An advanced glycation end product (AGE)-rich diet promotes Nε-carboxymethyl-lysine accumulation in the cardiac tissue and tendons of rats

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.

The AGE-RAGE Axis and the Pathophysiology of Multimorbidity in COPD

Chronic obstructive pulmonary disease (COPD) is a disease of the airways and lungs due to an enhanced inflammatory response, commonly caused by cigarette smoking. Patients with COPD are often multimorbid, as they commonly suffer from multiple chronic (inflammatory) conditions. This intensifies the burden of individual diseases, negatively affects quality of life, and complicates disease management. COPD and comorbidities share genetic and lifestyle-related risk factors and pathobiological mechanisms, including chronic inflammation and oxidative stress. The receptor for advanced glycation end products (RAGE) is an important driver of chronic inflammation. Advanced glycation end products (AGEs) are RAGE ligands that accumulate due to aging, inflammation, oxidative stress, and carbohydrate metabolism. AGEs cause further inflammation and oxidative stress through RAGE, but also through RAGE-independent mechanisms. This review describes the complexity of RAGE signaling and the causes of AGE accumulation, followed by a comprehensive overview of alterations reported on AGEs and RAGE in COPD and in important co-morbidities. Furthermore, it describes the mechanisms by which AGEs and RAGE contribute to the pathophysiology of individual disease conditions and how they execute crosstalk between organ systems. A section on therapeutic strategies that target AGEs and RAGE and could alleviate patients from multimorbid conditions using single therapeutics concludes this review.

Comparison of pharmacokinetics, biodistribution, and excretion of free and bound Nε-carboxymethyllysine in rats by HPLC-MS/MS

As a representative product of advanced glycation end products, Nɛ-carboxymethyllysine (CML) exists in free and bound forms in vivo and in food with different bioavailability. To thoroughly understand the bioavailability of free Nɛ-carboxymethyllysine (CML) and bovine serum albumin (BSA)-CML in vivo after intragastric administration, pharmacokinetics, biodistribution, and excretion of CML in rats were investigated by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Pharmacokinetics results revealed that free CML peaked at 1.83 h (1684.72 ± 78.08 ng/mL) and 1.33 h (1440.84 ± 72.48 ng/mL) in serum after intragastric administration of free CML and BSA-CML, demonstrating the higher absorption of free CML than BSA-CML. Besides, dietary free CML exhibited a relatively lower body clearance and tissue distribution than dietary BSA-CML based on the apparent volume of distribution and body clearance. Moreover, free CML was concentrated in the kidneys, indicating that kidneys were the target organ for the uptake of absorbed free CML. Additionally, the total excretion rate of CML in urine and feces were 37% and 60% after oral administration of free CML and BSA-CML. These results shed pivotal light on a better understanding of the biological effects of free and bound CML on health.

Identification of Pseudomonas asiatica subsp. bavariensis str. JM1 as the first Nε -carboxy(m)ethyllysine degrading soil bacterium

Thermal food processing leads to the formation of advanced glycation end products (AGE) such as N_ε -carboxymethyllysine (CML). Accordingly, these non-canonical amino acids are an important part of the human diet. However, CML is only partially decomposed by our gut microbiota and up to 30% are excreted via feces and, hence, enter the environment. In frame of this study, we isolated a soil bacterium that can grow on CML as well as its higher homologue N_ε -carboxyethyllysine (CEL) as sole source of carbon. Bioinformatic analyses upon whole genome sequencing revealed a subspecies of Pseudomonas asiatica, which we named 'bavariensis'. We performed a metabolite screening of P. asiatica subsp. bavariensis str. JM1 grown either on CML or CEL and identified N-carboxymethylaminopentanoic acid (CM-APA), and N-carboxyethylaminopentanoic acid (CE-APA), respectively. We further detected α-aminoadipate as intermediate in the metabolism of CML. These reaction products suggest two routes of degradation: While CEL seems to be predominantly processed from the α-C-atom, decomposition of CML can also be initiated with cleavage of the carboxymethyl group and under the release of acetate. Thus, our study provides novel insights into the metabolism of two important AGEs and how these are processed by environmental bacteria. This article is protected by copyright. All rights reserved.

In Vitro Methodologies to Study the Role of Advanced Glycation End Products (AGEs) in Neurodegeneration

Advanced glycation end products (AGEs) can be present in food or be endogenously produced in biological systems. Their formation has been associated with chronic neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and amyotrophic lateral sclerosis. The implication of AGEs in neurodegeneration is related to their ability to bind to AGE-specific receptors and the ability of their precursors to induce the so-called “dicarbonyl stress”, resulting in cross-linking and protein damage. However, the mode of action underlying their role in neurodegeneration remains unclear. While some research has been carried out in observational clinical studies, further in vitro studies may help elucidate these underlying modes of action. This review presents and discusses in vitro methodologies used in research on the potential role of AGEs in neuroinflammation and neurodegeneration. The overview reveals the main concepts linking AGEs to neurodegeneration, the current findings, and the available and advisable in vitro models to study their role. Moreover, the major questions regarding the role of AGEs in neurodegenerative diseases and the challenges and discrepancies in the research field are discussed.

