Effects of Different Types of Dietary Fibers on Lipid Metabolism and Bile Acids in Weaned Piglets

SIMPLE SUMMARY: Dietary fiber, as an essential nutrient, is very important for the health of animals and it is necessary to study the effects of dietary fiber on animal physiology and its mechanism. In the swine industry, the excessive deposition of body lipids in pigs has a negative impact on the lean meat rate. Previous studies showed that dietary fibers can inhibit lipid accumulation in pigs during the growing-finishing period, but it remains unclear what the effect of fiber is on the lipid metabolism in weaning pigs. Thus, this study investigated how dietary fiber regulates lipid metabolism in piglets. We found that dietary fibers had abilities to reduce lipid synthesis in piglets. Additionally, increasing evidence suggests that bile acids are associated with body lipid metabolism, and some bile acids (hyocholic acid species) are the key factors for the regulation of lipid metabolism. In this study, dietary fibers, especially fructooligosaccharides, elevate the levels of hyocholic acid species possibly via the gut microbiota and its enzymes. The bile acids altered by dietary fiber supplementation are closely correlated with lipid metabolism. These results deepen our understanding of dietary fibers and bile acids, and provide a reference for the application of them in the livestock industry. ABSTRACT: The aim of this study was to investigate the effects of dietary fiber on the serum biochemistry, bile acid profile, and gut microbiota in piglets. Twenty-four pigs (initial body weight: 10.53 ± 1.23 kg) were randomly divided into three treatments with eight replicate pens of one pig per pen for 21 d. The dietary treatments consisted of the following: (1) a fiber-free diet (NS); (2) a fiber-free diet + 3% fructooligosaccharides (SI); (3) a fiber-free diet + 3% dietary fiber mixture (fructooligosaccharides, long-chain inulin, and microcrystalline cellulose at the ratio 1:1:1; MIX). The results showed that compared with the NS group, the 3% SI diet reduced the serum total cholesterol (TC) concentration of the piglets (p < 0.05). The metabolomics results showed that the 3% SI diet increased the level of taurohyocholic acid (THCA) and α-muricholic acid, and the 3% MIX diet increased the level of THCA and cholic acid (p < 0.05). The use of 3% SI or MIX decreased the glycodeoxycholic acid (GDCA) level in the bile of the piglets (p < 0.05). The correlation analysis shows that the GDCA was positively related to the TC. The 16S rRNA gene sequencing results showed that UCG-002 and Holdemanella were enriched in the SI group, while Bacteroides was enriched in the MIX group. The microbial function prediction indicated that SI supplementation tended to elevate the relative abundance of gut bacteria capable of expressing bile acid-metabolizing enzymes. To sum up, the regulatory effect of dietary fiber on lipid metabolism is related to bile acids in piglets. Compared with MIX, SI is more likely to regulate bile acids through the gut microbiota.