Effects of gut microbes on nutrient absorption and energy regulation

Abstract

Malnutrition may manifest as either obesity or undernutrition. Accumulating evidence suggests that the gut microbiota plays an important role in the harvest, storage, and expenditure of energy obtained from the diet. The composition of the gut microbiota has been shown to differ between lean and obese humans and mice; however, the specific roles that individual gut microbes play in energy harvest remain uncertain. The gut microbiota may also influence the development of conditions characterized by chronic low-level inflammation, such as obesity, through systemic exposure to bacterial lipopolysaccharide derived from the gut microbiota. In this review, the role of the gut microbiota in energy harvest and fat storage is explored, as well as differences in the microbiota in obesity and undernutrition.

Figure 1

Microbial H₂-producing and consuming reactions in the human intestine. SCFA, short-chain fatty acid.

Figure 2

The gut microbiome has a regulatory function on host energy metabolism. By breaking down nondigestible polysaccharides, gut microorganisms produce monosaccharides and short-chain fatty acids (SCFAs). SCFAs bind to GPR 41/43 receptors and stimulate peptide YY (PYY) production, which inhibits gut motility and allows gut microbes to digest more polysaccharides. Gut microbes also regulate energy metabolism by reducing the expression of fasting-induced adipocyte factor (Fiaf) from gut epithelial cells. Suppressed Fiaf release results in the degradation of lipoproteins and deposition of free fatty acids in adipose tissues. The adiposity in liver and skeletal muscles is also regulated by microorganisms through the changes of phosphorylated adenosine monophosphate-activated protein kinase (AMPK) levels. LPL, lipoprotein lipase; VLDL, very low-density lipoprotein.