestinal microbial populations. Specifically, yeasts seem to act mainly upon fibrolytic and amylolytic bacteria, although, at the present moment, their role is still debated. The aim of this review is to provide the reader with an updated overview of the effects of yeast administration on nutrient digestibility and the intestinal microbial population, and to furnish an overview of yeast application in the field. ABSTRACT: To support the overall health of horses, it is essential to maintain an optimal gut health (GH) status, which encompasses several physiological and functional aspects, including the balance and functionality of intestinal microbial populations and, accordingly, the effective digestion and absorption of nutrients. Numerous biotic and abiotic stressors can lead to an imbalance of GH, such as the quality of forages and the composition of diet, e.g., the inclusion of high energy-dense feeds to meet the energy requirements of performance horses. To support the digestive function and the intestinal microbial populations, the diet can be supplemented with feed additives, such as probiotic yeasts, that promote the ability of cellulolytic bacteria in the hindgut to digest the available fiber fractions, finally increasing feed efficiency. Among the different yeasts available, S. cerevisiae is the most used in horses’ nutrition; however, results of digestibility trials, as well as data on intestinal and fecal microbial populations, are sometimes contradictory. Therefore, the purpose of this review is to summarize the effects of S. cerevisiae on in vivo and in vitro digestibility, providing an updated overview of its effects on the intestinal and fecal microbial population.