What are advanced glycation endproducts?

AGE are a very large and heterogeneous group of compounds that originate from the spontaneous reaction of reducing sugars with free amino groups in amino acids in the so-called Maillard or browning reaction^{(Reference Lin, Wu and Yen2)}. The Maillard reaction is well established, but AGE can also form through many other reactions, including oxidation of sugars, lipids and amino acids. Specific AGE such as pentosidine, N ^ε-carboxymethyllysine (CML), N ^ε-carboxyethyllysine (CEL) and methylglyoxal derivatives (MG-H1; N ^δ-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine) are some of the most commonly measured and well-described AGE in biological studies^{(Reference Vistoli, De Maddis and Cipak3)}.

Endogenous AGE form physiologically in the body at a continuous slow rate, which is markedly increased in conditions of hyperglycaemia or elevated OS. AGE, however, can also form exogenously, outside of the body, in any system when the required reagents for the above reactions are available^{(Reference O’Brien and Morrissey4-Reference Uribarri, Woodruff and Goodman6)}. AGE form spontaneously in food, but their rate of formation is markedly increased when food is processed and cooked with heat. Higher temperature, longer time of application, lower moisture, presence of trace metals and higher pH of the cooking system enhance AGE formation^{(Reference O’Brien and Morrissey4-Reference Uribarri, Woodruff and Goodman6)}. Therefore, cooking methods that use dry heat, such as broiling, grilling, searing and frying have been shown to have the highest content of AGE, while cooking methods that use low heat application in the presence of high water content such as stewing, poaching or steaming produce significantly less AGE^{(Reference O’Brien and Morrissey4-Reference Uribarri, Woodruff and Goodman6)}. Only a fraction of the ingested dietary AGE are absorbed into the body AGE pool, where they become indistinguishable from their endogenous counterparts, both in structure and function^{(Reference Koschinsky, He and Mitsuhashi7)}.

Dietary AGE intake can be easily decreased by changing the method of cooking from a high dry heat application to a low heat and high humidity, independent of nutrient composition. Databases with the AGE content of common foods have been published and can be used to calculate dietary AGE intake as well as to provide guidance on how to consume meals with less AGE content^{(Reference O’Brien and Morrissey4-Reference Uribarri, Woodruff and Goodman6)}. The essential concept in the low-AGE diet is that the same type and amount of foods consumed can provide very different amounts of AGE depending on the cooking method. For example, using published data^{(Reference Goldberg, Cai and Peppa5,Reference Uribarri, Woodruff and Goodman6)} in which the AGE content of food was measured with ELISA and expressed in arbitrary AGE kilounits (kU)/serving, a 90 g beefsteak contains 720 AGE kU when raw, 2199 kU when stewed and 6731 kU when broiled^{(Reference Goldberg, Cai and Peppa5,Reference Uribarri, Woodruff and Goodman6)} . The temperature and method of cooking appear more critical to AGE formation than time of cooking; for instance, meat samples broiled or grilled at 230°C for shorter cooking times have higher AGE content when compared with samples boiled in liquid media at 100°C for longer periods^{(Reference Goldberg, Cai and Peppa5)}. Unfortunately, there is no specific threshold temperature above which AGE start to generate and one can only make the general recommendation that the lower the temperature the less the amount of AGE generated^{(Reference O’Brien and Morrissey4-Reference Uribarri, Woodruff and Goodman6)}. At present, there are no official recommendations about acceptable dietary AGE intake. It has been previously proposed that half of the current mean AGE intake, or about 7500 kU/d, would be a realistic goal^{(Reference Goldberg, Cai and Peppa5,Reference Uribarri, Woodruff and Goodman6)} since some studies have shown that dietary AGE reduction of this magnitude is feasible and can significantly alter levels of circulating AGE, while at the same time reducing levels of markers of OS and inflammation and enhancing insulin sensitivity in diabetic patients^{(Reference Uribarri, Cai and Ramdas8)}. Because the low-AGE diet depends on the culinary technique, and not on the food being cooked, it can be applied for any group of individuals; the same principles will apply whether the patient has diabetes, CVD or not.

