Postprandial plasma fructose level is associated with retinopathy in patients with type 2 diabetes

Fasting Serum Fructose Levels Are Associated With Risk of Incident Type 2 Diabetes in Middle-Aged and Older Chinese Population

OBJECTIVE

We investigated the relationship between fasting serum fructose levels and the risk of incident type 2 diabetes in a prospective Chinese cohort.

RESEARCH DESIGN AND METHODS

Among 949 community-based participants aged ≥40 years without diabetes at baseline, fasting serum fructose levels were measured using liquid chromatography-tandem mass spectrometry. The participants were followed up for the occurrence of diabetes. Cox regression models were performed to analyze the effect of fasting serum fructose levels on risk of incident diabetes.

RESULTS

During a median of 3.5 years' follow-up, 179 of 949 (18.9%) participants developed type 2 diabetes. Elevated fasting serum fructose levels were associated with an increased risk of incident diabetes in a dose-response manner. After adjustment for age, sex, BMI, lipid profiles, blood pressure, liver function, smoking and drinking status, baseline glucose level, and sugar-sweetened beverage consumption, a 1-SD increased fasting fructose level was associated with a 35% (95% CI 1.08-1.67) increased risk of developing diabetes. After further adjustment for serum uric acid and estimated glomerular filtration rate, the association was partially attenuated (hazard ratio 1.33 [95% CI 1.07-1.65]). The association was similar by age, prediabetes status, BMI, and family history of diabetes but attenuated in women (P for heterogeneity = 0.037).

CONCLUSIONS

Elevated fasting serum fructose levels were independently associated with increased risk of incident type 2 diabetes in a middle-aged and older Chinese population. Our data suggest that higher fasting serum fructose levels might serve as a biomarker and/or a contributor to incident diabetes.

Exploring the Diversity of Sugar Compounds in Healthy, Prediabetic, and Diabetic Volunteers

SCOPE

Diabetes is thought to primarily represent a disturbance of carbohydrate metabolism; however, population studies employing metabolomics have mainly identified plasma amino acids and lipids, or their products, as biomarkers. In this pilot study, the aim is to analyze a wide spectrum of sugar compounds in the fasting state and during an oral glucose tolerance test (OGTT) in healthy, prediabetic, and type 2 diabetic volunteers.

METHODS AND RESULTS

The three volunteer groups underwent a standard OGTT. Plasma samples obtained in the fasting state, 30 and 90 min after the OGTT, are subjected to a semitargeted GC-MS (gas chromatography-mass spectrometry) sugar profiling. Overall, 40 sugars are detected in plasma, of which some are yet unknown to change during an OGTT. Several sugars (e.g., trehalose) reveal significant differences between the volunteer groups both in fasting plasma and in distinct time courses after the OGTT. This suggests an endogenous production from orally absorbed glucose and/or an insulin-dependent production/removal from plasma.

CONCLUSION

It is demonstrated that more sugars than expected can be found in human plasma. Since some of these show characteristic differences depending on health status, it may be worthwhile to assess their usability as biomarkers for diagnosing early-stage insulin resistance and type 2 diabetes.

Intestinal Fructose and Glucose Metabolism in Health and Disease

The worldwide epidemics of obesity and diabetes have been linked to increased sugar consumption in humans. Here, we review fructose and glucose metabolism, as well as potential molecular mechanisms by which excessive sugar consumption is associated to metabolic diseases and insulin resistance in humans. To this end, we focus on understanding molecular and cellular mechanisms of fructose and glucose transport and sensing in the intestine, the intracellular signaling effects of dietary sugar metabolism, and its impact on glucose homeostasis in health and disease. Finally, the peripheral and central effects of dietary sugars on the gut-brain axis will be reviewed.

Greater insulin response to acute fructose ingestion among Māori and Pacific people compared to European people living in Aotearoa New Zealand

BACKGROUND

Fructose consumption has been linked with insulin resistance, obesity and diabetes, which are more prevalent in those of Māori or Pacific ethnicity compared to New Zealand European.

AIM

To determine whether the acute effects of fructose consumption on serum glucose, insulin, lipids and C-reactive protein differs according to body mass index (BMI) and/or ethnicity.