Comparison of metabolic fate, target organs, and microbiota interactions of free and bound dietary advanced glycation end products

Increased intake of Western diets and ultra-processed foods is accompanied by increased intake of advanced glycation end products (AGEs). AGEs can be generated exogenously in the thermal processing of food and endogenously in the human body, which associated with various chronic diseases. In food, AGEs can be divided into free and bound forms, which differ in their bioavailability, digestion, absorption, gut microbial interactions and untargeted metabolites. We summarized the measurements and contents of free and bound AGE in foods. Moreover, the ingestion, digestion, absorption, excretion, gut microbiota interactions, and metabolites and metabolic pathways between free and bound AGEs based on animal and human studies were compared. Bound AGEs were predominant in most of the selected foods, while beer and soy sauce were rich in free AGEs. Only 10%-30% of AGEs were absorbed into the systemic circulation when orally administered. The excretion of ingested free and bound AGEs was approximately 90% and 60%, respectively. Dietary free CML has a detrimental effect on gut microbiota composition, while bound AGEs have both detrimental and beneficial impacts. Free and bound dietary AGEs changed amino acid metabolism, energy metabolism and carbohydrate metabolism. And besides, bound dietary AGEs altered vitamin metabolism, and glycerolipid metabolism.

Dietary N-epsilon-carboxymethyllysine as for a major glycotoxin in foods: A review

N-epsilon-carboxymethyllysine (CML), as a potential glycotoxin and general marker for dietary advanced glycation end products (dAGEs), exists in raw food and is formed via various formation routes in food processing such as Maillard reaction between the reducing sugars and amino acids. Although comprehensive cause-effect proof is not available yet, current research suggests a potential risk of chronic diseases such as diabetes is associated with exogenous CML. Thus, CML is causing public health concerns regarding its dietary exposure, but there is a lack of explicit guidance for understanding if it is detrimental to human health. In this review, inconsistent results of dietary CML contributed to chronic disease are discussed, available concentrations of CML in consumed foods are evaluated, measurements for dietary CML and relevant analytic procedures are listed, and the possible mitigation strategies for protecting against CML formation are presented. Finally, the main challenges and future efforts are highlighted. Further studies are needed to extend the dietary CML database in a wide category of foods, apply new identifying methods, elucidate the pathogenic mechanisms, assess its detrimental role in human health, and propose standard guidelines for processed food.

Dietary advanced glycation end-products elicit toxicological effects by disrupting gut microbiome and immune homeostasis

The aging immune system is characterized by a low-grade chronic systemic inflammatory state ("inflammaging") marked by elevated serum levels of inflammatory molecules such as interleukin (IL)-6 and C-reactive protein (CRP). These inflammatory markers were also reported to be strong predictors for the development/severity of Type 2 diabetes, obesity, and COVID-19. The levels of these markers have been positively associated with those of advanced glycation end-products (AGEs) generated via non-enzymatic glycation and oxidation of proteins and lipids during normal aging and metabolism. Based on the above observations, it is clinically important to elucidate how dietary AGEs modulate inflammation and might thus increase the risk for aging-exacerbated diseases. The present narrative review discusses the potential pro-inflammatory properties of dietary AGEs with a focus on the inflammatory mediators CRP, IL-6 and ferritin, and their relations to aging in general and Type 2 diabetes in particular. In addition, underlying mechanisms - including those related to gut microbiota and the receptors for AGEs, and the roles AGEs might play in affecting physiologies of the healthy elderly, obese individuals, and diabetics are discussed in regard to any greater susceptibility to COVID-19.

Dietary Advanced Glycation Endproducts and the Gastrointestinal Tract

The prevalence of inflammatory bowel diseases (IBD) is increasing in the world. The introduction of the Western diet has been suggested as a potential explanation of increased prevalence. The Western diet includes highly processed food products, and often include thermal treatment. During thermal treatment, the Maillard reaction can occur, leading to the formation of dietary advanced glycation endproducts (dAGEs). In this review, different biological effects of dAGEs are discussed, including their digestion, absorption, formation, and degradation in the gastrointestinal tract, with an emphasis on their pro-inflammatory effects. In addition, potential mechanisms in the inflammatory effects of dAGEs are discussed. This review also specifically elaborates on the involvement of the effects of dAGEs in IBD and focuses on evidence regarding the involvement of dAGEs in the symptoms of IBD. Finally, knowledge gaps that still need to be filled are identified.

Potential Markers of Dietary Glycemic Exposures for Sustained Dietary Interventions in Populations without Diabetes

There is considerable interest in dietary and other approaches to maintaining blood glucose concentrations within the normal range and minimizing exposure to postprandial hyperglycemic excursions. The accepted marker to evaluate the sustained maintenance of normal blood glucose concentrations is glycated hemoglobin A1c (HbA1c). However, although this is used in clinical practice to monitor glycemic control in patients with diabetes, it has a number of drawbacks as a marker of efficacy of dietary interventions that might beneficially affect glycemic control in people without diabetes. Other markers that reflect shorter-term glycemic exposures have been studied and proposed, but consensus on the use and relevance of these markers is lacking. We have carried out a systematic search for studies that have tested the responsiveness of 6 possible alternatives to HbA1c as markers of sustained variation in glycemic exposures and thus their potential applicability for use in dietary intervention trials in subjects without diabetes: 1,5-anhydroglucitol (1,5-AG), dicarbonyl stress, fructosamine, glycated albumin (GA), advanced glycated end products (AGEs), and metabolomic profiles. The results suggest that GA may be the most promising for this purpose, but values may be confounded by effects of fat mass. 1,5-AG and fructosamine are probably not sensitive enough to the range of variation in glycemic exposures observed in healthy individuals. Use of measures based on dicarbonyls, AGEs, or metabolomic profiles would require further research into possible specific molecular species of interest. At present, none of the markers considered here is sufficiently validated and sensitive for routine use in substantiating the effects of sustained variation in dietary glycemic exposures in people without diabetes.