AGE can be measured by several methods including fluorescence, ELISA and MS. Most of the early data on food AGE were based on ELISA measurements, which has raised significant criticism since these assays have more problems in assessing food matrices than methods using MS^{(Reference Assar, Moloney and Lima9)}. This controversy has prevented a faster development of this field and a general agreement in terms of actual quantification of the AGE content in different foods is still in discussion. At present, however, databases with the food AGE content measured by MS have been published and can be used by anyone^{(Reference Scheijen, Clevers and Engelen10)}.

A frequently ignored source of exogenous AGE is tobacco smoking^{(Reference Cerami, Founds and Nicholl11)}. Curing of tobacco generates large amounts of AGE that after smoke inhalation are absorbed into the circulation, increasing the body AGE pool similarly to food-derived AGE^{(Reference Cerami, Founds and Nicholl11)}. Once in the bloodstream, these AGE may react with serum proteins such as apoB as well as with vascular wall proteins and thereby accelerate the development of atherosclerosis, whose incidence is increased in smokers. These smoking-derived AGE have been shown to have the same actions as endogenous AGE and therefore may cross-link proteins and activate receptors throughout the body^{(Reference Cerami, Founds and Nicholl11)}.

Gastrointestinal absorption of dietary advanced glycation endproducts

Gastrointestinal metabolism of dietary AGE depends on two phenomena: hydrolysis of proteins and absorption of the resulting free amino acids or small peptides containing the AGE. The nutritional value of proteins is reduced by the Maillard reaction, mainly due to a reduction of the digestibility of proteins and the bioavailability of some amino acids^{(Reference Sharma, Kaur and Thind12,Reference Su, Li and Zhao13)} . Accessibility of digestive enzymes for hydrolysis is restricted by chemical modifications of the target amino acids and structural changes of proteins that limit enzyme–substrate interactions^{(Reference Su, Li and Zhao13-Reference Zhao, Le and Larsen15)}, all of which reduce the rate of protein hydrolysis^{(Reference Salazar-Villanea, Bruininx and Gruppen16)} and increase the molecular weight of the resulting peptides^{(Reference Zhao, Li and Le14,Reference Hellwig, Matthes and Peto17,Reference Salazar-Villanea, Bruininx and Gruppen18)} . The extent of hydrolysis of a protein containing Maillard modifications depends on the type of AGE that are formed^{(Reference Zhao, Le and Larsen15,Reference Hellwig, Matthes and Peto17)} , the extent of modification of the protein^{(Reference Zhao, Li and Le14)} and the protein source^{(Reference Hellwig, Matthes and Peto17)}. For example, after hydrolysis of a CML-modified casein under simulated gastric and intestinal conditions, most of the CML-containing peptides resulted in molecular weights between 250 and 1000 Da, which are assumed to be digested, yet not necessarily absorbable^{(Reference Hellwig, Matthes and Peto17)}.

Absorption of dietary AGE depends on the molecular weight of the products of protein hydrolysis^{(Reference Zhao, Li and Le14,Reference Hellwig, Matthes and Peto17)} , the type of transporters present in the gastrointestinal epithelium^{(Reference Stefanie, Michael and Madlen19,Reference Hellwig, Geissler and Matthes20)} and the type of AGE being transported^{(Reference Förster, Kühne and Henle21)}. Some free-form AGE can be absorbed by simple diffusion^{(Reference Grunwald, Krause and Bruch22)}, while peptide-bound AGE require peptide transporters to cross the epithelium^{(Reference Hellwig, Geissler and Matthes20)}, for example the di- and tripeptide transporter PEPT1^{(Reference Stefanie, Michael and Madlen19)}. Glycated dipeptides can cross a barrier of epithelial cells probably using the peptide transporter PEPT1^{(Reference Hellwig, Geissler and Matthes20)}. This was demonstrated for low molecular-weight peptides containing pyrraline that once inside the cell are hydrolysed allowing free pyrraline to enter the basolateral side by simple diffusion^{(Reference Hellwig, Geissler and Peto23)}. Furthermore, the transport of peptide-bound pyrraline across Caco-2 cells was shown to be 15-fold higher compared with free pyrraline^{(Reference Hellwig, Geissler and Peto23)}.

In one clinical study, approximately 10 % of the dietary intake of AGE reached the systemic circulation^{(Reference Koschinsky, He and Mitsuhashi7)}, which is consistent with a recent study in pigs^{(Reference Salazar-Villanea, Butré and Wierenga24)} showing that the apparent ileal digestibility of CML, N ^ε-carboxyethyllysine (CEL) and fructose-lysine ranged between 0 and 30 %.

