Relationship between obesity and gut microbiota

Abstract

In this review, the relationship between obesity and gut microbiota was discussed in the terms of current literatures in order to draw attention to the role of gut microbiota in the etiology of obesity. In obesity, it is accepted that the intestinal microbota profile has changed, and the amount of bacteria in the Firmicutes phylum increased compared to Bacteroides. Firmicutes hydrolyze dietary polysaccharides that cannot be digested by the host and they cause more energy harvest and fat accumulation. Although there are many studies showing the relationship between obesity and bacterial profiles of different genus and species in Firmicutes phylum, the answer to the question of which profile is playing a key role in obesity has not been fully obtained. Previous studies indicated the less microbial diversity in gut microbiota of obese than that of lean individuals, however the current knowledge indicates contrast results. Functional dysbiosis is now more important than the diversity or abundance of gut microbiota. Different microbial metabolites cause disrupting lipid and glucose homeostasis and affecting the regulation of satiety. Gut microbiota facilitates nutrient absorption by increasing the density of capillary vessels in small intestinal villi and by activating G protein receptors of short-chain fatty acids (SCFAs). Thus, caloric intake increases. In addition, interaction with these receptors is involved in energy metabolism by regulating fasting glycemia by affecting insulin sensitivity in adipose tissue and peripheral organs. On the other hand SCFAs reduce the appetite by stimulating the secretion of glucagon-like peptide-1. SCFAs also stimulate the secretion of glucagon-like peptide-2 to proliferate intestinal epithelial cells and maintain barrier integrity, thereby reducing metabolic endotoxemia. Gut bacteria play a role in the synthesis of bile acids. By this way they cause triglyceride deposition in the liver. It has been reported that conjugated linoleic acid produced by some species of Bifidobacterium spp. and Lactobacillus spp. has anti-obesity effect, in particular, obesity due to aging decreases with the consumption of probiotic foods containing these bacteria. As a result; disruption of the microbial balance in the gut leads to an increase in intestinal permeability, caloric intake and levels of proinflammatory cytokines, and causes endotoxemia. Dysbiosis increases the risk for development of insulin resistance, excessive fat accumulation, obesity and metabolic syndrome. Proteomics and metabolomics studies will be useful to clarify role of microbiota in obesity.