Formation of Fructose-Mediated Advanced Glycation End Products and Their Roles in Metabolic and Inflammatory Diseases^,

Abstract

Fructose is associated with the biochemical alterations that promote the development of metabolic syndrome (MetS), nonalcoholic fatty liver disease, and type 2 diabetes. Its consumption has increased in parallel with MetS. It is metabolized by the liver, where it stimulates de novo lipogenesis. The triglycerides synthesized lead to hepatic insulin resistance and dyslipidemia. Fructose-derived advanced glycation end products (AGEs) may be involved via the Maillard reaction. Fructose has not been a main focus of glycation research because of the difficulty in measuring its adducts, and, more importantly, because although it is 10 times more reactive than glucose, its plasma concentration is only 1% of that of glucose. In this focused review, I summarize exogenous and endogenous fructose metabolism, fructose glycation, and in vitro, animal, and human data. Fructose is elevated in several tissues of diabetic patients where the polyol pathway is active, reaching the same order of magnitude as glucose. It is plausible that the high reactivity of fructose, directly or via its metabolites, may contribute to the formation of intracellular AGEs and to vascular complications. The evidence, however, is still unconvincing. Two areas that have been overlooked so far and should be actively explored include the following: 1) enteral formation of fructose AGEs, generating an inflammatory response to the receptor for AGEs (which may explain the strong association between fructose consumption and asthma, chronic bronchitis, and arthritis); and 2) inactivation of hepatic AMP-activated protein kinase by a fructose-mediated increase in methylglyoxal flux (perpetuating lipogenesis, fatty liver, and insulin resistance). If proven correct, these mechanisms would put the fructose-mediated Maillard reaction in the limelight again as a contributing factor in chronic inflammatory diseases and MetS.

Introduction

The Western diet may be inducing the biochemical alterations that promote metabolic syndrome (MetS)³, type 2 diabetes, and nonalcoholic fatty liver disease (NAFLD). Fructose is a chief candidate for the following reasons: 1) the intake of fructose (especially in beverages) has greatly increased along with the incidence of MetS; 2) >90% of ingested fructose is metabolized by the liver at first pass, where it stimulates de novo lipogenesis to drive hepatic TG synthesis; and 3) this contributes to NAFLD, hepatic insulin resistance, and dyslipidemia (1–3).

Is there an as-yet undiscovered role for fructose-mediated advanced glycation end product (AGE) formation via the Maillard reaction in these processes? The Maillard reaction (adduct formation between reactive carbonyls in glucose, fructose, and their metabolites, such as methylglyoxal or deoxyglucosone, with amino groups in protein, DNA, and lipids) has been recognized as an important pathway at the root of diabetes complications (4–11). Fructose is 8–10 times more reactive than glucose in Maillard reaction product formation because of the higher stability of its open chain form and its keto group (12–17). It does not form the Amadori product, but, rather, the Heyns product. The popular clinical methods used for glucose glycation do not detect the Heyns product or other fructose-mediated adducts (18). This has compromised research on the potential role of fructose glycation in the pathogenesis of chronic disease in humans. Endogenous fructose formed in the sorbitol pathway was proposed early on as a source of AGEs in tissues implicated in diabetes complications (19–21). However, after much research on drugs targeting aldose reductase, the evidence for a critical role of endogenous fructose AGE formation in diabetic complications at the target tissue (endothelium, glomerulus, or neural) level is scant (22, 23). What about exogenous fructose? Given the role of hepatic insulin resistance in MetS, I believe that the recently proposed hypotheses for fructose AGE formation in the intestines before absorption (which can be considered to be premetabolism) and in the liver after portal flux should be given more attention. The putative mechanisms are summarized in this review, including a focused review of exogenous and endogenous fructose metabolism; fructose glycation; and in vitro, animal, and human data so far in order to frame the 2 new hypotheses in the appropriate context.

