Characterization of β-Glucans from Cereal and Microbial Sources and Their Roles in Feeds for Intestinal Health and Growth of Nursery Pigs

SIMPLE SUMMARY: The use of antibiotics in animal feeds has been phased out due to concerns surrounding microbial resistance to antibiotics. β-glucans have been shown to improve the intestinal health and growth performance of nursery pigs. β-glucans are non-starch polysaccharides originating from the cell walls of various sources including yeast, bacteria, fungi, and cereal grains. Depending on the sources and dose levels of β-glucans, however, their impacts on intestinal health and growth were not consistent due to the quantitative, compositional, and structural differences of β-glucans. Cereal grains-based diets provide high amounts of soluble fractions of β-glucans, causing digesta viscosity in the GIT of pigs and interfering with the nutrient digestion and intestinal health of pigs. Microbial β-glucans, however, showed positive effects on the intestinal health and growth of nursery pigs. Microbial β-glucans affect the intestinal immune system through activating dectin-1 and toll-like receptors related to the intestinal health of nursery pigs. Therefore, this review investigated the quantitative, compositional, and structural differences of β-glucans and the functional roles of β-glucans in the intestinal health and growth efficiency of nursery pigs. ABSTRACT: The objectives of this review are to investigate the quantitative, compositional, and structural differences of β-glucans and the functional effects of β-glucans on the intestinal health and growth of nursery pigs. Banning antibiotic feed supplementation increased the research demand for antibiotic alternatives to maintain the intestinal health and growth of nursery pigs. It has been proposed that β-glucans improve the growth efficiency of nursery pigs through positive impacts on their intestinal health. However, based on their structure and source, their impacts can be extensively different. β-glucans are non-starch polysaccharides found in the cell walls of yeast (Saccharomyces cerevisiae), bacteria, fungi (Basidiomycota), and cereal grains (mainly barley and oats). The total β-glucan content from cereal grains is much greater than that of microbial β-glucans. Cereal β-glucans may interfere with the positive effects of microbial β-glucans on the intestinal health of nursery pigs. Due to their structural differences, cereal β-glucans also cause digesta viscosity, decreasing feed digestion, and decreasing nutrient absorption in the GIT of nursery pigs. Specifically, cereal β-glucans are based on linear glucose molecules linked by β-(1,3)- and β-(1,4)-glycosidic bonds with relatively high water-soluble properties, whereas microbial β-glucans are largely linked with β-(1,3)- and β-(1,6)-glycosidic bonds possessing insoluble properties. From the meta-analysis, the weight gain and feed intake of nursery pigs increased by 7.6% and 5.3%, respectively, through the use of yeast β-glucans (from Saccharomyces cerevisiae), and increased by 11.6% and 6.9%, respectively, through the use of bacterial β-glucans (from Agrobacterium sp.), whereas the use of cereal β-glucans did not show consistent responses. The optimal use of yeast β-glucans (Saccharomyces cerevisiae) was 50 mg/kg in nursery pig diets based on a meta-analysis. Collectively, use of microbial β-glucans can improve the intestinal health of nursery pigs, enhancing immune conditions, whereas the benefits of cereal β-glucans on intestinal health were not consistent.