METHODS

Participants of Māori (n = 25), Pacific (n = 26) or New Zealand European (n = 25) ethnicity consumed a 64 g fructose/16 g glucose solution. Changes in lipids, glucose, insulin and C-reactive protein were analysed using mixed models for repeated measures.

RESULTS

After adjustment for age and gender, those with higher BMI had a higher glucose (P = 0.0064) and insulin (P = 0.0007) response than those with lower BMI. Those of Māori or Pacific ethnicity had similar glucose levels (P = 0.077) to those of New Zealand European ethnicity but higher insulin responses (P = 0.0005), which remained after additional adjustment for BMI (P = 0.001). Reported sugar-sweetened beverages (SSB) intake was higher among Māori and Pacific than New Zealand European (median 1.0 vs 0.0 SSB/day P = 0.002).

CONCLUSION

Even after adjustment for BMI, those of Māori and Pacific ethnicity have a significantly higher insulin response to fructose than New Zealand Europeans. Higher habitual SSB intake may be a contributing factor.

Determination of glyoxal and methylglyoxal in serum by UHPLC coupled with fluorescence detection

Glyoxal (GO) and methylglyoxal (MGO) are two important biomarkers in diabetes. Analytical methods for determination of GO and MGO in serum samples are either HPLC with UV-Vis (low sensitivity) or MS/MS (expensive) detection. These disadvantages have hampered the introduction of these biomarkers as a routine analyte for diabetes diagnostics into the clinical laboratory. In this study, we introduce a UHPLC method with fluorescence detection for the measurement of GO and MGO in serum samples by pre-column derivatization at neutral pH with 5, 6-diamino-2,4-dihydroxypyrimidine sulfate (DDP) to form lumazines. The method was validated as per FDA guidelines. Using this method, we have determined GO and MGO in a variety of animal serum samples, and for example, determined the GO and MGO concentration in adult bovine serum to be 852 ± 27 and 192 ± 10 nmol/L, respectively. In human serum, GO and MGO levels in non-diabetic subjects (n = 14) were determined to be 154 ± 88 and 98 ± 27 nmol/L, and in serum samples from subjects with diabetes (n = 14) 244 ± 137 and 190 ± 68 nmol/L, respectively. In addition, interference studies showed that physiological serum components did not lead to an artificial increase in the levels of GO and MGO.

Microbiome Alteration in Type 2 Diabetes Mellitus Model of Zebrafish

Understanding the gut microbiota in metabolic disorders, including type 2 diabetes mellitus (T2DM), is now gaining importance due to its potential role in disease risk and progression. We previously established a zebrafish model of T2DM, which shows glucose intolerance with insulin resistance and responds to anti-diabetic drugs. In this study, we analysed the gut microbiota of T2DM zebrafish by deep sequencing the 16S rRNA V3-V4 hypervariable regions, and imputed a functional profile using predictive metagenomic tools. While control and T2DM zebrafish were fed with the same kind of feed, the gut microbiota in T2DM group was less diverse than that of the control. Predictive metagenomics profiling using PICRUSt revealed functional alternation of the KEGG pathways in T2DM zebrafish. Several amino acid metabolism pathways (arginine, proline, and phenylalanine) were downregulated in the T2DM group, similar to what has been previously reported in humans. In summary, we profiled the gut microbiome in T2DM zebrafish, which revealed functional similarities in gut bacterial environments between these zebrafish and T2DM affected humans. T2DM zebrafish can become an alternative model organism to study host-bacterial interactions in human obesity and related diseases.

Intestinal Absorption of Fructose

Increased understanding of fructose metabolism, which begins with uptake via the intestine, is important because fructose now constitutes a physiologically significant portion of human diets and is associated with increased incidence of certain cancers and metabolic diseases. New insights in our knowledge of intestinal fructose absorption mediated by the facilitative glucose transporter GLUT5 in the apical membrane and by GLUT2 in the basolateral membrane are reviewed. We begin with studies related to structure as well as ligand binding, then revisit the controversial proposition that apical GLUT2 is the main mediator of intestinal fructose absorption. The review then describes how dietary fructose may be sensed by intestinal cells to affect the expression and activity of transporters and fructolytic enzymes, to interact with the transport of certain minerals and electrolytes, and to regulate portal and peripheral fructosemia and glycemia. Finally, it discusses the potential contributions of dietary fructose to gastrointestinal diseases and to the gut microbiome.