Exogenous Fructose Metabolism Skips Regulated Steps in Glycolysis and Enhances Lipogenesis

Exogenous fructose metabolism is >90% hepatic. Fructose is taken up by hepatocytes via glucose transporter (Glut) 2 and Glut8 (Figure 1, reaction 1). In our diets, fructose is usually accompanied by glucose [50%:50% in sucrose, ≤60%:40% in high-fructose corn syrup (HFCS), and 66%:33% in apple juice] (24, 25). Glucose will foster glycogenesis and stimulate insulin secretion, which fructose does not do. The crucial difference between glucose and fructose metabolism is that fructose leaps regulated steps in glycolysis-glucokinase and phosphofructokinase (24, 25). Phosphorylation by fructokinase (Figure 1, reaction 2), followed by an aldolase B splitting (Figure 1, reaction 3), leads swiftly and directly to trioses (Figure 1, reaction 4). When there is a simultaneous glucose flux, much of the triose pool becomes the backbone of TGs in de novo lipogenesis (Figure 1, reaction 5). These TGs are secreted with apoB100 as VLDL (Figure 1, reaction 6), or may initiate NAFLD. What has not attracted enough attention is the fact that an unregulated triose flux leads to excess methylglyoxal production (Figure 1, reaction 7) (26, 27). Methylglyoxal damages proteins by glycation. A notable exception is a murine study on hepatosteatosis and glycation by fructose or glucose in hepatocytes on which I will focus later in this review (28).

FIGURE 1.

Fructose metabolism in hepatocytes. Numbers indicate key reactions and are explained in text. Glut, glucose transporter; P, phosphate.

Exogenous Fructose Metabolism Depletes ATP and Increases Uric Acid Production

Because of the unregulated activity of fructokinase, a large fructose load depletes cytosolic ATP, especially when catalyzed by fructokinase C (Figure 1, reaction 2) (29, 30). This is relieved in part by adenylate kinase (Figure 1, reaction 8) with the production of AMP (Figure 1, reaction 9). AMP is turned into uric acid (Figure 1, reaction 10). This pathway has been demonstrated in animals, and data from humans coincide: fructose consumption correlates with hyperuricemia and hypertension (29–32). There is another effect of raising AMP concentrations or lowering ATP:AMP ratios: the activation of AMP-activated protein kinase (AMPK). AMPK is the master allosteric energy regulator of cells, and it senses low energy and promotes catabolic processes while arresting anabolic ones.

Endogenous Fructose Production: The Sorbitol Pathway

The contribution of fructose as an intracellular glycating agent was first suggested as a result of the polyol pathway. The intracellular concentration of fructose is high in the kidney, lens, and peripheral nerves of diabetic patients in whom the polyol pathway is active (19–21, 33). Aldose reductase has a high Michaelis constant for glucose, as depicted in Figure 2. The polyol pathway is inactive with normal glycemia. Conversely, in hyperglycemic conditions, glucose concentrations in insulin-independent tissues, such as neural tissue, glomerulus, lens, and erythrocytes, increase. Accordingly, the polyol pathway is activated. Sorbitol is acted upon by sorbitol dehydrogenase, which leads to fructose and potentially affects redox balance in the cell (19–21, 33). The newly synthesized fructose can be transported or leak passively across the plasma membrane. Acceleration of the polyol pathway is implicated in the pathogenesis of diabetic vascular complications (21–23, 34–36). Although fructose-derived AGEs may be formed intracellularly as a consequence of the polyol pathway, only a few studies have measured this directly. Increasing fructose concentrations in cultured pericytes necessitated >30 mM glucose and sorbitol dehydrogenase overexpression (37). This dampens our enthusiasm with regard to the actual pathogenic role of the process in humans. Moreover, after multiple clinical trials targeting aldose reductase, the evidence of a critical role of endogenous fructose AGE formation in diabetic complications at the target tissue (endothelium, glomerulus, or neural) level is meager, if there is any at all (22, 23).

FIGURE 2.

The sorbitol pathway as an intracellular source, and dietary sources of fructose and AGEs. Question marks indicate areas in which our knowledge or evidence is scant. Dietary fructose is mainly kept at the liver at first pass; therefore, the role of circulating fructose in AGE formation is questionable. AGE, advanced glycation end product; Fru-AGE, fructose-advanced glycation end product; GI, gastrointestinal; G-6-P, glucose-6-phosphate.

Fructose Glycation and AGE Formation

The simplified sequence for glucose and fructose mediated glycation of proteins is shown in Figure 3.

FIGURE 3.

The Maillard reaction by glucose (A) and fructose (B) produce different early and late glycation products. Fru-AGE, fructose-advanced glycation end product; Glc-AGE, glucose-derived advanced glycation end product.

