Modulation of protein fermentation does not affect fecal water toxicity: a randomized cross-over study in healthy subjects.

Karen Windey,Frans Schuit,Greet Vansant,Kristin Verbeke,Jean Herman,Vicky De Preter,Thierry Louat,Daniel Tomé

doi:10.1371/journal.pone.0052387

Abstract

ObjectiveProtein fermentation results in production of metabolites such as ammonia, amines and indolic, phenolic and sulfur-containing compounds. In vitro studies suggest that these metabolites might be toxic. However, human and animal studies do not consistently support these findings. We modified protein fermentation in healthy subjects to assess the effects on colonic metabolism and parameters of gut health, and to identify metabolites associated with toxicity.DesignAfter a 2-week run-in period with normal protein intake (NP), 20 healthy subjects followed an isocaloric high protein (HP) and low protein (LP) diet for 2 weeks in a cross-over design. Protein fermentation was estimated from urinary p-cresol excretion. Fecal metabolite profiles were analyzed using GC-MS and compared using cluster analysis. DGGE was used to analyze microbiota composition. Fecal water genotoxicity and cytotoxicity were determined using the Comet assay and the WST-1-assay, respectively, and were related to the metabolite profiles.ResultsDietary protein intake was significantly higher during the HP diet compared to the NP and LP diet. Urinary p-cresol excretion correlated positively with protein intake. Fecal water cytotoxicity correlated negatively with protein fermentation, while fecal water genotoxicity was not correlated with protein fermentation. Heptanal, 3-methyl-2-butanone, dimethyl disulfide and 2-propenyl ester of acetic acid are associated with genotoxicity and indole, 1-octanol, heptanal, 2,4-dithiapentane, allyl-isothiocyanate, 1-methyl-4-(1-methylethenyl)-benzene, propionic acid, octanoic acid, nonanoic acid and decanoic acid with cytotoxicity.ConclusionThis study does not support a role of protein fermentation in gut toxicity. The identified metabolites can provide new insight into colonic health.Trial RegistrationClinicalTrial.gov NCT01280513

Highlights

Protein fermentation is widely recognized to be detrimental to gut health
Dietary protein intake was significantly higher during the high protein (HP) diet compared to the normal protein intake (NP) and low protein (LP) diet
Fecal water cytotoxicity correlated negatively with protein fermentation, while fecal water genotoxicity was not correlated with protein fermentation

Summary

Introduction

Protein fermentation is widely recognized to be detrimental to gut health. Protein fermentation or putrefaction is the anaerobic digestion of protein by the microbiota residing in the colon. Protein fermentation results in the production of branched chain fatty acids (BCFA; isobutyric and isovaleric acid) and short chain fatty acids (SCFA; acetic acid, propionic acid and butyric acid) and of metabolites such as ammonia (NH3), amines, indolic, phenolic and sulfur-containing compounds [3]. Ammonia was found to increase cell proliferation and decrease cell permeability in colonic adenocarcinoma cells (CaCo-2) at concentrations between 10 and 100 mM [4,5]. Phenol (1– 10 mmol/L) increases cell permeability [6]. Attene-Ramos et al showed that hydrogen sulfide (H2S; 250 mmol/L) induces genotoxic damage in colonic adenocarcinoma cells (HT-29) [7] and suggests that this could be radical-mediated [8]. H2S impairs butyrate oxidation, which is the most important energy pathway in colonocytes [9,10]

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