Serum levels of advanced glycation end products and their receptors sRAGE and Galectin-3 in chronic pancreatitis

BACKGROUND

/Objectives: AGE and their receptors like RAGE and Galectin-3 can activate inflammatory pathways and have been associated with chronic inflammatory diseases. Several studies investigated the role of AGE, Galectin-3 and sRAGE in pancreatic diseases, whereas no comprehensive data for chronic pancreatitis (CP) are available.

METHODS

Serum samples from CP patients without an active inflammatory process (85 ACP; 26 NACP patients) and 40 healthy controls were collected. Levels of AGE, sRAGE and Galectin-3 were measured by ELISA. To exclude potential influences of previously described RAGE SNPs on detected serum levels, we analyzed variants rs207128, rs207060, rs1800625, and rs1800624 by melting curve technique in 378 CP patients and 338 controls.

RESULTS

AGE and Galectin-3 serum levels were significantly elevated in both ACP and NACP patients compared to controls (AGE: 56.61 ± 3.043 vs. 31.71 ± 2.308 ng/mL; p < 0.001; Galectin-3: 16.63 ± 0.6297 vs. 10.81 ± 0.4835 ng/mL; p < 0.001). In contrast, mean serum sRAGE levels were significantly reduced in CP patients compared to controls (sRAGE: 829.7 ± 37.10 vs. 1135 ± 55.74 ng/mL; p < 0.001). All results were consistent after correction for gender, age and diabetes mellitus. No genetic association with CP was found.

CONCLUSIONS

Our extensive analysis demonstrated the importance of aging related pathways in the pathogenesis of CP. As the results were consistent in ACP and NACP, both entities most likely share common pathomechanisms. Most probably the involved pathways are a general hallmark of an inflammatory state in CP that is even present in symptom-free intervals.

Advanced Glycation End Products: Building on the Concept of the "Common Soil" in Metabolic Disease

The role of advanced glycation end products (AGEs) in promoting and/or exacerbating metabolic dysregulation is being increasingly recognized. AGEs are formed when reducing sugars nonenzymatically bind to proteins or lipids, a process that is enhanced by hyperglycemic and hyperlipidemic environments characteristic of numerous metabolic disorders including obesity, diabetes, and its complications. In this mini-review, we put forth the notion that AGEs span the spectrum from cause to consequence of insulin resistance and diabetes, and represent a "common soil" underlying the pathophysiology of these metabolic disorders. Collectively, the surveyed literature suggests that AGEs, both those that form endogenously as well as exogenous AGEs derived from environmental factors such as pollution, smoking, and "Western"-style diets, contribute to the pathogenesis of obesity and diabetes. Specifically, AGE accumulation in key metabolically relevant organs induces insulin resistance, inflammation, and oxidative stress, which in turn provide substrates for excess AGE formation, thus creating a feed-forward-fueled pathological loop mediating metabolic dysfunction.

The fate of dietary advanced glycation end products in the body: from oral intake to excretion

Advanced glycation end products (AGEs), which are closely associated with various chronic diseases, are formed through the Maillard reaction when aldehydes react with amines in heated foods or in living organisms. The fate of dietary AGEs after oral intake plays a crucial role in regulating the association between dietary AGEs and their biological effects. However, the complexity and diversity of dietary AGEs make their fate ambiguous. Glycated modifications can impair the digestion, transport and uptake of dietary AGEs. High and low molecular weight AGEs may exhibit individual differences in their distribution, metabolism and excretion. Approximately 50-60% of free AGEs are excreted after dietary intake, whereas protein-bound AGEs exhibit a limited excretion rate. In this article, we summarize several AGE classification criteria and their abundance in foods, and in the body. A standardized static in vitro digestion method is strongly recommended to obtain comparable results of AGE digestibility. Sophisticated hypotheses regarding the intestinal transportation and absorption of drugs, as well as calculated physicochemical parameters, are expected to alleviate the difficulties determining the digestion, transport and uptake of dietary AGEs. Orally supplied AGEs with low or high molecular weights must be supported by well-defined amounts in investigations of excretion. Furthermore, unequivocal evidence should be obtained regarding the degradation and metabolism products of dietary AGEs.

Effects of Long-Term Physical Activity and Diet on Skin Glycation and Achilles Tendon Structure

Advanced glycation end-products (AGEs) accumulate with aging and have been associated with tissue modifications and metabolic disease. Regular exercise has several health benefits, and the purpose of this study was to investigate the effect of regular long-term exercise and diet on skin autofluorescence (SAF) as a measure of glycation and on Achilles tendon structure. In connection with the 2017 European Masters Athletics Championships Stadia, high-level male athletes (n = 194) that had regularly trained for more than 10 years were recruited, in addition to untrained controls (n = 34). SAF was non-invasively determined using an AGE Reader. Achilles tendon thickness and vascular Doppler activity were measured by ultrasonography, and diet was assessed by a questionnaire. There was no significant difference in SAF between the athletes and controls. However, greater duration of exercise was independently associated with lower SAF. Diet also had an effect, with a more "Western" diet in youth being associated with increased SAF. Furthermore, our data demonstrated that greater Achilles tendon thickness was associated with aging and training. Together, our data indicate that long-term exercise may yield a modest reduction in glycation and substantially increase Achilles tendon size, which may protect against injury.

