Bi-directional interactions between the gut microbiota and dietary glycation products

Glycation products comprise a heterogenous group of compounds which can be formed upon thermal processing of food products and include advanced glycation end products (AGEs), such as carboxymethyllysine, and their precursors such as the Amadori product fructoselysine and α-dicarbonyl compounds like methylglyoxal, glyoxal and 3-deoxyglucosone. Besides being present in exogenous dietary sources, glycation products can also be produced endogenously inside the human body. AGEs have been associated with the development of multiple adverse health effects, such as diabetes and cardiovascular diseases, but whether exposure to exogenous dietary AGEs and their precursors contributes to these effects remains debated. In this thesis, an overview was presented of the toxicokinetics and toxicodynamics of exogenous and endogenous AGEs and their precursors which are of relevance to consider when evaluating whether exposure to exogenous dietary glycation products can contribute to the adverse health effects associated with AGEs. It was concluded that better characterization of the tested AGEs and precursors, and to distinguish between their low molecular mass (LMM) and high molecular mass (HMM) forms, as well as quantification of the exposure from endogenous formation relative to that resulting from dietary intake are all of importance to provide a definite conclusion on whether dietary exposure to glycation products can contribute to the adverse health effects associated to AGEs. In addition, it was concluded that part of the AGEs and their precursors remain unabsorbed upon dietary exposure and can reach the colon. Consequently at least part of the AGEs and their precursors can interact with the gut microbiota in a bi-directional manner. This can affect the toxicokinetics of the AGEs and their precursors but on the other hand can also result in exerted toxicodynamic effects by altering the gut microbiota composition and possibly function. The aim of this project was to characterize these interactions of the gut microbiota with selected dietary glycation products and vice versa, and to characterize inter- and intraindividual differences in gut microbial reactions using the Amadori product fructoselysine and the AGE carboxymethyllysine as model compounds. This was achieved by the application of an in vitro model where collected individual human fecal samples were incubated with the substrate of interest. Michaelis-Menten kinetic parameters could be obtained, in addition to information on metabolite formation (short chain fatty acid; SCFA). In addition to interindividual differences, intra-individual differences were quantified as well of microbial degradation of the AGE carboxymethyllysine and its precursor fructoselysine. The gut microbiota is a dynamic ecosystem and thus intraindividual temporal differences are of possible relevance for its functioning. The results revealed pronounced inter- and intraindividual variation in both carboxymethyllysine and fructoselysine degradation while the two degradation rates were not correlated (R2=0.08), showing the need to evaluate toxicokinetics for individual AGEs and precursors instead of evaluating them on a group level. In addition, multiple bacterial genera were correlated with the individuals’ carboxymethyllysine and fructoselysine degradation activities, corroborating that showing that the potential to convert fructoselysine and carboxymethyllysine may originate from multiple bacterial genera. The effect of different dietary exposures on microbial degradation was evaluated by comparing functional differences in microbial fructoselysine degradation between breast-fed (BF) and formula-fed (FF) infants, in view of their different exclusive diets and consequent different dietary fructoselysine exposures. This was done by evaluating a publicly available metagenome dataset analysis with metagenome assembled genomes (MAGs) from infant fecal samples which showed that genes involved in microbial fructoselysine degradation were present in multiple taxa in both BF and FF infant fecal samples but were higher prevalent in fecal samples from FF infants compared to the BF infants. Further collection of individual fecal samples from exclusively BF and FF infants showed that both groups were able to degrade fructoselysine anaerobically but fecal samples from the FF infants resulted in a significantly higher degradation activity compared to the BF infants. This indicated that the infant gut microbiota adapts towards dietary fructoselysine exposure. This dynamic adaptive aspect of the gut microbiota was also observed in mice which were exposed to a heated diet high in AGEs which resulted in an altered gut microbiota composition compared to mice exposed to the control diet. Exposure to the heated diet high in AGEs followed by the control diet (i.e. the switch group) showed that the altered gut microbiota composition was reversible and adapted to the dietary exposure. This reversibility was also observed for the accumulation of the tested AGEs in plasma, kidney and liver (as analyzed in their protein-bound and free form), a result that also pointed to at least partial bioavailability of the dietary AGEs and/or their precursors that appeared to be enriched in the heated chow diet.

Overall, it can be concluded that the bi-directional relation of the gut microbiota with exogenous AGEs and their precursors is of relevance when evaluating their toxicokinetic and toxicodynamic characteristics. To accurately evaluate the hazards and risks of dietary exposure to AGEs and precursors, better characterization and quantification of the applied test substances and biological outcomes, and also of the exposure from endogenous formation relative to that resulting from dietary intake, are essential.