Dietary Fiber, Gut Microbiota, Short-Chain Fatty Acids, and Host Metabolism

With the rapid development of gut microbiological research and high-throughput sequencing technology, we have gained a better understanding of the effects of the gut microbiota and its metabolites such as short-chain fatty acids (SCFAs) on the metabolism of hosts. This effect was found closely related with the consumed dietary fiber by hosts. Dietary fiber has been proven to be very important for hosts. However, hosts such as human, chickens and other monogastric animals cannot digest dietary fiber due to a lack of endogenous fiber-degrading enzymes; therefore, they must rely on gut microorganisms who own endogenous fiber-degrading enzymes such as carbohydrate-active enZymes (CAZymes) encoded by gene. Excellent fiber-degrading bacteria include members of Bacteroidetes phylum such as Bacteroides and Prevotella and members of Firmicutes phylum including Ruminococcus, Fibrobacter, Butyrivibrio, Ruminiclostridium and so on. These fiber-degrading bacteria degrade fiber into monosaccharides via different degrading mechanisms. For instance, Bacteroidetes degrade a dozen kinds of plant fiber using its unique arm-polysaccharide utilization locus (PUL). In contrast to Bacteroidetes, members of the Firmicutes use gram-positive PULs (gp PULs) to process fiber. Some members of the Firmicutes can degrade cellulose and hemicellulose through the cellulosome pathway. And then some oligosaccharides and glucose produced by dietary fiber degradation can be used as carbon and energy sources for microbial growth, thus increasing the diversity of microorganisms. Dietary fiber is the substrate of gut microorganisms. The left monosaccharides are fermented into short-chain fatty acids (SCFAs) by SCFA-producing bacteria including Bifidobacterium, Phascolarctobacterium, Faecalibacterium and so on via different pathways. SCFAs mainly include acetate, propionate and butyrate. SCFAs can further regulate the host's metabolism including energy metabolism, host appetite, liver metabolism and the glucose balance via SCFA receptors including GPR41 and GPR43 or other mechanisms. Therefore, gut microorganisms are also called our "second genome" or "forgotten organs". In this paper, we provide an overview of the interactions among dietary fiber, gut microbiota, SCFAs and host metabolism.