The intestinal in situ formation of fructose-derived AGE has been postulated, which once absorbed, become part of the body AGE pool, similar to dietary AGE^{(Reference DeChristopher, Uribarri and Tucker25)}. Intraluminal formation of fructose-derived AGE may be facilitated by increased intake of beverages and food containing high-fructose maize syrup in which the ratio of fructose to glucose is higher than 1:1, therefore creating conditions of potential fructose malabsorption favouring intraluminal generation of fructose-derived AGE^{(Reference DeChristopher, Uribarri and Tucker25)}. Preliminary in vitro studies have confirmed this postulation^{(Reference Bains, Gugliucci and Caccavello26,Reference Martinez-Saez, Fernandez-Gomez and Weijing27)} . Increased fructose intake and absorption as such may also potentially increase the endogenous formation of AGE and therefore increase the body AGE pool with all its subsequent effects^{(Reference Aragno and Mastrocola28)}.

Once exogenous AGE are absorbed into the circulation, they act in the circulation and at different cellular and tissue levels inducing either protein cross-linking or a variety of cellular effects that eventually lead to increased OS and inflammation, precursors of most non-infectious chronic diseases.

Fig. 1 illustrates the metabolism of dietary AGE within the gastrointestinal tract.

Fig. 1.

Metabolism of dietary advanced glycation endproducts (AGE). Panel 1A: Food AGE are ingested. Panel 1B: Some AGE may form in situ within the lumen of the intestine in conditions of high local fructose. Panel 2: Both exogenous and in situ-formed AGE may be partially absorbed through the small intestine into the circulation from where they can be excreted in the urine by the kidneys or reach the tissues where they can produce direct protein cross-linking or react with cellular receptors to induce inflammatory cytokines or oxidative stress. Panel 3: AGE not absorbed in the small intestine move to the colon where several things can happen. 3a: AGE may be broken down by bacteria of the microbiota. 3b: AGE may react with bacteria, releasing compounds that may be absorbed across the colon. 3c: AGE may potentially be absorbed through the colon mucosa or locally activate RAGE. 3d: AGE can be eliminated in the stools. For a colour figure, see the online version of the paper.

The role of unabsorbed dietary advanced glycation endproducts in the colon

The major part of the dietary AGE escapes gastrointestinal absorption with a potential effect of unabsorbed dietary AGE in the colon microbiota and even directly in the colon wall. Some of these AGE could bind to the receptor for AGE (RAGE) or Toll-like receptors in the colon cells inducing a local inflammatory response with subsequent release of inflammatory mediators into the circulation eventually producing chronic diseases, or they could alter the microbiome profile in the gut, which in turn leads to release of toxins or inflammatory mediators into the circulation^{(Reference Snelson and Coughlan29)}. We know from studies in mice that activation of RAGE-dependent signalling is key in creating mucosal barrier dysfunction after haemorrhagic shock^{(Reference Raman, Sappington and Yang30)}. One could postulate that this RAGE-dependent disruption of the intestinal epithelial barrier may contribute to systemic inflammation by facilitating translocation of endotoxin, microbial fragments and other factors produced locally by the gut flora^{(Reference Vaziri, Zhao and Pahl31)}.

Degradation of CML and pyrraline by human gut microbiota has been well documented^{(Reference Hellwig, Auerbach and Müller32)}. The products of this degradation of dietary AGE in the large intestine could in turn modulate the profile of the bacterial population and the metabolites produced by them, which in turn could facilitate systemic illnesses^{(Reference Raman, Sappington and Yang30)}.

A 2‐week randomised two‐period cross-over study, in which two different diets were consumed by male adolescents, showed negative correlation between faecal lactobacilli and enterobacteria numbers and dietary CML content^{(Reference Seiquer, Rubio and Peinado33)}. A different study analysed the effects of a low- v. high-AGE diet for 1 month in a small group of peritoneal dialysis patients^{(Reference Yacoub, Nugent and Cai34)}. Dietary AGE restriction altered the bacterial gut microbiota with a significant reduction in Prevotella copri and Bifidobacterium animalis relative abundance and increased Alistipes indistinctus, Clostridium citroniae, C. hathewayi and Ruminococcus gauvreauii relative abundance^{(Reference Yacoub, Nugent and Cai34)}. The exact significance and systemic effects of these changes in the faecal microbiota, however, remain to be determined.

