Consistent with the overall understanding of AGEs as perilous molecules, several studies have demonstrated that the dietary intake of AGEs induces the dysfunction of the gut microbiota along with unfavorable effects in the host organism. Specifically, in a human study, it was demonstrated that the increased exposure to glycated BSA significantly affected the colonic microbiota sampled from the feces of both healthy subjects and patients with ulcerative colitis, with a profound decrease in beneficial bacteria (eubacteria and bifidobacteria) and an increased abundance of more hazardous phyla (clostridia, bacteroides, sulfate-reducing bacteria) (42). The detailed study by Seiquer et al (43) revealed the significant association between AGEs intake and gut microbiota composition both in humans and rats. Specifically, the relative abundance of lactobacteria was inversely associated with dietary hydroxymethylfurfural and carboxymethyl-lysine, whereas the relative numbers of bifidobacterial were inversely associated with the intake of Amadori compounds in adolescents. Similarly, in rats, the dietary intake of the Amadori compounds, hydroxymethylfurfural and carboxymethyl-lysine, was negatively associated with both total bacteria and lactobacteria (43).

In a laboratory study, it was demonstrated that AGE-rich food significantly aggravated alterations in gut microbiota biodiversity in a time-dependent manner, being more profound at 18 weeks of exposure, as compared to 6 and 12 weeks. Furthermore, it was shown that the high-AGE group ws characterized by a significant decrease in the abundance of Ruminococcaceae, Lachnospiraceae, Alloprevotella, Mollicutes, Christensenellaceae, Treponema, Prevotellaceae, Sphaerochaeta, Elusimicrobium, Butyrivibrio and Anaeroplasma, whereas that of Oscillibacter, Allobaculum, Anaerotruncus, Barnesiella, Fusicatenibacter and Veillonellaceae was elevated (44). Feeding rats with a heat-treated diet rich in AGEs resulted in a significant increase in the relative abundance of Parabacteroides, Alloprevotella, Helicobacter, Ruminococcaceae_UCG-014, and unclassified Rhodospirillaceae, whereas populations of Desulfovibrio, Rikenellaceae, Lachnospiraceae and Alistipes, were significantly reduced, being associated with perturbations of microbial metabolites and certain metabolic pathways, including impaired carbohydrate and amino acid metabolism (45). Correspondingly, in another study, the administration of an AGE-rich diet, obtained by heating the standard chow, to C57BL/6 mice resulted in a significant increase in circulating and tissue AGEs levels, systemic inflammation, and the alteration of the gut microbiota mainly characterized by the increased abundance of Clostridium_sensu_stricto_1, Turicibacter and Parasutterella, and in particular, Dubosiella at the genera level, as well as the increased abundance of Clostridiaceae_1, Erysipelotrichaceae and Burkholderiaceae families (46).

Dietary AGE-induced effects on gut microbiota have also been linked to several diseases. Specifically, a previous study demonstrated that feeding C57BL/6 mice with an AGE-rich diet significantly increased systemic AGE (CML) levels, protein glycosylation, receptor for AGEs (RAGE) expression in the ileum and submandibular glands, as well as complex alterations in gut microbiota composition. AGE intake significantly increased Lawsonia, Parabacteroides and Ruminococcus abundance, whereas the relative numbers of Lactobacillus, Prevotella, Anaerostipes and Candidatus Arthromitus were significantly reduced, altogether being associated with impaired insulin signal transduction (47). It has been also demonstrated that feeding rats with casein glycated with methylglyoxal 5-hydro-5-methylimidazolone significantly affects the intestinal microbiota, which may at least in part be responsible for a reduction in systemic gastric inhibitory polypeptide and glucagon-like peptide-1 levels, consistent with an altered incretin-insulin axis, as well as with the overproduction of the pro-inflammatory cytokines, interleukin (IL)-1β and IL-17 and plasminogen activator inhibitor-1 (48).

