An in vitro model for microbial fructoselysine degradation shows substantial interindividual differences in metabolic capacities of human fecal slurries

Dietary glycation compounds - implications for human health

The term "glycation compounds" comprises a wide range of structurally diverse compounds that are formed endogenously and in food via the Maillard reaction, a chemical reaction between reducing sugars and amino acids. Glycation compounds produced endogenously are considered to contribute to a range of diseases. This has led to the hypothesis that glycation compounds present in food may also cause adverse effects and thus pose a nutritional risk to human health. In this work, the Senate Commission on Food Safety (SKLM) of the German Research Foundation (DFG) summarized data on formation, occurrence, exposure and toxicity of glycation compounds (Part A) and systematically assessed potential associations between dietary intake of defined glycation compounds and disease, including allergy, diabetes, cardiovascular and renal disease, gut/gastrotoxicity, brain/cognitive impairment and cancer (Part B). A systematic search in Pubmed (Medline), Scopus and Web of Science using a combination of keywords defining individual glycation compounds and relevant disease patterns linked to the subject area of food, nutrition and diet retrieved 253 original publications relevant to the research question. Of these, only 192 were found to comply with previously defined quality criteria and were thus considered suitable to assess potential health risks of dietary glycation compounds. For each adverse health effect considered in this assessment, however, only limited numbers of human, animal and in vitro studies were identified. While studies in humans were often limited due to small cohort size, short study duration, and confounders, experimental studies in animals that allow for controlled exposure to individual glycation compounds provided some evidence for impaired glucose tolerance, insulin resistance, cardiovascular effects and renal injury in response to oral exposure to dicarbonyl compounds, albeit at dose levels by far exceeding estimated human exposures. The overall database was generally inconsistent or inconclusive. Based on this systematic review, the SKLM concludes that there is at present no convincing evidence for a causal association between dietary intake of glycation compounds and adverse health effects.

Physiologically based kinetic (PBK) modeling as a new approach methodology (NAM) for predicting systemic levels of gut microbial metabolites

There is a clear need to develop new approach methodologies (NAMs) that combine in vitro and in silico testing to reduce and replace animal use in chemical risk assessment. Physiologically based kinetic (PBK) models are gaining popularity as NAMs in toxico/pharmacokinetics, but their coverage of complex metabolic pathways occurring in the gut are incomplete. Chemical modification of xenobiotics by the gut microbiome plays a critical role in the host response, for example, by prolonging exposure to harmful metabolites, but there is not a comprehensive approach to quantify this impact on human health. There are examples of PBK models that have implemented gut microbial biotransformation of xenobiotics with the gut as a dedicated metabolic compartment. However, the integration of microbial metabolism and parameterization of PBK models is not standardized and has only been applied to a few chemical transformations. A challenge in this area is the measurement of microbial metabolic kinetics, for which different fermentation approaches are used. Without a standardized method to measure gut microbial metabolism ex vivo/in vitro, the kinetic constants obtained will lead to conflicting conclusions drawn from model predictions. Nevertheless, there are specific cases where PBK models accurately predict systemic concentrations of gut microbial metabolites, offering potential solutions to the challenges outlined above. This review focuses on models that integrate gut microbial bioconversions and use ex vivo/in vitro methods to quantify metabolic constants that accurately represent in vivo conditions.

Intraintestinal fermentation of fructo- and galacto-oligosaccharides and the fate of short-chain fatty acids in humans

Consumption of fructo- (FOS) and galacto-oligosaccharides (GOS) has health benefits which have been linked in part to short-chain fatty acids (SCFA) production by the gut microbiota. However, detailed knowledge of this process in the human intestine is lacking. We aimed to determine the acute fermentation kinetics of a FOS:GOS mixture in healthy males using a naso-intestinal catheter for sampling directly in the ileum or colon. We studied the fate of SCFA as substrates for glucose and lipid metabolism by the host after infusion of 13C-SCFA. In the human distal ileum, no fermentation of FOS:GOS, nor SCFA production, or bacterial cross-feeding was observed. The relative composition of intestinal microbiota changed rapidly during the test day, which demonstrates the relevance of postprandial intestinal sampling to track acute responses of the microbial community toward interventions. SCFA were vividly taken up and metabolized by the host as shown by incorporation of 13C in various host metabolites.

The Intervention of Probiotics on Type 2 Diabetes Mellitus in Animal Models

Type 2 diabetes accounts for more than 90% of diabetes patients with the incidence and prevalence continuously rising globally. As a prospective therapy strategy for type 2 diabetes, probiotics have shown beneficial effects both in animal experiments and human clinical trials. This review summarizes the commonly used animal models in probiotic intervention research and presents the evidence and mechanism of diabetes intervention with probiotics in these animal models. Probiotics can help maintain glucose homeostasis, improve lipid metabolism, promote the production of short-chain fatty acids, and reduce inflammatory reactions in animal models. However, the clinical translation of benefits from probiotics is still challenged by intrinsic differences between experimental animal models and humans, and the application of humanized non-rodent diabetic animal models may contribute to the clinical translation of probiotics in the future.