The reaction between D-glucose or D-fructose and the N-terminal amino-acid and/or ε-amino groups of a protein forms Schiff base adducts. In the case of glucose, the Schiff base then undergoes Amadori rearrangement to yield a more stable adduct. With fructose, the reaction is similar, but the reaction is termed a Heyns rearrangement product (with carbon 2 instead of carbon 1 of the hexose), and results in the formation of 2 products (13–17). These early glycation products undergo further rearrangement, dehydration, and condensation reactions to produce irreversibly crosslinked fluorescent derivatives, or AGEs. N-carboxymethyllysine stems from degradation of the Amadori products. Several glucose-derived AGEs have been characterized, such as N-carboxymethyllysine, pentosidine, crosslines, and glucosepane. There is overlap between glucose and fructose AGEs (N-carboxymethyllysine, carboxyethyl-lysine, and pentosidine), as shown in Figure 3. Fructose-specific AGEs, however, have not been well characterized (13–17).

In Vitro Studies of Fructose Glycation

Although well known by food chemists for decades, the Maillard reaction by fructose at physiologic temperatures and pressures started to be studied only in the 1980s. Early studies helped establish the fact that the potential harmful effects of fructose on proteins was far more potent than those of glucose. A wide array of in vitro model proteins was used, as well as tissue culture conditions (13–15, 38–48). Several endogenous compounds, as well as those from food, were shown to inhibit this glycation (49–54). Although providing a chemical framework to explain why fructose may be harmful via the Maillard reaction, all these studies were conducted with concentrations far beyond the ranges found in vivo, and with proteins that may not encounter fructose at all. Indeed, fructose concentrations in plasma are in the lower micromolar range, whereas glucose is 5 mM, 2–3 orders of magnitude higher. The lack of a good circulating marker of fructose AGE adducts or the Heyns product continues to prevent the effective translation of the in vitro information to actual data of clinical relevance.

Animal Studies

AGEs accumulate in the tissues of fructose-fed rats.

In the past 2 decades, a plethora of studies have been conducted in animal models, chiefly rats. The studies in lenses may be the most physiologic, because the sorbitol pathway has a high level of activity in this tissue, and relevant concentrations of fructose may be produced (44). The role of fructose in cataracts has been observed (11, 55). The majority of the rest of the studies used the fructose-fed insulin-resistant rat model. The accumulation of pentosidine and other AGEs in tissues was documented, and its possible association with tendon and vascular rigidity was reported (6–8, 56).

Drugs and natural substances decrease AGEs in the tissues of fructose-fed rats.

The effects of taurine, genistein, aminoguanidine, glycyrrhizin, ursodeoxycolic acid, soy protein, eucalyptus, caffeine, and betaine, ferulic, and lipoic acid on outcomes related to AGE formation or their surrogates were explored in the same model in several studies (56–81). Although valuable in providing proof of concept and underscoring the effects of several antiglycation agents in this model, these studies have several pitfalls that prevent them from translation into human disease. First and foremost, they used very large amounts of free fructose (300 g/L) ad libitum, even though humans do not consume free fructose, but rather consume it usually accompanied by glucose at a ratio of between 50:50 and 66:33 (and sometimes more in pure honey, which is not, however, used by the majority of people to sweeten foods and beverages). Second, these studies did not address modifications in the key organ that may indeed be affected by fructose: the liver. Third, they did not take into account the pleiotropic effects of the compounds studied, which largely may have been acting as antioxidants and not inhibitors of fructose AGE formation. Recent studies showing neural metabolism of fructose, although interesting, were artificial, with fructose being injected directly into cells (59).

Animal studies focusing on the main target of fructose: the liver.