Metabolic and Renal Effects of Dietary Advanced Glycation end Products in Pregnant Rats – A Pilot Study

Thermally processed food contains advanced glycation end products (AGEs) including N-(carboxymethyl)lysine (CML). Higher AGEs or circulating CML were shown to be associated with pregnancy complications such as preeclampsia and gestational diabetes. It is unclear whether this association is causal. The aim of our study was to analyze the effects of dietary CML and CML-containing thermally processed food on metabolism in pregnant rats. Animals were fed with standard or with AGE-rich diet from gestation day 1. Third group received standard diet and CML via gavage. On gestation day 18, blood pressure was measured, urine and blood were collected and the oral glucose tolerance test was performed. Plasma AGEs were slightly higher in pregnant rats fed with the AGE-rich diet (p=0.09). A non-significant trend towards higher CML in plasma was found in the CML group (p=0.06). No significant differences between groups were revealed in glucose metabolism or markers of renal functions like proteinuria and creatinine clearance. In conclusion, this study does not support the hypothesis that dietary AGEs such as CML might induce harmful metabolic changes or contribute to the pathogenesis of pregnancy complications. The short duration of the rodent gestation warrants further studies analyzing long-term effects of AGEs/CML in preconception nutrition.

Metabolization of the Advanced Glycation End Product N-ε-Carboxymethyllysine (CML) by Different Probiotic E. coli Strains

N-ε-Carboxymethyllysine (CML) is formed during glycation reactions (synonym, Maillard reaction). CML is degraded by the human colonic microbiota, but nothing is known about the formation of particular metabolites. In the present study, six probiotic E. coli strains were incubated with CML in the presence or absence of oxygen in either minimal or nutrient-rich medium. CML was degraded by all strains only in the presence of oxygen. HPLC-MS/MS was applied for identification of metabolites of CML. For the first time, three bacterial metabolites of CML have been identified, namely N-carboxymethylcadaverine (CM-CAD), N-carboxymethylaminopentanoic acid (CM-APA), and the N-carboxymethyl-Δ¹-piperideinium ion. During 48 h of incubation of CML with five different E. coli strains in minimal medium in the presence of oxygen, 37-66% of CML was degraded, while CM-CAD (1.5-8.4% of the initial CML dose) and CM-APA (0.04-0.11% of the initial CML dose) were formed linearly. Formation of the metabolites is enhanced when dipeptide-bound CML is applied, indicating that transport phenomena may play an important role in the "handling" of the compound by microorganisms.

Effects of Catechins on Nε-(Carboxymethyl)lysine and Nε-(Carboxyethyl)lysine Formation in Green Tea and Model Systems

The effects of catechins on N^ε-(carboxymethyl)lysine (CML) and N^ε-(carboxyethyl)lysine (CEL) formation in green tea and related model systems were investigated in this study. Since the first step of green tea processing entails enzyme inactivation, the catechin content was maintained at a high level during processing. However, drying still had a great effect on CML and CEL formation, while other steps also contributed. Hence, model systems were developed to analyze the effects of catechins on CML and CEL formation. Catechins ((-)-epicatechin gallate, (-)-epigallocatechin, and (-)-epigallocatechin gallate) could inhibit CML formation in the model imitating the condition of green tea processing, though the inhibitory efficiency was reduced by transition metals. This suggested that catechins could inhibit CML formation in the real tea system, though the inhibitory efficiency may be reduced by tea components which promote its synthesis. However, CEL formation was not always inhibited by the tested catechins, though catechins could significantly decrease the content of methylglyoxal which is considered an important intermediate. Consequently, the main pathway of CEL formation may not be through methylglyoxal.

Effect of fatty acids and triglycerides on the formation of lysine-derived advanced glycation end-products in model systems exposed to frying temperature

Lysine-derived advanced glycation end-products (AGEs), including N^ε-carboxymethyllysine (CML), N^ε-carboxyethyllysine (CEL) and pyrraline, are prevalent in processed foods. The impact of lipids on AGE formation is still contentious and most of our current knowledge is based only on CML. Little is known about how lipids impact the formation of CEL and pyrraline, or their involvement in formation pathways. This study investigated the effect of lipids (fatty acids and triglycerides) on the formation of CML, CEL and pyrraline simultaneously in model systems subjected to frying temperature. The results showed the presence of the lipids promoted the formation of CML, CEL and pyrraline. The promotion effects on CML and CEL were not dependent on the unsaturation degree and addition level (50, 100 or 150 μmol) of lipid, while pyrraline formation depended on lipid type (fatty acids vs. triglycerides) and level. The concentration of CML and CEL was statistically correlated with the concentration of glyoxal (GO) and methylglyoxal (MGO), respectively. The pyrraline concentration was statistically correlated with fructoselysine concentration. These results suggested that lipids promote the formation of lysine-derived AGEs by impacting the generation of Amadori products and α-dicarbonyl compounds. This information may be useful for optimizing the formulation and processing conditions to create tastier and healthier foods.

