## 4 Discussion
### Chemical Metabolites Profiling the Microbial Fermentation  
Various bioactive chemical molecules from food digestion, xenobiotics processing, or directly generated by the gut inhabitants drive the cell-to-cell communication in the gastrointestinal tract. In fact, some crucial metabolites are mainly (or exclusively) produced by microorganisms and released to the intestinal environment to act locally or systematically [96; 97]. Signal transduction, energy production and storage, host immunity, and epigenetic regulation are among the crucial roles of gut microbiota metabolites [98]. In this study, the colon microbiota metabolic capacity was profiled by analyzing important molecules derived from processing dietary fiber (i.e., SCFA, BCFA) and amino acids (i.e., biogenic amines). SCFAs are the main metabolites produced in the colon over dietary fibers or nondigestible carbohydrate fermentation [99]. Fatty acids are chemotaxonomic markers of metabolic health, and their increased production in the colon is linked to the improvement of gut barrier function and a reduction in intestinal inflammation [100; 101]. A low percentage of SCFA produced in the colon is excreted in the feces (~5%). In fact, when detected in stool, they could be a sign of gut dysbiosis and permeability, as well as a risk factor associated with various diseases [102]. Once produced, most SCFAs are absorbed by the colonocytes or else incorporated into systemic regulation through the portal vein into the liver. Systemic concentrations of SCFA are much lower than colonic concentrations. Thus, rather dissimilar systemic availabilities of acetate (~36%), propionate (~9%), and butyrate (~2%) have been observed by stable isotope technologies using \({}^{13}\)C-labelled fibers [103]. The production of SCFA is shaped by the individual microbiota composition, which is largely determined by dietary patterns. In the colon of adults between 18 and 50 years old, the total concentrations of SCFAs are estimated to be from 64-105 mM [104]. Meanwhile, the BCFA concentrations commonly reach a maximum of 5 mM [104]. The SCFA and BCFA measurements in the present study concurred with these suggested values, as observed in Figure 3g,h, respectively. Moreover, the relative percentage of the most abundant SCFAs (i.e., C2, C3, C4) reached levels in line with previous reports of in vitro colonic simulations [105; 72]. At a local level, SCFAs are the major energy substrates of colonocytes, and multiple beneficial roles are attributed to them in the intestine. For instance, butyrate is a well-known regulator of mucosal inflammation, transepithelial fluid transport, gut barrier functions, and motility [106]. Valerate, although less explored, seems to have an important role in the intestinal barrier and a synergistic effect with the rest of SCFA to regulate intestine homeostasis [107]. The presence of acetate and propionate, as major microbial fermentation metabolites, modulate the entire gut ecosystem. Propionate participates in the control of satiety signals and in lowering cholesterol, and propionate-producing bacteria might alter gut microbe-dependent T cell differentiation [108]. Acetic acid-producing bacteria have been positively correlated with colonic motility and absorption, and thus with constipation, as they control the transit rate of the small intestine and the water content of the stool [109]. Furthermore, the effects of SCFA reach far beyond the intestinal context. Thus, they have a role in cardio-metabolic health [110], the pathophysiology of obesity [111], and type 2 diabetes mellitus [112]. Cell-based molecular methods have shown that AhR signaling pathways are activated by SCFAs, being the inhibition of histone deacetylase (HDAC), a plausible associated mechanism [34]. On the other hand, amino acid decarboxylation by microbial enzymes leads to the production of biogenic amines, nine of which were quantified in this work.