The first hard evidence for a role of hepatic fructose glycation in major clinically relevant outcomes comes from a recent study in mice and hepatosteatosis (28). After 30 wk of an intervention feeding experiment with fructose or glucose, mice that consumed both fructose and glucose ad libitum in drinking water that contained 100 g either fructose or glucose/L had higher fasting glycemia, glucose intolerance, hyperlipidemia, and hepatosteatosis than did controls that drank water alone. Mice that consumed fructose had higher liver TG concentrations than did those that consumed glucose, which was associated with higher expression and activity of sterol regulatory element–binding protein 1, the transcription factor responsible for de novo lipogenesis. LC-MS analysis uncovered a different profile of AGE production in hepatocytes between mice that consumed fructose and those that consumed glucose. Fructose generated more AGEs derived from glyoxal, such as glyoxal-lysine dimer and N-carboxymethyllysine, whereas glucose generated more AGEs derived from methylglyoxal and methylglyoxal-lysine dimer. The high concentrations of N-carboxymethyllysine and activation of sterol regulatory element–binding protein pathway induced by fructose support an important role of this signaling pathway in mediating fructose-induced lipogenesis. To my knowledge, this is the first relevant study linking a major pathway in a key tissue responsible for insulin resistance with fructose and the Maillard reaction. The same authors found similar results for skeletal muscle, but again, this tissue only receives micromolar concentrations of fructose (82).

Human Studies

Fructose consumption is associated with MetS, diabetes, and obesity

The association of high fructose diets with MetS, diabetes, and obesity is undeniable (3, 83), which large epidemiologic studies have demonstrated, notably with soda and fruit juice consumption. Nevertheless, correlation does not necessarily mean causation, and some researchers maintain that this correlation is just due to extra calories, not to specific mechanistic action (3, 83–86). The role of fructose as a lipogenic substrate is well known (3, 83–92). Increased de novo lipogenesis may lead to hepatic steatosis or increased VLDL production associated with small-dense LDL formation and what is called the atherogenic dyslipidemia complex (2). One intervention that increased dietary fructose consumption in adults documented worsening lipid profiles (88). This and many other mechanistic studies were flawed by the fact that they substituted glucose for fructose, when, as stated above, we rarely sweeten food and drinks with pure fructose. Studies that add fructose to the diet also lead to and are confounded by weight change.

Isocaloric fructose restriction decreases insulin resistance, lipogenesis, and the atherogenic dyslipidemia complex

We thought that isocaloric fructose restriction was a better model to mechanistically prove that fructose overconsumption leads to MetS (1, 2). To better demonstrate a primary effect of fructose restriction unrelated to energy intake or weight change, we substituted dietary added fructose calorie-for-calorie with glucose (in starch) in order to retain equivalence for calories, total carbohydrate content, and body weight. We recently reported the effects of a controlled dietary intervention study of isocaloric substitution of starch for sugar on metabolic markers in children with obesity and metabolic comorbidities. We studied 50 children and explored the effects of such a diet, maintained for just 9 d (1, 2). Significant changes in glucose, insulin resistance, the atherogenic dyslipoproteinemia complex (TGs, LDL cholesterol, apoB, apoC-III, apoC-II, and small-dense LDL), and blood pressure were found (1, 2). Notably, another study pointed to opposite effects of fructose overconsumption in adults: serum concentrations of non-HDL cholesterol, LDL cholesterol, apoB, and apoC-III increased in a dose-dependent manner in young adults who consumed beverages that provided ≤25% of calories as HFCS for 2 wk (88). The crossdirectionality of the effects found by us and this group strengthen the validity of the data, which clearly show the mechanisms involved.

Although this is very interesting, is there a role for AGEs in these processes?

As stated earlier, because of the paucity of appropriate practical methods to measure fructose-derived early and late glycation adducts, there is scant information in regard to their presence or their role in humans. With the use of a fructose AGE antibody, researchers found that serum fructose AGEs were 4-fold higher in diabetic subjects than in control subjects (37). Nevertheless, these concentrations did not correlate to glycated hemoglobin, fasting blood glucose, TGs, or total cholesterol concentrations in type 1 diabetic patients, which complicates interpretation. The data, however, if confirmed, lend support to the contention that fructose AGEs are formed in humans especially under hyperglycemic conditions. Are these AGEs formed in serum, or do they leak from high sorbitol pathway tissues? As stated earlier, serum fructose concentrations are very low (10–35 μM), making the in situ production of fructose AGEs in plasma less likely. Would AGEs leak from hepatocytes, given the fact that they get most of the load? Would they come from food?

Indeed, fructose AGE content was measured in >100 commercially available products. The highest concentrations of fructose AGEs were observed in yogurt beverages. Fructose glycation adducts can then be absorbed (≤10% of dietary AGEs are absorbed) and detected in serum (37).

Time to focus on putative roles of AGE formation by exogenous fructose in tissues in which its concentration is high: the intestines and liver

Intestinal fate of fructose: are fructose AGEs formed in the lumen of the small intestine?