Fatty acids and triglycerides impact lysine-derived AGE formation through modulating the formation of α-dicarbonyl compounds and Amadori products.

Lower tendon stiffness in very old compared with old individuals is unaffected by short-term resistance training of skeletal muscle

Aging negatively affects collagen-rich tissue, like tendons, but in vivo tendon mechanical properties and the influence of physical activity after the 8th decade of life remain to be determined. This study aimed to compare in vivo patellar tendon mechanical properties in moderately old (old) and very old adults and the effect of short-term resistance training. Twenty old (9 women, 11 men, >65 yr) and 30 very old (11 women, 19 men, >83 yr) adults were randomly allocated to heavy resistance training (HRT) or no training (CON) and underwent testing of in vivo patellar tendon (PT) mechanical properties and PT dimensions before and after a 3-mo intervention. Previous measurements of muscle properties, blood parameters, and physical activity level were included in the analysis. Data from 9 old HRT, 10 old CON, 14 very old CON, and 12 old HRT adults were analyzed. In addition to lower quadriceps muscle strength and cross-sectional area (CSA), we found lower PT stiffness and Young's modulus ( P < 0.001) and a trend toward the lower mid-portion PT-CSA ( P = 0.09) in very old compared with old subjects. Daily step count was also lower in very old subjects ( P < 0.001). Resistance training improved muscle strength and cross-sectional area equally in old and very old subjects ( P < 0.05) but did not affect PT mechanical properties or dimension. We conclude that PT material properties are reduced in very old age, and this may likely be explained by reduced physical activity. Three months of resistance training however, could not alter PT mechanical properties in very old individuals. NEW & NOTEWORTHY This research is the first to quantify in vivo tendon mechanical properties in a group of very old adults in their eighties. Patellar tendon stiffness was lower in very old (87 yr on average) compared with moderately old (68 yr on average) individuals. Reduced physical activity with aging may explain some of the loss in tendon stiffness, but regular heavy resistance training for 3 mo was not sufficient to change tendon mechanical properties.

Dietary Advanced Glycosylation End-Products (dAGEs) and Melanoidins Formed through the Maillard Reaction: Physiological Consequences of their Intake

The main purpose of this review is to clarify whether the consumption of food rich in melanoidins and dietary advanced glycosylation end-products (dAGEs) is harmful or beneficial for human health. There are conflicting results on their harmful effects in the literature, partly due to a methodological issue in how dAGEs are determined in food. Melanoidins have positive functions particularly within the gastrointestinal tract, whereas the intake of dAGEs has controversial physiological consequences. Most of the in vivo intervention trials were done comparing boiled versus roasted diet (low and high dAGE, respectively). However, these studies can be biased by different lipid oxidation and by different calorie density of foods in the two conditions. The attraction that humans have to cooked foods is linked to the benefits they have had during mankind's evolution. The goal for food technologists is to design low-energy-dense products that can satisfy humans' attraction to rewarding cooked foods.

An advanced glycation endproduct (AGE)-rich diet promotes accumulation of AGEs in Achilles tendon

Advanced Glycation Endproducts (AGEs) accumulate in long‐lived tissue proteins like collagen in bone and tendon causing modification of the biomechanical properties. This has been hypothesized to raise the risk of orthopedic injury such as bone fractures and tendon ruptures. We evaluated the relationship between AGE content in the diet and accumulation of AGEs in weight‐bearing animal Achilles tendon. Two groups of mice (C57BL/6Ntac) were fed with either high‐fat diet low in AGEs high‐fat diet (HFD) (n = 14) or normal diet high in AGEs (ND) (n = 11). AGE content in ND was six to 50‐fold higher than HFD. The mice were sacrificed at week 40 and Achilles and tail tendons were carefully excised to compare weight and nonweight‐bearing tendons. The amount of the AGEs carboxymethyllysine (CML), methylglyoxal‐derived hydroimidazolone (MG‐H1) and carboxyethyllysine (CEL) in Achilles and tail tendon was measured using ultraperformance liquid chromatography tandem mass spectrometry (UPLC‐MS/MS) and pentosidine with high‐pressure liquid chromatography (HPLC) with fluorescent detection. AGEs in Achilles tendon were higher than in tail tendon for CML (P < 0.0001), CEL (P < 0.0001), MG‐H1 and pentosidine (for both ND and HFD) (P < 0.0001). The AGE‐rich diet (ND) resulted in an increase in CML (P < 0.0001), MG‐H1 (P < 0.001) and pentosidine (P < 0.0001) but not CEL, in Achilles and tail tendon. This is the first study to provide evidence for AGE accumulation in injury‐prone, weight‐bearing Achilles tendon associated with intake of an AGE‐rich diet. This indicates that food‐derived AGEs may alter tendon properties and the development of tendon injuries.