The role of the advanced glycation endproduct–receptor for advanced glycation endproduct–reactive oxygen species loop

During the last three decades, compelling evidence derived from both clinical and experimental research has revealed that AGE have profound effects on human pathology, far beyond diabetes complications^{(Reference Uribarri, del Castillo and de la Maza1,Reference Lin, Wu and Yen2,Reference Yan, Ramasamy and Schmidt35)} . High dietary AGE intake produces a sustained oxidant overload that may overwhelm host defences and lead to unopposed OS and chronic subclinical inflammation, which over time raises susceptibility to a variety of chronic clinical conditions^{(Reference Vlassara and Uribarri36)}.

AGE exert their action through different mechanisms; one is based on the capacity of these compounds to alter the function of many biomolecules, either by reducing its biological activity or by inducing cross-linking with proteins^{(Reference Rojas, González, Añazco and Uribarri37)}. Additionally, several AGE-binding proteins have been identified. Some of these proteins play an important role in the removal of AGE, including the AGE receptor complex, AGE-R1/OST-48, AGE-R2/80K-H, AGE-R3/galectin-3, as well as some members of the scavenger receptor family^{(Reference Yan, Ramasamy and Schmidt35,Reference Rojas, González, Añazco and Uribarri37)} . Very interestingly, the activity of AGE-R1 is not only restricted to clearance of AGE, but also it has further activities linked to counteract the pro-oxidant activities of AGE^{(Reference Cai, He and Zhu38,Reference Cai, He and Zhu39)} . Furthermore, AGE also bind to RAGE on different cell types, thus activating key cell signalling pathways with subsequent modulation of inflammation-related gene expression. The RAGE–ligand interaction triggers activation of NF-κB and other signalling pathways through stimulation of p21ras, extracellular signal-regulated kinase (ERK) 1/2, p38 mitogen-activated protein kinase (p38 MAPK), stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), Rho GTPases, phosphoinositide 3-kinase (PI3K) and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathways^{(Reference Rojas, Delgado-López and González40-Reference Ott, Jacobs and Haucke44)}. Subsequently, expression of many inflammatory gene products is increased, leading to the establishment of an inflammatory niche^{(Reference Hofmann, Drury and Fu45)}.

RAGE binds ligands other than AGE, particularly high-mobility group protein box-1 (HMGB1), calgranulins, amyloid-β-protein, triggering cellular responses extensively involved in different pathophysiological scenarios including inflammation, proliferation, angiogenesis, migration and fibrosis^{(Reference Wautier, Guillausseau and Wautier46-Reference Alleyn, Breitzig and Lockey49)}. Binding of RAGE not only causes an inflammatory gene expression profile, but also a feed-forward loop, in which inflammatory stimuli activate NF-κB, which induces RAGE expression, subsequently followed by sustained NF-κB activation. Thus, RAGE can function as a master switch, converting short-lasting pro-inflammatory responses into long-lasting cellular dysfunction^{(Reference Bierhaus, Schiekofer and Schwaninger50,Reference Kierdorf and Fritz51)} .

The complexity of signalling is further demonstrated by the presence of alternative adaptor molecules such as DAP10, and even by the recruitment of TIRAP (Toll/IL-1 receptor domain-containing adapter protein) and MyD88, well-known adaptor proteins for downstream signal transduction of Toll-like receptor (TLR)-2 and TLR-4, which are critical for enhancing RAGE-mediated signalling^{(Reference Sakaguchi, Murata and Aoyama52,Reference Sakaguchi, Murata and Yamamoto53)} .

RAGE and TLR also share common ligands and signalling pathways, and accumulating evidence points towards their cooperative interaction. This fact is supported by several experimental findings, such as the capacity of AGE to induce PPARγ down-regulation-mediated inflammatory signalling via TLR4 and RAGE^{(Reference Chen, Sheu and Tsai54)}, as well as that AGE-modified molecules activate TLR signalling through binding to either TLR receptors on the cell surface or the intracellular adaptor proteins of TLR^{(Reference Cheng, Dong and Zhu55)}.