In contrast to the aforementioned findings, a number of studies have demonstrated that administration of dietary AGEs can afford beneficial effects, both to the gut microbiota and host metabolism. It is notable that the majority of results demonstrating the positive influence of AGEs on gut microbiota were attributable to administration of glycated fish proteins. Specifically, the glycation of dietary grass carp myofibrillar proteins resulting in increased furosine levels in glycoconjugates significantly increased gut microbiota biodiversity and butyrate production that was positively associated with the relative abundance of Mitsuokella, Lachnospiraceae_UCG-004, Sutterella, Salinimicrobium, Fodinibius and Nitriliruptor, being inversely associated with that of Enterococcus, Dorea, Escherichia-Shigella and Phascolarctobacterium, thus being indicative of the potential beneficial effects of protein glycation on gut health (49). Correspondingly, another study demonstrated that the administration of glycated fish protein to rats significantly increased the relative abundance of Allobaculum, Akkermansia, Turicibacter and Lactobacillus animalis, and reduced that of Escherichia-Shigella, Fusobacterium and Erysipelatoclostridium in the caecum, altogether being associated with the increased production of butyrate from fructoselysine (50). Similar findings were obtained in another study, where the administration of fish peptide glycated with galactooligosaccharide resulted in increased abundance of the Veillonellaceae, Prevotellaceae and Coriobacteriaceae families, and increased the abundance of the genera, Anaerovibrio, Collinsella, Prevotella_9, as well as reduced Alloprevotella, Holdemanella, Escherichia-Schigella and Streptococcus when compared to the control rats (51). Increasing fish protein glycation through heating for 24 or 48 h with its subsequent in vitro fermentation in a model of human distal colon significantly increased the relative abundance of Lactococcus in parallel with a decrease in Bacteroides. Moreover, the exposure of gut bacteria to glycated fish protein heated for 48 h resulted in a greater abundance of Holdemania, Streptococcus, Enterococcus and Lactobacillus, as well as a reduction of Parabacteroides when compared to a less glycated protein (24 h of heating). These changes, and particularly a decrease in Bacteroides, Dialister and Parabacteroides, were associated with reduced ammonia and indole production (52). The intake of glycated fish protein in high-fat fed rats also decreased relative abundance of Ruminiclostridium and Desulfovibrio, as well as dose-dependent effects, including increased Ruminococcus and Roseburia abundance in low glycated protein diet and decreased Helicobacter and Lachnospiraceae upon high-dose glycated protein intake. Moreover, the observed AGE-induced modulation of gut microbiota composition was associated with a significant reduction of systemic proinflammatory cytokine (IL-1β and IL-6) levels and lipid profile improvement, thus indicative of the potential beneficial effects on gut and metabolism (53).

It has been also demonstrated that the glycation of the milk proteins, β-lactoglobulin and casein, significantly increased fermentability of the proteins by Lactobacillus and Bifidobacterium thus promoting their growth, although the effect was more profound at the initial steps of Maillard reaction (54). β-lactoglobulin-galactose conjugate was also shown to promote Clostridium coccoides-Eubacterium rectal group growth, as well as increase bacterial acetate production (55). Notably, the modification of β-lactoglobulin by glycation and ultrasonication has been shown to reduce milk protein allergenicity, which may be mediated by lower digestibility, modification of allergenic epitopes on the protein molecule, as well as modulation of gut microbiota composition (56).

The administration of glycated whey proteins to aged male non-obese diabetic mice with autoimmune prostatitis significantly increased mice survival, reduced prostatic inflammation, as well as an increased abundance of Allobaculum, Anaerostipes, Bacteroides, Parabacteroides and Prevotella and reduced abundance Adlercreutzia and Roseburia, whereas the population of Bacteroides acidifaciens significantly correlated with the observed effects, indicative of the role of gut microbiota modulation in protective effects of glycated whey proteins (57). Similarly, a beneficial effect on the gut microbiota was demonstrated for glycated pea protein which increased Bacteroides, Lactobacillus/Enterococcus and Bifidobacterium growth as well as short chain fatty acids (SCFAs), acetate, propionate, lactate, and butyrate production (58).

The dietary restriction of AGEs was shown to reduce systemic CML and MG levels, as well as affect the gut microbiota by increasing the relative abundance of Alistipes indistinctus, Clostridium citroniae, Clostridium hathewayi, and Ruminococcus gauvreauii, in parallel with a decrease in Prevotella copri and Bifidobacterium animalis in peritoneal dialysis patients (59). Concomitantly, in another study, a reduction in dietary AGEs intake did not have a significant effect on the most abundant gut bacteria in healthy obese subjects. As compared to the group with a high AGE intake, the low dietary AGE group was characterized by a greater abundance of Tyzzerella, Family_XII_UCG-001 and Christensenellaceae_R-7 Group, as well as a lower abundance of Negativibacillus, Oscillibacter and Anaerostipes (60).