Kidney, heart and brain: three organs targeted by ageing and glycation

Advanced glycation end-product (AGE) is the generic term for a heterogeneous group of derivatives arising from a non-enzymatic reaction between reducing sugars and proteins. In recent years, evidence has accumulated that incriminates AGEs in pathogenic processes associated with both chronic hyperglycaemia and age-related diseases. Regardless of their exogenous or endogenous origin, the accumulation of AGEs and their derivatives could promote accelerated ageing by leading to protein modifications and activating several inflammatory signalling pathways via AGE-specific receptors. However, it remains to be demonstrated whether preventing the accumulation of AGEs and their effects is an important therapeutic option for successful ageing. The present review gives an overview of the current knowledge on the pathogenic role of AGEs by focusing on three AGE target organs: kidney, heart and brain. For each of these organs we concentrate on an age-related disease, each of which is a major public health issue: chronic kidney disease, heart dysfunction and neurodegenerative diseases. Even though strong connections have been highlighted between glycation and age-related pathogenesis, causal links still need to be validated. In each case, we report evidence and uncertainties suggested by animal or epidemiological studies on the possible link between pathogenesis and glycation in a chronic hyperglycaemic state, in the absence of diabetes, and with exogenous AGEs alone. Finally, we present some promising anti-AGE strategies that are currently being studied.

Dietary Advanced Glycation End Products and Cardiometabolic Risk

PURPOSE OF REVIEW

This report analyzes emerging evidence about the role of dietary advanced glycation end products (AGEs) as a cardiometabolic risk factor. Two important aspects are discussed: First, the modulation of AGE load by dietary AGEs; second, if the evidence of clinical and observational studies is enough to make dietary recommendations towards lowering AGE intake.

RECENT FINDINGS

Clinical studies in subjects with diabetes mellitus have shown that high intake of dietary AGEs increases inflammation markers, oxidative stress, and could impair endothelial function. In subjects at risk for cardiometabolic diseases (with overweight, obesity, or prediabetes), dietary AGE restriction decreases some inflammatory molecules and improves insulin sensitivity. However, studies in healthy subjects are limited, and not all of the studies have shown a decrease in circulating AGEs. Therefore, it is still unclear if dietary AGEs represent a health concern for people potentially at risk for cardiometabolic diseases. The evidence shows that dietary AGEs are bioavailable and absorbed, and the rate of excretion depends on dietary intake. The metabolic fate of most dietary AGEs remains unknown. Regardless, most studies have shown that by diminishing AGE intake, circulating levels will also decrease. Thus, dietary AGEs can modulate the AGE load at least in patients with DM, overweight, or obesity. Studies with specific clinical outcomes and large-scale observational studies are needed for a better risk assessment of dietary AGEs and to establish dietary recommendations accordingly.

Carboxymethyl-lysine: thirty years of investigation in the field of AGE formation

In 1985 carboxymethyl-lysine (CML), the first glycoxidation product, was discovered by Dr Ahmed while trying to identify the major products formed in reactions of glucose with lysine under physiological conditions. From that moment, a significant number of researchers have joined efforts to study its formation routes both in foods and in living beings, and the possibility of the existence of an additive action between food-occurring and in vivo produced CML and to explore all the implications associated with its appearance in the biological systems, regardless of its origin. This review presents interesting information on the latest advances in the research on CML sources, mitigation strategies, intake, metabolism and body fluid and tissue delivery, its possible in vivo synergy with highly modified advanced glycation end products-protein, and the physio-pathological implications derived from the presence of this compound in body fluids and tissues.

Assessing the effects of model Maillard compound intake on iron, copper and zinc retention and tissue delivery in adult rats

The behaviour of dietary Maillard reaction compounds (MRP) as metal chelating polymers can alter mineral absorption and/or retention. Our aim in this study was to analyse the long-term effects of the consumption of model MRP from glucose-lysine heated for 90 min at 150 °C (GL) on iron, copper and zinc whole-body retention and tissue delivery. For 88 days, weaning rats were fed a Control diet or one containing 3% GL, until reaching the adult stage. During the experimental period a mineral balance was conducted to investigate the mineral retention. At day 88, the animals were sacrificed, blood was drawn for haemoglobin determination and some organs were removed. Copper and zinc balances were unaffected (Cu: 450 vs. 375 μg; Zn: 6.7 vs. 6.2 mg for Control and GL groups, respectively) and no change was observed in whole-body delivery. Iron retention, too, was unaltered (11.2 mg for Control and GL groups) but due to the tendency toward decreased body weight in the GL group (248 vs. 233 g for the Control and GL groups), whole-body iron concentration was 13% higher in the GL group than in the Control group. Absorbed iron accumulated particularly in the liver (144 vs. 190 μg g(-1) for the Control and GL groups), thus reducing haemoglobin levels. The long-term intake of MRP induced iron accumulation in the body but this did not result in enhanced iron functionality, since the haemoglobin concentration declined. Taking into account the findings of our research group's studies of young and adult rats, we now corroborate the hypothesis that the negative effect of GL MRP consumption on iron functionality takes place regardless of the animals' stage of life.

Diet low in advanced glycation end products increases insulin sensitivity in healthy overweight individuals: a double-blind, randomized, crossover trial

BACKGROUND

The consumption of advanced glycation end products (AGEs) has increased because of modern food processing and has been linked to the development of type 2 diabetes in rodents.

OBJECTIVE

We determined whether changing dietary AGE intake could modulate insulin sensitivity and secretion in healthy, overweight individuals.