The RAGE gene undergoes extensive alternative splicing to produce a variety of transcripts with diverse functions. In this context, soluble variants include a secreted form RAGE v1 (previously named as sRAGE, secretory C-truncated RAGE, esRAGE, hRAGE sec or sRAGE1/2/3) and an N-terminally truncated isoform RAGE v2 (previously named Nt-RAGE, N-RAGE or N-truncated RAGE)^{(Reference González, Romero and Rodríguez56)}. However, endogenous soluble RAGE isoforms may be generated by mechanisms other than alternative splicing, such as membrane-associated proteases, including the sheddase A disintegrin, metalloprotease 10 (ADAM-10) and matrix metalloproteinase-9 (MMP-9)^{(Reference Raucci, Cugusi and Antonelli57,Reference Zhang, Bukulin and Kojro58)} .

Soluble RAGE is thought by many authors to function as a decoy receptor, and thus preventing the interaction with the membrane-anchored full-length RAGE^{(Reference Salazar-Villanea, Butré and Wierenga24)}. In this context, there is evidence supporting a role for soluble RAGE contributing to the removal/detoxification of AGE^{(Reference Santilli, Vazzana and Bucciarelli59)}.

In the 1990s, it was initially reported that RAGE activation by AGE was associated with increased OS, as demonstrated either by the use of blocking antibodies to RAGE or antioxidants, both in vivo and in vitro ^{(Reference Yan, Schmidt and Anderson60)}. As a consequence of RAGE engagement, multiple signalling pathways are induced, including the generation of reactive oxygen species (ROS), mainly due to the activation of the NADPH oxidase (NOX) pathway^{(Reference Guo, Ananthakrishnan and Qu61,Reference Wautier, Wautier and Schmidt62)} . Interestingly, AGE–RAGE-induced cytosolic ROS production also facilitates mitochondrial superoxide production in hyperglycaemic environments, which play a central role in inducing diabetic-associated organ damage^{(Reference Coughlan, Thorburn and Penfold63)}. Of note, the cytoplasmic domain of RAGE binds to the formin molecule mDia1, through its conserved poly-proline rich formin homology-1 (FH1) domain, and this interaction is strictly required for RAGE ligands to activate cell signalling responses. Formins such as mDia1 are actin-binding molecules that contribute to signal transduction mechanisms, in part via Rho GTPase signals^{(Reference Young and Copeland64)}, and particularly Rac1, which is a key component in NADPH oxidase activation^{(Reference Hordijk65-Reference Acevedo and González-Billault67)}. There is increasing evidence that ROS induced by NADPH oxidases not only causes cell damage, but also acts as a second messenger implicated in proliferation, differentiation and apoptosis, which finally may lead to disease onset and progression^{(Reference Elnakish, Hassanain and Janssen68-Reference Hoffmann and Griffiths72)}.

OS and inflammation are inseparable actors in the pathogenesis of many human diseases. For instance, by activating NF-κB, a redox-sensitive transcription factor of major importance in inflammation, OS promotes the expression of pro-inflammatory cytokines and chemokines, increasing the recruitment and activation of leucocytes and resident cells, thereby fuelling inflammatory processes^{(Reference Gloire, Legrand-Poels and Piette73-Reference Gloire and Piette77)}. Additionally, plasma proteins are prone to be oxidised by ROS. These oxidised protein products, termed advanced oxidation protein products (AOPP), have been linked to vascular lesions in diabetes, chronic renal disease, obesity, immune-mediated inflammatory diseases, neurodegenerative diseases, cancer, the metabolic syndrome and atherosclerosis^{(Reference Cao, Hou and Nie78-Reference Witko-Sarsat, Friedlander and Nguyen Khoa81)}.

Of note, RAGE has also been described as a receptor of AOPP-modified albumin^{(Reference Guo, Niu and Hou82,Reference Zhou, Cao and Xie83)} . Furthermore, RAGE expression on kidney podocytes is highly increased in response to chronic loading of AOPP. The AOPP–RAGE interaction activates NADPH oxidase, leading to ROS production and to a perpetuation of renal OS^{(Reference Yamamoto and Yamamoto84-Reference Wu, Zhong and Zhu86)}.

Finally, oxidants, derived from either the phagocyte NADPH oxidase or the myeloperoxidase–H₂O₂–chloride system, may lead to AGE production, particularly CML. These CML adducts of proteins act as ligands for RAGE and could potentially generate an important amplifying loop in tissue damage at inflammation sites^{(Reference Anderson, Requena and Crowley87,Reference Anderson and Heinecke88)} (Fig. 2).

Fig. 2.