Functional food supplements to ameliorate the secondary complications in high fructose fed diabetic rats

Poly-herbal functional food supplements inhibited high fructose induced glycation in diabetic rats and showed promise for effective management of secondary complications of diabetes such as improved lipid profile, kidney function and reduction of oxidative stress.

Added sugars drive chronic kidney disease and its consequences: A comprehensive review

The consumption of added sugars (e.g. sucrose [table sugar] and high-fructose corn syrup) over the last 200 years has increased exponentially and parallels the increased prevalence of chronic kidney disease (CKD). Data for animals and humans suggest that the consumption of added sugars leads to kidney damage and related metabolic derangements that increase cardiovascular risk. Importantly, the consumption of added sugars has been found to induce insulin resistance and increase uric acid in humans, both of which increase the conversion of glucose to fructose (i.e. fructogenesis) via the polyol pathway. The polyol pathway has recently been implicated in the contribution and progression of kidney damage, suggesting that even glucose can be toxic to the kidney via its endogenous transformation into fructose in the proximal tubule. Consuming added fructose has been shown to induce insulin resistance, which can lead to hyperglycaemia, oxidative stress, inflammation and the activation of the immune system, all of which can synergistically contribute to kidney damage. CKD guidelines should stress a reduction in the consumption of added sugars as a means to prevent and treat CKD as well as reduce CKD–related morbidity and mortality.

Bioactivity-Guided Identification of Botanical Inhibitors of Ketohexokinase

Objective

In developed countries with westernized diets, the excessive consumption of added sugar in beverages and highly refined and processed foods is associated with increased risk for obesity, diabetes, and cardiovascular diseases. As a major constituent of added sugars, fructose has been shown to cause a variety of adverse metabolic effects, such as impaired insulin sensitivity, hypertriglyceridemia, and oxidative stress. Recent studies have shown that ketohexokinase isoform C is the key enzyme responsible in fructose metabolism that drive’s fructose's adverse effects. The objective of this study was to identify botanical ingredients with potential for inhibitory activity against ketohexokinase-C and fructose-induced metabolic effects by using a series of in vitro model systems.

Methods

Extracts from 406 botanicals and 1200 purified phytochemicals were screened (initial concentration of 50 μg/mL and 50 μM, respectively) for their inhibitory activity using a cell free, recombinant human ketohexokinase-C assay. Dose response evaluations were conducted on botanical extracts and phytochemicals that inhibited ketohexokinase-C by > 30% and > 40%, respectively. Two different extract lots of the top botanical candidates were further evaluated in lysates of HepG2 cells overexpressing ketohexokinase-C for inhibition of fructose-induced ATP depletion. In addition, extracts were evaluated in intact Hep G2 cells for inhibition of fructose-induced elevation of triglyceride and uric acid production.

Results

Among the botanical extracts, phloretin (Malus domestica) extracts were the most potent (IC₅₀: 8.9–9.2 μg/mL) followed by extracts of Angelica archangelica (IC_50: 22.6 μg/mL—57.3 μg/mL). Among the purified phytochemicals, methoxy-isobavachalcone (Psoralea corylifolia, IC₅₀ = 0.2 μM) exhibited the highest potency against ketohexokinase isoform C activity followed by osthole (Angelica archangelica, IC₅₀ = 0.7 μM), cratoxyarborenone E (Cratoxylum prunifolium, IC₅₀ = 1.0 μM), and α-/γ-mangostin (Cratoxylum prunifolium, IC₅₀ = 1.5 μM). Extracts of Angelica archangelica, Garcinia mangostana, Petroselinum crispum, and Scutellaria baicalensis exhibited ketohexokinase inhibitory activity and blocked fructose-induced ATP depletion and fructose-induced elevation in triglyerides and uric acid.