DESIGN

We performed a double-blind, randomized, crossover trial of diets in 20 participants [6 women and 14 men; mean ± SD body mass index (in kg/m(2)): 29.8 ± 3.7]. Isoenergetic- and macronutrient-matched diets that were high or low in AGE content were alternately consumed for 2 wk and separated by a 4-wk washout period. At the beginning and end of each dietary period, a hyperinsulinemic-euglycemic clamp and an intravenous glucose tolerance test were performed. Dietary, plasma and urinary AGEs N(€)-(carboxymethyl)lysine (CML), N(€)-(carboxyethyl)lysin (CEL), and methylglyoxal-derived hydroimadazolidine (MG-H1) were measured with the use of mass spectrometry.

RESULTS

Participants consumed less CML, CEL, and MG-H1 during the low-AGE dietary period than during the high-AGE period (all P < 0.05), which was confirmed by changes in urinary AGE excretion. There was an overall difference in insulin sensitivity of -2.1 mg · kg(-1) · min(-1) between diets (P = 0.001). Insulin sensitivity increased by 1.3 mg · kg(-1) · min(-1) after the low-AGE diet (P = 0.004), whereas it showed a tendency to decrease by 0.8 mg · kg(-1) · min(-1) after the high-AGE diet (P = 0.086). There was no difference in body weight or insulin secretion between diets (P = NS).

CONCLUSIONS

A diet that is low in AGEs may reduce the risk of type 2 diabetes by increasing insulin sensitivity. Hence, a restriction in dietary AGE content may be an effective strategy to decrease diabetes and cardiovascular disease risks in overweight individuals. This trial was registered at clinicaltrials.gov as NCT00422253.

Toxicological evaluation of advanced glycation end product Nε-(carboxymethyl)lysine: Acute and subacute oral toxicity studies

Nε-(carboxymethyl)lysine (CML) as a novel potential noxious compound in various food products has aroused extensive concern in recent years. This study aimed to investigate the oral acute and subacute toxicity of CML in mice as per OECD 420 and 407 guidelines. Acute administration of 2000 and 5000 mg/kg CML did not induce any mortality within 14 days, nevertheless some toxicological symptoms and histopathological changes were observed. The estimated LD50 of CML was >5000 mg/kg. In subacute toxicity test, CML was dosed at 200, 500 and 1000 mg/kg in both genders for 28 days. The body weights reduced which was accompanied with the decrease of food consumptions. Hematology parameters viz. RBC, HGB and MCH showed minor alteration but these were still within normal range. Biochemical analysis of hepatic and renal function markers showed significant elevating in AST, ALT, Cr and BUN etc. Histopathological alterations were observed in lung, liver, kidney and spleen. Subacute toxicity of CML involved oxidative stress caused by reducing antioxidant enzyme (SOD and GSH-Px) activities, and significantly increasing lipid peroxide (MDA) level. In conclusion, CML was unlikely to present an acute hazard, but repeated administration could produce deleterious effects on mice especially inducing liver and kidney damage through oxidative stress.

Measurement of urinary advanced glycation end-products (AGEs) using a fluorescence assay for metabolic syndrome-related screening tests

AIMS

The simple screening test of advanced glycation end-products (AGEs) has not been established yet. We aimed to clarify the usefulness of simple measurement of AGEs for screening tests.

METHODS

The subjects were healthy participants and patients with metabolic syndrome. Urine samples were diluted from 1:10 to 1:200 using phosphate-buffered saline, and the fluorescence intensity was measured at 440nm after excitation at 370nm in a 96-well microplate spectrophotometer. The obtained intensities were adjusted according to the urinary creatinine levels.

RESULTS

In patients with metabolic syndrome, urinary AGE levels were significantly higher than in healthy individuals (median [range], 168.25 [82.51-1276.15] AU/g creatinine [n=37] versus 134.67 [37.86-776.31] AU/g creatinine [n=350], respectively; p=0.0066). We found significant positive correlations between urinary AGEs and systolic and diastolic blood pressures (Spearman's correlation r=0.119 [p=0.019] and r=0.128 [p=0.012], respectively). There was no significant correlation between estimated glomerular filtration rate and urinary AGEs (r=0.018 [p=0.744]), confirming that renal dysfunction did not influence results of urinary AGE measurements. When all of the participants in the study were classified into four groups according to the numbers of components of metabolic syndrome, we found a significant tendency (p=0.0127) for urinary AGE levels to be higher with the increasing number of metabolic syndrome components.

CONCLUSION

These results suggested that measurement of urinary AGE levels may be useful for evaluating the risk of metabolic syndrome.

Effects of Long-Term Exposure to Free Nε-(Carboxymethyl)lysine on Rats Fed a High-Fat Diet

Recently correlation studies between dietary advanced glycation end products (AGEs) and the progression of chronic diseases have attracted much attention. To explore the impact of dietary AGEs on the health risk of people consuming a high-fat diet (HFD), male Sprague-Dawley rats were used as the research subject. Under HFD, free N(ε)-(carboxymethyl)lysine (CML, a major AGE, 60 mg/kg body weight/day) was administered by gavage for 12 consecutive weeks. The results indicated that protein-bound CML accumulation in the kidney, heart, lung, pancreas, and muscle significantly increased to 178 ± 36, 161 ± 2, 106 ± 11, 39 ± 8, and 141 ± 20 μg/g dry matter, respectively, compared to HFD control levels of 86 ± 9, 127 ± 10, 89 ± 6, 23 ± 2, and 95 ± 3 μg/g dry matter, whereas no statistical increase was found in the liver and spleen. An increase of the protein-bound CML might be due to free CML binding to proteins in tissues by covalent bonds. Moreover, the rats' serum low-density lipoprotein cholesterol concentration, fasting blood glucose levels, and energy expenditure also increased obviously. These findings indicated that long-term intake of high-dose free CML might be a health risk factor for rats with HFD. This could provide valuable information for further study on the possible effects of long-term consumption of CML-rich fatty foods on human health, involving the progression of chronic disease.

