Impact van proteine fermentatie op de darmgezondheid

Dietary and endogenous protein that escape digestion and absorption in the small intestine are fermented by the colonic bacteria to a wide range of metabolites including BCFA, SCFA, ammonia, sulfides, phenolic and indolic compounds. Some of those compounds have been shown to have a toxic capacity in in vitro studies and animal studies. Therefore, protein fermentation is often considered as a risk factor for the development of CRC. However, evidence in humans is currently lacking. Considering the increasing popularity of high protein weight loss diet and the high protein intake associated with our Western-style diet , the aim of this PhD-project was to investigate the role of protein fermentation in gut health. Several randomized controlled studies, with a cross-over or parallel design, were conducted, in which protein fermentation was modulated by changing dietary protein intake or by administering prebiotics. Colonic protein fermentation was evaluated using urinary and fecal markers of protein fermentation, urinary p-cresol concentration and fecal BCFA and p-cresol concentrations. The biomarker lactose-[15N,15N]-ureide was used to study the colonic ammonia metabolism, denaturating gradient gel electrophoresis was applied to study changes in the predominant bacteria and real time PCR to quantify selected bacteria. Fecal metabolite patterns were measured using GC-MS to investigate total colonic fermentation. Fecal water genotoxicity was evaluated using the Comet Assay and fecal water cytotoxicity using the WST-1 assay. The relation between colonic fermentation and gut health was investigated by performing cluster analysis on the fecal metabolite patterns based on fecal water cytotoxicity and genotoxicity. In Chapter 2, we reviewed the literature on the evidence implicating protein fermentation in gut health. Indications on the toxic character of protein fermentation can be found in in vitro and animal studies. However, the available evidence in humans is insufficient to decide on the relevance of protein fermentation in gut health. In Chapter 3 and 4, protein fermentation was modified by changing dietary protein intake. In Chapter 3, normal-weight healthy subjects consumed a high (27%) and low (12%) protein diet isocaloric to their normal protein diet (15%) , while in Chapter 4, a high (30% protein) and standard (15% protein) protein calorie restricted weight loss diet were consumed by overweight subjects. The isocaloric high protein diet successfully stimulated protein fermentation as urinary p-cresol excretion was significantly increased. After the high protein weight loss diet, protein fermentation remained similar to baseline but was higher than after the standard protein weight loss diet. Fecal water genotoxicity and cytotoxicity were not significantly affected by the isocaloric diets or the weight loss diets. In both Chapters 3 and 4, fecal sulfide levels were associated with high genotoxicity. As colonic sulfide-production depends on the presence of sulfate-reducing bacteria, the relation between the number of these bacteria in the colon and the change in fecal water genotoxicity after a high protein diet was further explored in Chapter 5. Increased numbers of sulfate-reducing bacteria correlated with an increase in fecal water genotoxicity after an isocaloric high protein diet, but not after a high protein weight loss diet, suggesting that the combination of sufficient protein intake and high numbers of sulfate-reducing bacteria increase the risk on colonic toxicity. Therefore, high protein diets with high absolute amounts of protein should be discouraged in subjects carrying high numbers of sulfate-reducing bacteria. In Chapters 6 and 7, prebiotics were administered as a strategy to reduce colonic protein fermentation. In Chapter 6, a moderate dose of AXOS (10g/day) was administered, while in Chapter 7, a high dose (30g/day) of AXOS-containing WBE and FOS were compared. In both studies, protein fermentation was reduced after intake of AXOS. The impact of FOS on protein fermentation was less pronounced. In Chapter 6, intake of AXOS induced a decreased urinary ammonia excretion and an increased fecal ammonia excretion, suggesting a reduced exposure of the colonocytes to toxic ammonia, which can be explained by a stimulation of colonic bacterial growth or activity. This functional change was supported by a change in the microbial composition as evidenced by an increase in the numbers of Bifidobacterium adolescentis after AXOS-intake.A high dose of WBE reduced fecal water cytotoxicity, while a moderate dose of AXOS and a high dose of FOS were less effective. None of the prebiotics affected fecal water genotoxicity. Consistent in these two chapters was the finding that prebiotic intake was associated with higher fecal concentrations of cycloalkanes and cycloalkenes. These metabolites were also associated with low cytotoxicity in Chapters 3, 6 and 7. Finally, in Chapter 8, metabolite patterns generated from 4 different sample matrices (urine, feces, fecal water and lyophilized feces) were compared. Metabolite patterns in urine samples were clearly distinct from metabolite patterns in samples originating from feces, which can be attributed to the more important contribution of the host metabolism to the urine metabolome. Also the different fecal sample types displayed distinct metabolite patterns. The impact of prebiotic intake was most pronounced in the fecal metabolome.In conclusion, the results obtained in this PhD-project did not provide evidence for a role of protein fermentation in gut toxicity in healthy human subjects. Although it is without doubt that protein fermentation yields intrinsically toxic luminal compounds that affect epithelial cell metabolism and barrier function, we hypothize that the impact of protein fermentation is overshadowed by other dietary or lifestyle factors.status: publishe