Conclusions

Angelica archangelica, Garcinia mangostana, Petroselinum crispum, and Scutellaria baicalensis were the top four botanical candidiates identified with inhibitory activity against ketohexokinase-C. Future studies are needed to show proof of mechanism and the efficacy of these botanical extracts in humans to blunt the negative metabolic effects of fructose-containing added sugars.

Effect of dietary fructose on portal and systemic serum fructose levels in rats and in KHK-/- and GLUT5-/- mice

Elevated blood fructose concentrations constitute the basis for organ dysfunction in fructose-induced metabolic syndrome. We hypothesized that diet-induced changes in blood fructose concentrations are regulated by ketohexokinase (KHK) and the fructose transporter GLUT5. Portal and systemic fructose concentrations determined by HPLC in wild-type mice fed for 7 days 0% free fructose were <0.07 mM, were independent of time after feeding, were similar to those of GLUT5(-/-), and did not lead to hyperglycemia. Postprandial fructose levels, however, increased markedly in those fed isocaloric 20% fructose, causing significant hyperglycemia. Deletion of KHK prevented fructose-induced hyperglycemia, but caused dramatic hyperfructosemia (>1 mM) with reversed portal to systemic gradients. Systemic fructose in wild-type and KHK(-/-) mice changed by 0.34 and 1.8 mM, respectively, for every millimolar increase in portal fructose concentration. Systemic glucose varied strongly with systemic, but not portal, fructose levels in wild-type, and was independent of systemic and portal fructose in KHK(-/-), mice. With ad libitum feeding for 12 wk, fructose-induced hyperglycemia in wild-type, but not hyperfructosemia in KHK(-/-) mice, increased HbA1c concentrations. Increasing dietary fructose to 40% intensified the hyperfructosemia of KHK(-/-) and the fructose-induced hyperglycemia of wild-type mice. Fructose perfusion or feeding in rats also caused duration- and dose-dependent hyperfructosemia and hyperglycemia. Significant levels of blood fructose are maintained independent of dietary fructose, KHK, and GLUT5, probably by endogenous synthesis of fructose. KHK prevents hyperfructosemia and fructose-induced hyperglycemia that would markedly increase HbA1c levels. These findings explain the hyperfructosemia of human hereditary fructosuria as well as the hyperglycemia of fructose-induced metabolic syndrome.

Added fructose: a principal driver of type 2 diabetes mellitus and its consequences

Data from animal experiments and human studies implicate added sugars (eg, sucrose and high-fructose corn syrup) in the development of diabetes mellitus and related metabolic derangements that raise cardiovascular (CV) risk. Added fructose in particular (eg, as a constituent of added sucrose or as the main component of high-fructose sweeteners) may pose the greatest problem for incident diabetes, diabetes-related metabolic abnormalities, and CV risk. Conversely, whole foods that contain fructose (eg, fruits and vegetables) pose no problem for health and are likely protective against diabetes and adverse CV outcomes. Several dietary guidelines appropriately recommend consuming whole foods over foods with added sugars, but some (eg, recommendations from the American Diabetes Association) do not recommend restricting fructose-containing added sugars to any specific level. Other guidelines (such as from the Institute of Medicine) allow up to 25% of calories as fructose-containing added sugars. Intake of added fructose at such high levels would undoubtedly worsen rates of diabetes and its complications. There is no need for added fructose or any added sugars in the diet; reducing intake to 5% of total calories (the level now suggested by the World Health Organization) has been shown to improve glucose tolerance in humans and decrease the prevalence of diabetes and the metabolic derangements that often precede and accompany it. Reducing the intake of added sugars could translate to reduced diabetes-related morbidity and premature mortality for populations.

Dietary sugars: their detection by the gut-brain axis and their peripheral and central effects in health and diseases

Background

Substantial increases in dietary sugar intake together with the increasing prevalence of obesity worldwide, as well as the parallels found between sugar overconsumption and drug abuse, have motivated research on the adverse effects of sugars on health and eating behaviour. Given that the gut–brain axis depends on multiple interactions between peripheral and central signals, and because these signals are interdependent, it is crucial to have a holistic view about dietary sugar effects on health.