It is well known that the absorption of fructose via Glut5 in the enterocyte is slower when unaccompanied by glucose. Fructose malabsorption from a high intake of HFCS has been shown (93–96). It is not rare to achieve a bolus dose of 40 g fructose in liquid form, as is found in 600 mL soda or juice, in many cases along with a large intake of protein and fat to delay gastric motility. As proposed by DeChristopher (97), given fructose high reactivity, would that not lead to luminal AGE formation? Would this not be another source of dietary AGEs formed in situ? Dietary AGEs are believed to be proinflammatory via engagement of the receptor for advanced glycation end products (RAGE). They may participate in chronic inflammation enhancing insulin resistance in MetS and diabetes, as well as in renal failure (97). Studies have been conducted to provide epidemiologic evidence for the luminal fructose AGE formation hypothesis. These studies led to the discovery of previously unsuspected associations of fructose consumption with nonmetabolic inflammatory diseases (98–100). Associations between childhood asthma and consumption of beverages that contain excess fructose (vis-a-vis glucose) have been unveiled. With the use of data from 2801 adults aged 20–55 y from the 2003–2006 NHANES, researchers showed that, independent of all covariates, intake of nondiet soda ≥5 times/wk (compared with the consumption of diet soda) was associated with nearly twice the likelihood of having chronic bronchitis (OR: 1.80; 95% CI: 1.01, 3.20). Results support the hypothesis that fructose malabsorption and fructose reactivity in the intestines could contribute to asthma and chronic bronchitis. Notably, the association is not found when sucrose is involved. The same authors also found that young adults consuming any combination of high-fructose beverages (sodas or fruit juices) 5 times/wk (but not diet soda) were 3 times as likely to have arthritis as nonconsumers or low consumers (OR: 3.01; 95% CI: 1.20, 7.59), independent of all covariates, including physical activity, other dietary factors, blood glucose, and smoking.

The authors suggest that these associations may be caused by ingesting beverages or foods with a fructose-to-glucose ratio that is >1:1, leading to fructose malabsorption and prolonged life of fructose in the gut (97–100). Fructose may then lead to in situ formation of fructose AGEs, which may contribute to lung or joint disease via engagement of RAGE, as depicted on the left in in Figure 4. Are fructose AGEs free adducts forming in the intestinal lumen? Research on the Maillard reaction between fructose and amino acids was conducted by food chemists >50 y ago. But, to my knowledge, no research seems to exist that explores the reaction at millimolar concentrations, with a physiologic pH and ion composition at 37°C. If fructose AGEs are formed in a period of minutes to a few hours, then they can be a source of dietary AGEs, which we have not considered so far. Given the huge consumption of HFCS at least in the United States, the likelihood that AGEs formed in the intestinal lumen may contribute to the circulating AGE load may be of pathogenic significance. With regard to the above-described association, longitudinal and biochemical research is needed to confirm and clarify the mechanisms involved. I believe this Maillard biochemistry should be revisited.

FIGURE 4.

Two areas of fructose-mediated Maillard reaction that have been overlooked. Enteral formation of fructose AGEs, generating an RAGE inflammatory response (A); inactivation of hepatic AMPK by a fructose-mediated increase in methylglyoxal flux (B). AcCoA, acetyl coenzyme A; AGE, advanced glycation end product; AMPK, AMP-activated protein kinase; MG, methylglyoxal; RAGE, receptor for advanced glycation end products.

Hepatic fate of fructose: are fructose-derived AGE precursors involved in hepatic damage?

As stated earlier, fructose is essentially metabolized by the liver to the exclusion of the rest of the body. Fructose is taken up by hepatocytes via Glut2 and Glut8 (100, 101) (Figure 1). What has been overlooked is that an augmented, unregulated triose flux would raise methylglyoxal production (100, 101). Concomitantly, unregulated phosphorylation of large fructose loads depletes cytosolic ATP. Changes in the AMP:ADP ratio greatly increase the activity of AMPK. Fructose flux increases uric acid production, ergo its precursor AMP (3). When following this sequence of events, a high liver flux of fructose would activate AMPK. AMPK induces a cascade of events within cells in response to the fluxes and availability of metabolites. The role of AMPK in regulating cellular energy status [by sensing low energy–using (AMP) as its signal] and activating catabolic pathways while inhibiting anabolic routes places this enzyme at a central control point in maintaining energy homeostasis (102). When activated, AMPK-mediated downstream phosphorylation events switch cells from active ATP consumption to active ATP production. Consequently, an active AMPK raises glucose transport, glycolysis, and β oxidation, and lowers lipogenesis and cholesterol biosynthesis (101). The activation of AMPK also has long-term effects on gene expression and protein synthesis, inhibits mechanistic target of rapamycin pathways (and protein synthesis), and activates mitochondrial biogenesis and autophagy. The latter is an important pathway that prevents liver steatosis (102).

