Colonic bacterial metabolites and human health

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Highlights

  • Diet-driven changes in microbially produced SCFA can influence health via signalling.

  • Gut microbiota mediates the release and transformation of many bioactive phenolics.

  • Gut microbiota degrades dietary choline to methylamines.

  • Interactions between the microbiota, inflammasomes and host influence liver disease.

The influence of the microbial–mammalian metabolic axis is becoming increasingly important for human health. Bacterial fermentation of carbohydrates (CHOs) and proteins produces short-chain fatty acids (SCFA) and a range of other metabolites including those from aromatic amino acid (AAA) fermentation. SCFA influence host health as energy sources and via multiple signalling mechanisms. Bacterial transformation of fibre-related phytochemicals is associated with a reduced incidence of several chronic diseases. The ‘gut–liver axis’ is an emerging area of study. Microbial deconjugation of xenobiotics and release of aromatic moieties into the colon can have a wide range of physiological consequences. In addition, the role of the gut microbiota in choline deficiency in non-alcoholic fatty liver disease (NAFLD) and insulin resistance is receiving increased attention.

Introduction

The human large intestine is colonised by dense microbial communities that utilise both diet-derived and host-derived energy sources for growth, predominantly through fermentative metabolism. This highly diverse community has the capacity to perform an extraordinary range of biochemical transformations that go well beyond those encoded by the host genome, and these activities exert an important influence upon many aspects of human health. Metabolites formed by the gut microbiota are largely determined by the composition of the diet and the pattern of food intake, and it is now clear that the species composition of the colonic microbiota is itself altered by the diet [1•, 2, 3••]. This review will consider selected examples where recent progress has been made in understanding the links between diet, gut microbial activity and metabolites relevant to health.

Section snippets

Bacterial metabolites derived from the fermentation of plant-derived carbohydrates and their impact on the host

Many carbohydrates (CHOs) present in plant-derived foods are digested slowly, if at all, in the small intestine, making them available for microbial fermentation in the large intestine. Intake of starch that is resistant to digestion in the small intestine (resistant starch) can have benefits for metabolic health [4] and results in changes in the gut microbiota [1]. Recent work also shows a beneficial influence of whole grain intake upon inflammation, again with concomitant changes in the gut

Fibre-related phytochemicals

It is suggested that the inverse relationship between the intake of fibre-rich diets and the incidence of several chronic diseases is mediated in part by the gut microbiota. Microbial release of phytochemical metabolites may be a contributing factor and most widely studied for disease prevention are the aromatic metabolites produced by the phenylpropanoid pathway [25, 26]. Increasing the fibre (NSP) content of the diet from 8.8 to 12.8 g day−1 in a human volunteer study resulted in significantly

The ‘gut–liver’ axis

Given the exposure of the liver to intestinal-derived catabolites and the microbiota to biliary/waste products, the ‘gut–liver’ axis is receiving great attention with respect to host health and its potential to affect systemic host processes [36]. A recent study has nicely demonstrated the direct involvement of the gut microbiota in the development of obesity-independent non-alcoholic fatty liver disease (NAFLD), and the microbiota's influence on whole body glucose homeostasis and liver lipid

Conclusion

Microbial–mammalian co-metabolism is shaping human health in many ways. In this review, we have covered recent findings on SCFA, AAA and methylamine metabolism and their consequences on human health and disease, which are illustrating particularly well this metabolic symbiosis. With the constant refinement of metagenomics and metabolomics, further insights will become available from cohort studies, bearing promises for personalised nutrition and healthcare in the future.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

LH is funded by EU-FP7 METACARDIS (HEALTH-F4-2012-305312), M-E.D. is funded by Nestlé, Institut Mérieux and EU-FP7 (METACARDIS). HF and WR acknowledge support from the Scottish Government (RESAS).

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    These authors made equal contributions to this review.

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