Methods

Recent data on the effects of dietary sugars (i.e. sucrose, glucose, and fructose) at both peripheral and central levels and their interactions will be critically discussed in order to improve our understanding of the effects of sugars on health and diseases. This will contribute to the development of more efficient strategies for the prevention and treatment for obesity and associated co-morbidities.

Results

This review highlights opposing effects of glucose and fructose on metabolism and eating behaviour. Peripheral glucose and fructose sensing may influence eating behaviour by sweet-tasting mechanisms in the mouth and gut, and by glucose-sensing mechanisms in the gut. Glucose may impact brain reward regions and eating behaviour directly by crossing the blood–brain barrier, and indirectly by peripheral neural input and by oral and intestinal sweet taste/sugar-sensing mechanisms, whereas those promoted by fructose orally ingested seem to rely only on these indirect mechanisms.

Conclusions

Given the discrepancies between studies regarding the metabolic effects of sugars, more studies using physiological experimental conditions and in animal models closer to humans are needed. Additional studies directly comparing the effects of sucrose, glucose, and fructose should be performed to elucidate possible differences between these sugars on the reward circuitry.

Photoluminescent C-dots@RGO for sensitive detection of hydrogen peroxide and glucose

We have demonstrated sensitive detections of hydrogen peroxide (H2O2) and glucose using reduced graphene oxide decorated with carbon dots (C-dots@RGO). The C-dots@RGO prepared from catechin (reducing agent and carbon source) and graphene oxide via hydrothermal routes possesses excitation-wavelength-dependence photoluminescence (PL) characteristics, with maximum excitation and emission wavelengths of 365 and 440 nm, respectively. The C-dots@RGO is stable in solution containing NaCl up to 350 mM, but is quenched by reactive oxygen species (ROS). ROS reacts with H2O2 and thus its PL quenching toward the C-dots@RGO is minimized. When using C-dots@RGO and glucose oxidase (GOx), the PL assay allows detection of glucose in the presence of 10 µM of bovine serum albumin, with linearity over a concentration range from 1 to 60 µM (r=0.99) and a limit of detection (at a signal-to-noise ratio of 3) of 140 nM. The practicality of this assay has been validated by determining the concentrations of glucose in serum and saliva samples, with results of 5.1 ± 0.6mM (n=3) and 117.9 ± 8.1 μM (n=3), respectively. Our simple and sensitive assay opens a new avenue of developing assays for various analytes using C-dots@RGO in conjunction with different enzymes.

SGLT5 reabsorbs fructose in the kidney but its deficiency paradoxically exacerbates hepatic steatosis induced by fructose

Although excessive fructose intake is epidemiologically linked with dyslipidemia, obesity, and diabetes, the mechanisms regulating plasma fructose are not well known. Cells transfected with sodium/glucose cotransporter 5 (SGLT5), which is expressed exclusively in the kidney, transport fructose in vitro; however, the physiological role of this transporter in fructose metabolism remains unclear. To determine whether SGLT5 functions as a fructose transporter in vivo, we established a line of mice lacking the gene encoding SGLT5. Sodium-dependent fructose uptake disappeared in renal brush border membrane vesicles from SGLT5-deficient mice, and the increased urinary fructose in SGLT5-deficient mice indicated that SGLT5 was the major fructose reabsorption transporter in the kidney. From this, we hypothesized that urinary fructose excretion induced by SGLT5 deficiency would ameliorate fructose-induced hepatic steatosis. To test this hypothesis we compared SGLT5-deficient mice with wild-type mice under conditions of long-term fructose consumption. Paradoxically, however, fructose-induced hepatic steatosis was exacerbated in the SGLT5-deficient mice, and the massive urinary fructose excretion was accompanied by reduced levels of plasma triglycerides and epididymal fat but fasting hyperinsulinemia compared with fructose-fed wild-type mice. There was no difference in food consumption, water intake, or plasma fructose between the two types of mice. No compensatory effect by other transporters reportedly involved in fructose uptake in the liver and kidney were indicated at the mRNA level. These surprising findings indicated a previously unrecognized link through SGLT5 between renal fructose reabsorption and hepatic lipid metabolism.

