Review

The gut microbiota at the intersection of diet and human health

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Science  16 Nov 2018:
Vol. 362, Issue 6416, pp. 776-780
DOI: 10.1126/science.aau5812

Figures

  • Fig. 1 MAC fermentors produce SCFAs that can have multiple interactions with host tissues.

    Butyrate is taken up by epithelial cells and used as a primary source of energy for these cells. Butyrate (and to a lesser degree, propionate) can block histone deacetylases (HDAC) to regulate gene expression. All of the SCFAs can bind with varying affinities to G protein receptors in the intestines and other cells to regulate energy metabolism, intestinal homeostasis, and immune responses. Acetate and propionate are primarily metabolized in the liver, where propionate is used as a substrate for gluconeogenesis and acetate is used as an energy source and for fatty acid synthesis.

  • Fig. 2 Gut bacteria play an important role in bile acid modification.

    Primary bile acids deposited into the small intestine are deconjugated and dehydroxylated by enzymes from bile-modifying bacteria. These changes influence total bile acid pools available for reabsorption and recycling through enterohepatic circulation. In addition, bile acids can act as regulatory molecules by binding to cell surface or nuclear receptors, influencing host factors such as energy expenditure and lipid metabolism.

  • Fig. 3 Interactions between amino acids and the gut microbiota.

    Microbial metabolism of the amino acid carnitine produces trimethylamine (TMA), which is subsequently oxidized in the liver to TMAO in a reaction catalyzed by flavin-containing monooxygenase (FMO). Increased levels of circulating TMAO have been linked to metabolic disease. Gut microbes metabolize the amino acid tryptophan into various substances, including indolepropionic acid (IPA) and indole-3-acetic acid (I3A), both of which can enter the general circulation. The metabolic effects of IPA, I3A, and other microbially derived amino acid metabolites are only now beginning to emerge.

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