As clearly detailed in previous studies, the effects of dietary AGEs on the gut microbiota vary significantly, depending on their characteristics, including both the dose, source and chemical properties. The study by Cao et al (61) proposed that the observed inconsistencies in the reported effects of AGE intake on the gut microbiota and overall health were dependent on the dose of the glycated protein in the diet. The intake of low levels of glycated fish proteins for 15 weeks in mice resulted not only in an increased abundance of the butyrate-producing bacteria, Lachnospiraceae and Allobaculum, but also increased intestinal tight junction protein expression (occludin and Zonula occludens-1), reduced pro-inflammatory cytokine production (IL-1β and IL-6) and improved insulin sensitivity. By contrast, inverse effects were observed upon exposure to high-dose glycated fish protein in parallel with a reduction in Bifidobacterium and Lactobacillus abundance (61). It has been posited that free AGEs, such as carboxymethyllysine have detrimental effects on the composition and functions of the gut microbiota, whereas bound AGEs have a more beneficial effect, although certain detrimental effects may be also observed (62). It was hypothesized that the positive effects of glycated proteins, namely fish proteins on the gut microbiota are due to the use of such proteins as a slow fermentable protein source or carbonyl donors providing additional energy to gut microbiota (46). In addition, free and protein-bound AGEs in the diet have distinct effects on the gut microbiota due to differences in digestibility (63). In addition to the AGE species, the molecular weight of the ligand has a significant impact on digestibility. Specifically, in a previous study, in a model of glyoxal-glycated casein digests, CML was degraded predominantly in the low molecular weight fraction (38.7%) followed by medium (21.7%) and high molecular weight fractions (9.6%) which may be mediated by the lower activity of proteases (64).

The understanding of the pathogenesis of diabetes mellitus sheds light onto the toxicological effects of AGEs and their role in metabolic diseases (10). Although the dysfunction of the gut microbiota has been known to play a significant role in diabetogenesis (65), the potential interplay between gut microbiota dysfunction and excessive protein glycation in diabetes has been studied only recently. Wu et al (66) demonstrated that dietary AGE exposure significantly affected the gut microbiota, with an irreversible increase in Bacteroidetes populations and a decrease in Firmicutes abundance at the phylum level, whereas at the genera level, a high-AGE diet stimulated Helicobacter, Bacteroides, Rikenella, Alistipes, Bifidobacterium, Candidatus Saccharimonas, Faecalibaculum, Clostridiales, Erysipelatoclostridium and Intestinimonas, and decreased unidentified Lachnospiraceae, Roseburia, Oscillibacter, Anaerotruncus, Blautia, Mucispirillum, Angelakisella, Lachnoclostridium, Lachnospira, Ruminiclostridium, Acetatifactor, and Desulfovibrio. These perturbations were shown to contribute to diabetes pathogenesis through the modulation of glyceraldehyde and pyruvate production with the subsequent aggravation of insulin resistance and other alterations in carbohydrate and lipid metabolism, as well as inflammation due to higher systemic lipopolysaccharide (LPS) levels (66). A comparative analysis demonstrated that despite a significant increase in circulating AGEs and the induction of insulin resistance in mice exposed to both an AGE-rich diet or purified methylglyoxal-bovine serum protein (exogenous AGE), profound alterations of intestinal permeability and microbiota structure were observed only upon exogenous AGE intake. A high AGE intake was shown to reduce the abundance of Bacteroidales_S24-7, Bacteroidaceae, Porphyromonadaceae, Odoribacteraceae, Lachnospiraceae, Rikenellaceae, and Erysipelotrichaceae in parallel with an increase in Desulfovibrionaceae abundance (67). It was proposed that a decrease in butyrate production by butyrate-producing bacteria may promote the impairment of the intestinal epithelial barrier and induce inflammation, thus contributing to systemic insulin resistance (67). The observed increase in Desulfovibrio abundance generally corresponds to the early observed positive association between these bacteria with blood glucose indices (68) and Parkinson's disease (69), although the results of the Guangdong Gut Microbiome Project demonstrated that the abundance of Desulfovibrio may be inversely associated with body mass index and triglyceride levels (70). Moreover, earlier findings in diabetic db/db mice exposed to high levels of dietary AGEs demonstrated in a significant increase in gut permeability, as well as an elevation of the Firmicutes-to-Bacteroidetes ratio, altogether being associated with kidney damage and albuminuria, whereas the administration of resistant starch ameliorated these effects (71). It is also notable that the formation of Maillard reaction products with subsequent protein aggregation in bacterial species shares certain similarity to that observed in Parkinson's and Alzheimer's disease (72). Therefore, preliminary data demonstrate that AGE-induced alterations in the gut microbiota can contribute to the aggravation of insulin resistance through a number of mechanisms, including the impairment of the intestinal epithelial barrier and subsequent increase in circulating LPS levels.