Markedly increased serum and urinary fructose concentrations in diabetic patients with ketoacidosis or ketosis

To investigate fructose concentrations in diabetic patients with ketoacidosis or ketosis, serum fructose concentrations and daily urinary fructose excretion were measured in 23 patients with ketoacidosis (n = 16) and ketosis (n = 7) on the first day of admission. Seventeen patients were diagnosed with type 1, one patient with mitochondrial, and 4 patients with atypical diabetes. In 16 of the 23 patients, serum and urinary fructose could be assessed after starting treatments. Mean serum fructose concentration was 71.6 ± 108.1 μmol/l, and mean daily urinary fructose excretion was 352.1 ± 473.7 μmol/day. Serum fructose levels in patients with atypical diabetes were much higher (205.0 ± 213.3 μmol/l) than those in patients with type 1 diabetes (45.1 ± 44.5 μmol/l), while urinary fructose levels in atypical diabetes (249.7 ± 92.4 μmol/day) tended to be lower than those in type 1 diabetes (382.6 ± 533.2 μmol/day). Serum fructose concentrations decreased significantly (P < 0.05) from 88.1 ± 126.3 to 18.0 ± 11.0 μmol/l, and daily urinary fructose excretion also decreased significantly (P < 0.05) from 459.8 ± 530.9 to 75.1 ± 62.0 μmol/day in accordance with glycemic normalization after treatment. Marked and reversible increases in serum and urinary fructose concentrations were observed in diabetics with ketoacidosis and ketosis.

Designed glucose-responsive microgels with selective shrinking behavior

We report on the synthesis of various glucose-responsive microgels based on N-alkylacrylamide derivatives and phenylboronic acid (PBA) as a glucose sensing moiety. Depending on their chemical composition, the microgels exhibit opposite behaviors in response to glucose concentration increase: they can either swell or shrink, using two different mechanisms for glucose recognition. Both behaviors may be suitable for glucose sensing and insulin delivery. When glucose binds a single boronate receptor, the microgel swells as glucose concentration increases. This mechanism can be used to deliver a drug by diffusion through the network. In other cases, glucose binds specifically to two boronates, which creates additional cross-links within the network and provokes shrinkage. Such systems are promising for the development of sensors with improved selectivity and also as potential "intelligent" valves in microfabricated delivery systems. By a rational choice of the constituting units of the network structure, we show how to favor one or the other type of response to glucose variation. Therefore, glucose-swelling microgels operating under physiological conditions have been obtained by copolymerization with an appropriate choice of alkylacrylamide monomer and boronate derivative. At a pH above the pK(a) of the boronic acid derivative, the same structures shrink in response to glucose concentration. The nature of the cross-linker is a key parameter to enable this dual behavior. In other microgels, an amine group is introduced in the vicinity of the boronic acid, which lowers its pK(a) and favors microgel contraction at physiological pH. This work has allowed us to give some general rules to control the swelling/shrinking behavior of glucose-responsive microgels.

Fructose in fetal cord blood and its relationship with maternal and 48-hour-newborn blood concentrations

BACKGROUND

Studies have suggested that different non-glucose sugars and sugar alcohols play a role in placental and fetal metabolism. However, the role of fructose in the fetal and newborn metabolism is unclear and studies are scarce.

AIM

Our objective was to investigate the presence of fructose in umbilical cord blood in full-term gestation and its relationship with maternal and 48-hour-old- newborn blood concentrations, to evaluate fructose production by the fetus and newborn infant.

METHODS

Blood fructose and glucose concentrations were determined by HPLC in 26 paired samples of maternal blood, umbilical cord vein, and peripheral newborn blood at 48 h after birth. ANOVA, the Friedman Analysis of Variance on Ranks and the Pearson correlation with p<0.05 were used.

RESULTS

Fructose concentration in umbilical cord blood was higher than maternal blood (p=0.024), suggesting endogenous fructose production by the fetal-placental unit via the sorbitol pathway. Fructose concentrations were higher in newborns at 48 h after birth than in the fetal umbilical cord blood (p=0.004), suggesting that fructose production is a continuous process from fetus to newborn.