However, essentially the opposite occurs in chronic fructose overconsumption. Actually, reduced glucose transport (insulin resistance) and increased lipogenesis and cholesterol biosynthesis are found both in animal models and in humans when fructose consumption is excessive, which leads to fatty liver and dyslipoproteinemia (89).

Fructose-induced increased methylglyoxal flux may silence AMPK.

To explain this apparent paradox, I recently proffered the hypothesis that surges of fructose in the portal vein lead to increased unregulated flux to trioses accompanied by unavoidable methylglyoxal production (100) (Figure 4). The new sudden flux exerts carbonyl stress on proteins, including the 3 arginines on the γ subunits AMP binding site of AMPK, irreversibly blocking some of the enzyme molecules to allosteric modulation. The above explains why, even when fructose increases AMP and should therefore activate AMPK, the effects of fructose are compatible with the inactivation of AMPK

During evolution, liver AMPK was certainly not exposed to fluxes in fructose such as those effortlessly achieved with 600 mL of juices or soda (in many cases several times per day). The increased flux of trioses produces fragmentation of glyceraldehyde-3-phosphate to methylglyoxal. Methylglyoxal forms stable imidazolone adducts with arginine residues. There are precisely 3 of them in the AMP allosteric site of the enzyme. Therefore, if the above happens, a methylglyoxal-modified AMPK would not be activated by AMP. Although methylglyoxal is detoxified by the glyoxalase system, this pathway could be overwhelmed by the current large fructose loads. De novo lipogenesis, (which should be inhibited by AMPK activation by high AMP) will be not inhibited during fructose surges because of defective allosteric activation of methylglyoxal-modified AMPK, as depicted on the right in Figure 4. The pathways inhibited by AMPK will be there for activated, i.e., gluconeogenesis, glucose output, and cholesterol biosynthesis, all of which are relevant to MetS. Autophagy, which is involved in the control of lipid accumulation in the liver, will now be inhibited.

The accumulated epidemiologic evidence with respect to fructose consumption and metabolic health—together with animal studies and recent mechanistic studies in humans, including those from our group (1–3, 83, 87, 100)—indicates that fructose overconsumption may well be an initiating factor in the development of fatty liver, liver insulin resistance, MetS, and finally diabetes. One of the multiple mechanisms by which fructose may be causative is by abrupt excessive carbonyl pressure on AMPK, which in part loses its allosteric regulation, thereby leading to unrestrained lipogenesis. The Maillard reaction, which is so important in the pathogenesis of long-term diabetic complications, may also participate as an initial causative event via fructose glycation. Research is warranted to prove this mechanism.

Conclusions

Fructose has not been a main focus of glycation research, mainly because of the difficulty of measuring its adducts in a practical way, and, more importantly, because even though fructose is 10 times more reactive than glucose, its concentration in plasma is 1% that of glucose. Intracellularly, fructose is elevated in a number of tissues of diabetic patients in which the polyol pathway is active (101), reaching the same order of magnitude as glucose. It is plausible that the high reactivity of fructose and its metabolites may contribute to the formation of intracellular AGEs and to vascular complications. The evidence is, however, still unconvincing. I believe that there are 2 areas that have been overlooked so far: 1) enteral formation of fructose AGEs (when unpaired excess free fructose is ingested), generating an RAGE inflammatory response (which may explain the strong association between fructose consumption and asthma, chronic bronchitis, and arthritis); and 2) inactivation of hepatic AMPK by a fructose-mediated increase in methylglyoxal flux (perpetuating lipogenesis, fatty liver, and insulin resistance) should be actively explored. If proven correct, these mechanisms would put the fructose-mediated Maillard reaction in the limelight again, as a contributing factor in chronic inflammatory diseases and MetS.