Amadori compounds: analysis, composition in food and potential health beneficial functions

Amadori compounds (ACs) are key intermediates of the Maillard reaction, and found in various thermally processed foods. Simultaneous analysis of multiple ACs is challenging due to the complex amino acid and carbohydrate compositions, and the different food matrices. Most studies focus on the effects of ACs on food flavor and related sensory properties, but not their biological functions. However, increasing evidence shows that ACs possess various beneficial effects on human health, thus a comprehensive review on the various biological activities is warranted. In this review, we summarized the composition and content of ACs in different foods, their formation and degradation reactions, and discussed the latest advances in analytical methods of ACs and their biological functions related to human health. Limitations and research gaps were identified and future perspectives on ACs research were proposed. This review points to the needs of systematic and comprehensive in vitro and in vivo studies on human health related biological functions of ACs and their mechanisms of action, particularly the synergistic effects with other food components and drugs, and roles in intestinal health and metabolic syndrome.

The Maillard reaction end product Nε-carboxymethyllysine is metabolized in humans and the urinary levels of the microbial metabolites are associated with individual diet

During food processing most of the thermally-driven chemical reactions start off on the side chain amino group of lysine generating structurally modified compounds with specific metabolic routes. Upon human digestion, dietary N^ε-carboxymethyllysine (CML) may enter the colon and undergo gut microbial metabolism. However, little is known about the in vivo metabolic fate of dietary CML and its relationship with the habitual diet. We explored by hydrophilic interaction liquid chromatography tandem mass spectrometry the metabolites of CML in urine samples collected from 46 healthy subjects and studied the associations with diet. Mean concentration of N-carboxymethylcadaverine (CM-CAD), N-carboxymethylaminopentanoic acid (CM-APA), N-carboxymethylaminopentanol (CM-APO), and the N-carboxymethyl-Δ1-piperideinium ion were 0.49 nmol mg^-1 creatinine, 1.45 nmol mg^-1 creatinine, 4.43 nmol mg^-1 creatinine and 4.79 nmol mg^-1 creatinine, respectively. The urinary concentration of CML, its metabolites and lysine were positively correlated. Dietary intake of meat products negatively correlated with urinary excretion of CML and CM-APA; conversely dietary plant-to-animal proteins ratio positively correlated with urinary CML and its metabolites. The identification and quantification of CML metabolites in urine and the associations with diet corroborate the hypothesis that CML, an advanced glycation end-product, can undergo further biochemical transformations in vivo. The gut microbiome may have a major role in human metabolism of dietary CML.

Differences in gut microbial fructoselysine degradation activity between breast-fed and formula-fed infants

The Amadori product fructoselysine is formed upon heating of food products and is abundantly present in infant formula while being almost absent in breast milk. The human gut microbiota can degrade fructoselysine for which interindividual differences have been described for adults. The aim of this study is to compare functional differences in microbial fructoselysine degradation between breast-fed and formula-fed infants, in view of their different diets and resulting different fructoselysine exposures. First, a publicly available metagenomic dataset with metagenome-assembled genomes (MAGs) from infant fecal samples was analyzed and showed that query genes involved in fructoselysine degradation (frlD/yhfQ) were abundantly present in multiple bacterial taxa in the fecal samples, with a higher prevalence in the formula-fed infants. Next, fecal samples collected from exclusively breast-fed and formula-fed infants were anaerobically incubated with fructoselysine. Both groups degraded fructoselysine, however the fructoselysine degradation activity was significantly higher by fecal samples from formula-fed infants. Overall, this study provides evidence that infant formula feeding, leading to increased dietary fructoselysine exposure, seems to result in an increased fructoselysine degradation activity in the gut microbiota of infants. This indicates that the infant gut microbiota adapts towards dietary fructoselysine exposure.

Inter- and Intraindividual Differences in the Capacity of the Human Intestinal Microbiome in Fecal Slurries to Metabolize Fructoselysine and Carboxymethyllysine

The advanced glycation endproduct carboxymethyllysine and its precursor fructoselysine are present in heated, processed food products and are considered potentially hazardous for human health. Upon dietary exposure, they can be degraded by human colonic gut microbiota, reducing internal exposure. Pronounced interindividual and intraindividual differences in these metabolic degradations were found in anaerobic incubations with human fecal slurries in vitro. The average capacity to degrade fructoselysine was 27.7-fold higher than that for carboxymethyllysine, and degradation capacities for these two compounds were not correlated (R² = 0.08). Analysis of the bacterial composition revealed that interindividual differences outweighed intraindividual differences, and multiple genera were correlated with the individuals' carboxymethyllysine and fructoselysine degradation capacities (e.g., Akkermansia, Alistipes).

Differences in kinetics and dynamics of endogenous versus exogenous advanced glycation end products (AGEs) and their precursors

Advanced glycation end products (AGEs) and their precursors, referred to as glycation products, are a heterogenous group of compounds being associated with adverse health effects. They are formed endogenously and in exogenous sources including food. This review investigates the roles of endogenously versus exogenously formed glycation products in the potential induction of adverse health effects, focusing on differences in toxicokinetics and toxicodynamics, which appeared to differ depending on the molecular mass of the glycation product. Based on the available data, exogenous low molecular mass (LMM) glycation products seem to be bioavailable and to contribute to dicarbonyl stress and protein cross-linking resulting in formation of endogenous AGEs. Bioavailability of exogenous high molecular mass (HMM) glycation products appears limited, while these bind to the AGE receptor (RAGE), initiating adverse health effects. Together, this suggests that RAGE-binding in relevant tissues will more likely result from endogenously formed glycation products. Effects on gut microbiota induced by glycation products is proposed as a third mode of action. Overall, studies which better discriminate between LMM and HMM glycation products and between endogenous and exogenous formation are needed to further elucidate the contributions of these different types and sources of glycation products to the ultimate biological effects.