Pathologic role of dietary advanced glycation end products in cardiometabolic disorders, and therapeutic intervention

Reactive derivatives from nonenzymatic glucose-protein condensation reactions, as well as lipids and nucleic acids exposed to reducing sugars, form a heterogeneous group of irreversible adducts called AGEs (advanced glycation end products). The glycation process begins with the conversion of reversible Schiff base adducts to more stable, covalently bound Amadori rearrangement products. Over the course of days to weeks, these Amadori products undergo further rearrangement and condensation reactions to form irreversibly cross-linked macroprotein derivatives known as AGEs. The formation and accumulation of AGEs have been known to progress in a physiological aging process and at an accelerated rate under hyperglycemic and oxidative stress conditions. There is growing evidence that AGEs play a pathologic role in numerous disorders. Indeed, glycation and/or cross-linking modification of circulating or organic matrix proteins by AGEs the senescence of moieties and deteriorate their physiological function and structural integrity in multiple organ systems. Moreover, AGEs elicit oxidative stress and inflammatory reactions through the interaction with the receptor for advanced glycation products in a variety of cells, thereby contributing to the development and progression of various aging- or diabetes-related disorders, such as cardiovascular disease, chronic kidney disease, insulin resistance, and Alzheimer's disease. Recently, diet has been recognized as a major environmental source of AGEs that could cause proinflammatory reactions and organ damage in vivo. Therefore, this review summarizes the pathophysiological role of dietary AGEs in health and disease, especially focusing on cardiometabolic disorders. We also discuss the potential utility in targeting exogenously derived AGEs for therapeutic intervention.

Increased accumulation of protein-bound N(ε)-(carboxymethyl)lysine in tissues of healthy rats after chronic oral N(ε)-(carboxymethyl)lysine

In recent years, chronic diseases related to advanced glycation end products (AGEs) have attracted more attention. Because diet is an important exogenous source of AGEs, this study aimed to investigate the effects of chronic oral administration of pure N(ε)-(carboxymethyl)lysine (CML) (a major AGE) at 60 mg kg(-1) per day on healthy Sprague-Dawley rats. After administration for 12 weeks, the levels of protein-bound CML were increased to 202 ± 17, 167 ± 47, 217 ± 44, 107 ± 4, 144 ± 23, and 33 ± 7 μg/g dry matter in the kidneys, heart, liver, lungs, spleen, and pancreas, respectively, in comparison with control values of 98 ± 1, 90 ± 15, 140 ± 42, 76 ± 18, 115 ± 15, and 30 ± 4 μg/g dry matter. The difference was significant (p < 0.05) for the kidneys, heart, liver, and lungs, whereas no significant increase was seen in the spleen and pancreas. Furthermore, serum blood urea nitrogen (BUN), creatinine (CREA), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) values increased significantly (p < 0.05), as evidence of impaired kidney and liver function. Additionally, the rats' fasting blood glucose (FBG) levels remained within the normal range, indicating that chronic intake of CML does not promote a rise in blood glucose. These results clearly indicate that a CML-rich diet might be a potential health risk in humans, particularly with respect to kidney and liver function.

Systemic stiffening of mouse tail tendon is related to dietary advanced glycation end products but not high-fat diet or cholesterol

Tendon pathology is related to metabolic disease and mechanical overloading, but the effect of metabolic disease on tendon mechanics is unknown. This study investigated the effect of diet and apolipoprotein E deficiency (ApoE(-/-)) on mechanical properties and advanced glycation end product (AGE) cross-linking of non-weight-bearing mouse tail tendons. Twenty ApoE(-/-) male mice were used as a model for hypercholesterolemia along with 26 wild-type (WT) mice. One-half of the mice from each group was fed a normal diet (ND) and the other half was fed a high-fat diet (HFD) to induce obesity. All were killed at 40 wk, and tail tendon fascicles were mechanically tested to failure and analyzed for AGEs. Diets were also analyzed for AGEs. ApoE(-/-) mice displayed a 14% increase in plateau modulus compared with WT mice (P < 0.05), whereas HFD mice displayed a 13% decrease in plateau modulus (P < 0.05) and a 12% decrease in total modulus (P < 0.05) compared with ND mice. Tail tendons of HFD mice had significantly lower concentrations of AGEs [carboxymethyllysine (CML): 26%, P < 0.0001; methylglyoxal-derived hydroimidazolone 1 (MG-H1): 15%, P < 0.005; pentosidine: 13%, P < 0.0005]. The HFD had ∼44-fold lower content of CML (P < 0.01), ∼29-fold lower content of carboxyethyllysine (P < 0.005), and ∼16-fold lower content of MG-H1 (P < 0.05) compared with ND. ApoE(-/-) increased, whereas HFD decreased mouse tail tendon stiffness. Dietary AGE content may be a crucial determinant for accumulation of AGE cross-links in tendons and for tissue compliance. The results demonstrate how systemic metabolic factors may influence tendon health.