Advanced glycation endproduct–receptor for advanced glycation endproduct–reactive oxygen species (AGE–RAGE–ROS) loops. RAGE engagement by AGE produces an early increase in NADPH oxidase-derived ROS which in turn, activate the redox-sensitive transcription factor NF-κB, and thus favouring a transcriptional pro-inflammatory profile fuelling inflammation and a positive feed-forward loop for RAGE itself. Additionally, ROS are also involved in another relevant amplifying loop, by promoting the formation of AGE, particularly N ^ε-carboxymethyllysine. CREB, cAMP response element-binding protein; AP-1, activator protein 1. For a colour figure, see the online version of the paper.

Potential role of dietary advanced glycation endproducts in chronic diseases

The effect of dietary AGE on a variety of clinical conditions in human subjects has been studied by many, using either acute oral loads of AGE^{(Reference Koschinsky, He and Mitsuhashi7,Reference Schiekofer, Franke and Andrassy89-Reference Davis, Prasad and Vijayagopal93)} or dietary interventions^{(Reference Vlassara, Cai and Goodman94-Reference Yacoub, Nugent and Cai106)}, ranging from a few weeks up to 1 year consisting essentially of changing the way of cooking food without interfering with the actual energy and macronutrient intake. A summary of these studies with their main findings is listed in Table 1. One of the critiques of these clinical studies is that changing the way of cooking may change not only AGE but also many other factors such as vitamins and phytochemicals contained in food that may confound the interpretation of the effects resulting only as the results of the changes in food AGE content^{(Reference Kellow and Coughlan107)}. In order to address this critique, studies were performed in mice by providing a chronic load of a pure synthetic AGE precursor that produced the same effect as those of a high-AGE diet created by cooking with high heat^{(Reference Cai, Ramdas, Zhu and Chen108)}. However, no similar studies in human subjects are available.

Table 1.

Dietary advanced glycation endproduct (AGE) interventions in human subjects

CML, N ^ε-carboxymethyllysine; FMD, flow-mediated vasodilatation; MG-H1, N ^δ-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine; PAI-1, plasminogen activator-1; VCAM-1, vascular cell adhesion protein-1; OS, oxidative stress; HOMA-IR, homeostatic model assessment of insulin resistance; sAGE, serum AGE; CEL, N ^ε-carboxyethyllysine; CKD, chronic kidney disease; ESRD, end-stage renal disease.

The variability of method used in estimating dietary AGE content, mentioned above, also extends to measurements of AGE is serum, plasma or urine. Different methodologies including ELISA with commercially prepared kits or in-house developed antibodies as well as more precise MS/HPLC-based assays have been used by different groups. In order to give the readers a more balanced view of the different outcomes we are also including the AGE measurement methodology for each study listed in Table 1.

Four of the six acute oral AGE load studies showed that this load significantly increased circulating AGE levels in parallel with increases in markers of inflammation and endothelial dysfunction^{(Reference Koschinsky, He and Mitsuhashi7,Reference Stirban, Negrean and Stratmann90-Reference Negrean, Stirban and Stratmann92)} , while two studies did not find this association^{(Reference Schiekofer, Franke and Andrassy89,Reference Davis, Prasad and Vijayagopal93)} .

In Table 1 we also briefly mention two studies with energy restriction that demonstrated that this intervention is associated with reduced dietary AGE intake, probably related to the low AGE content of the replacement meals used for the studies^{(Reference Iwashige, Kouda and Kouda109,Reference Gugliucci, Kotani and Taing110)} . Another study looked at the effect of energy restriction with a Mediterranean-type diet and showed a significant fall of serum CML levels; subjects were not given the actual meals but only instructions on how to eat^{(Reference Rodriguez, Leiva Balich and Concha111)}. The effect of the Mediterranean diet was tested in healthy elderly subjects in Spain and also showed that this intervention decreased serum AGE levels, although the magnitude of the change was small^{(Reference Lopez-Moreno, Quintana-Navarro and Delgado-Lista112)}.

In the next few sections we will review the above information, first in healthy subjects and in individuals with the metabolic syndrome, diabetes and kidney disease where evidence for a potential effect from dietary AGE comes from intervention studies (Table 1), and then in conditions such as CVD, dementia, sarcopenia and cancer in which intervention studies have not been performed, but there is a rationale for the potential involvement of dietary AGE.