Recent studies have demonstrated that alterations in the gut microbiota, as well as increased levels of AGEs are associated with aging, contributing to the development of age-related diseases (73,74). Age-related changes in the gut microbiota characterized by a reduced Firmicutes-to-Bacteroidetes ratio at the phylum level, as well as by the increased abundance of Turibacter, Alloprevotella, Parasutterella, Bifidobacterium, Macellibacteroides, Alistipes sensu stricto 1, Peptostreptococcaceae incertae sedis and Parabacteroides, and the lower abundance of Pantoea, Anoxybacillus, Lachnospiraceae incertae sedis, Cutrobacterium and Acetatifactor at the genera level, were shown to contribute to the accumulation of N⁶-carboxymethyllysine in microglia and subsequent oxidative stress and mitochondrial dysfunction by increasing intestinal permeability, whereas germ-free mice brain microglia were characterized by lower oxidative stress and mitochondrial damage (75). In corroboration, the translocation of fecal microbiota from aged to young rats impaired cognition, induced synaptic dysfunction, along with oxidative stress and inflammation, which may be at least partially mediated by an increased AGE production and RAGE expression (76). Correspondingly, the antibiotic treatment of 5xFAD mice, a model of Alzheimer's disease that is known to be age-related, resulted in a significant decrease in intestinal Lactobacillaceae abundance, being also associated with reduced hippocampal plaque formation, antidiabetic effect and decreased RAGE expression (77). Taken together, even these limited data demonstrate that age-related alterations of the gut microbiota may contribute to AGE accumulation, particularly in brain tissues, indicating the gut microbiota-AGE interplay in age-related neurodegeneration.

A recent study in ethanol-fed mice demonstrated an increased abundance of Bacteroidetes and a decrease in Firmicutes numbers, which was associated with an elevation in AGE and RAGE levels in colonic tissues, and considered a potential mechanism of ethanol-related colorectal cancer pathogenesis (78).

Several studies have demonstrated that the modulation of the gut microbiota using probiotics is also associated with reduced AGE accumulation and toxicity, thus also supporting the role of the gut microbiota in AGE toxicity. Specifically, in a previous study, in a model of Alzheimer's disease, the modulation of the gut microbiota through the administration of SLAB51 probiotic significantly reduced brain AGE accumulation and tau phosphorylation, and also improved insulin signaling through the Akt/AMPK pathway (79). In another laboratory in vivo study, the administration of probiotic Lactobacillus paraplantarum BGCG11 significantly reduced AGE accumulation in parallel with inhibiting hyperglycemia, oxidative stress, DNA damage, liver and kidney fibrosis in rats with streptozotocin-induced diabetes (80). In another study, the administration of the commercial probiotic, Protexin^®, in Cd-exposed rats significantly reduced the serum MGO levels, as well as decreased tissue Cd accumulation and Cd-induced oxidative stress (81). Lactococcus lactis KF140 supplementation has also been shown to reduce serum CML levels and hepatic CML accumulation that may be at least partially mediated by activity of bacteria-derived β-galactosidase (82).

In addition to probiotics, it has been demonstrated that treatment with prebiotics may also modulate AGE accumulation and toxicity. Specifically, the administration of the prebiotic, resistant dextrin, has been shown to significantly reduce carboxymethyl lysine, soluble RAGE, as well as several other cardiometabolic risk factors in adult women (83).

At the same time, additional studies, including clinical trials are required to address the impact of microbiota modulation by probiotics and prebiotics on AGE toxicity and RAGE signaling, as well as the clinical validity of these interventions.

Polyphenols have also been shown to have a significant beneficial effect on gut microbiota and glycative stress, and these effects appear partially interrelated (84). Specifically, the administration of Physalis alkekengi L. fruit polysaccharide to AGE-fed mice was shown to modulate gut microbiota by increasing the abundance of Rikenallaceae, Alistipes, Nocardiaceae, Rhodococus, Bacilli, Lactobacillaceae, Bacteroidaceae and Burkholderiaceae, improving the Bacteroidetes/Firmicutes ratio, as well as decreasing LPS production. Treatment with Physalis alkekengi polysaccharide also improved the bacterial production of SCFAs, namely acetic and propionic acids, which may at least partially mediate the treatment-induced reduction of insulin resistance (85). It has also been demonstrated that the administration of quercetin to AGE-fed mice significantly ameliorated cognitive dysfunction through the reduction of tau phosphorylation, cathepsin B and neuroinflammation, as well as increased gut microbiota biodiversity and reduced the abundance of Verrucomicrobia phylum, and Blautia and Anaerotruncus genera (86). In addition, it has been proposed that an increase in Lactobacteria and particularly Bifidobacteria by Geranium dielsianum extract may at least partially mediate antiglycative effect of the extract (87). These data demonstrate that the protective effects of phytochemicals against AGE toxicity are mediated by its influence on the composition of the gut microbiota.