CONCLUSIONS

Fructose production by the sorbitol pathway, present in the fetus and newborn, is an alternative pathway in glucose metabolism probably used to maintain redox balance in the fetus. We suggest that endogenous fructose, similar to dietary ingested fructose, under physiological conditions produces the backbone for triacylglycerol and lipid synthesis in the fetus and newborn. Therefore the route for metabolizing fructose is already present in the early steps of human development.

Elevated dietary sugar and the heart: experimental models and myocardial remodeling

A dramatic rise in the prevalence of insulin resistance has been paralleled by increasing dietary consumption of sugar. The use of added sweeteners containing fructose (sucrose and high-fructose corn syrup) has increased by 25% over the past 3 decades. High fructose intake has the potential to adversely influence systemic and cellular metabolism via insulin resistance and glycolytic dysregulation. As a tissue that is both insulin sensitive and glycolysis dependent, the heart may be especially vulnerable to fructose over-consumption. In this review, experimental studies of elevated dietary sugar intake are evaluated, including sucrose and fructose dietary manipulation models. The possible role of the GLUT5 transporter as a mediator of cardiomyocyte fructose uptake is considered. The impact of dietary sucrose and fructose on cardiac insulin-dependent signaling in the context of perturbed systemic metabolic response is detailed. Myocardial dysfunction, modified growth, and oxidative stress responses associated with high dietary sugar intake are discussed. Finally, the involvement of the renin-angiotensin system in mediating fructose cardiopathology is considered. This review highlights the importance of obtaining new mechanistic data that can contribute to a more developed understanding of how high sugar intake directly contributes to structural and functional cardiomyopathy.

Elevated Serum Sorbitol and not Fructose in Type 2 Diabetic Patients

Reductions in fasting serum fructose or erythrocyte sorbitol have been proposed as markers for early proof of mechanism in clinical development of aldose reductase (AR) inhibitors. However fructose is significantly impacted by meals and evaluation of erythrocyte sorbitol poses technical challenges. To more accurately assess the performance of these markers in biological samples, a gas chromatography-mass spectrometry assay was modified and validated. Serum was collected on three consecutive days from 13 healthy volunteers (HV) and 14 patients with type 2 diabetes mellitus (T2DM), and assayed for sorbitol and fructose using this assay. Serum fructose and sorbitol were relatively constant across the three days. Fasting fructose levels were comparable between the two groups (T2DM: 1.48 ± 0.49 mg/L; HV: 1.39 ± 0.38 mg/L, mean ± standard deviation, P = 0.61), but fasting sorbitol levels were significantly higher in diabetics (T2DM: 0.280 ± 0.163 mg/L; HV: 0.164 ± 0.044 mg/L, P = 0.02). Feeding resulted in a 5–6 fold increase in serum fructose levels, but only a 5%–10% increase in sorbitol. Only sorbitol remained significantly elevated pre- and post feeding in T2DM patients relative to HV. These data suggest that serum sorbitol may be a robust proof of mechanism biomarker and facilitate dose selection for clinical development of AR inhibitors.

Measurement of glucose and fructose in clinical samples using gas chromatography/mass spectrometry

OBJECTIVE

The impact of increased fructose consumption on carbohydrate metabolism is a topic of current interest, but determination of serum level has been hindered due to low concentration and interference from serum glucose. We are reporting a method for the quantification of glucose and fructose in clinical samples using gas chromatography/mass spectrometry (GC/MS). The accuracy and precision of GC/MS and an enzymatic assay were compared.

DESIGN AND METHODS

Mass spectrometry fragmentation patterns of methyloxime peracetate derivatized aldose and ketose were determined. Unique fragments for glucose and fructose were used for quantitative analysis using isotope labeled recovery standards.

RESULTS

Methyloxime peracetate derivatives of glucose and fructose showed characteristic loss of acetate (M-60) or ketene (M-42) under chemical ionization (CI). Under electron impact (EI) ionization, a unique C1-C2 fragment of glucose was formed, while a C1-C3 fragment was formed from keto-hexoses. These unique fragments were used in the quantitative assay of glucose and fructose in clinical samples. In clinical samples, the GC/MS assay has a lower limit of detection than that of the enzymatic assay. In plasma samples from patients evaluated for diabetes the average serum glucose and fructose were 6.19+/-2.72 mM and 46+/- 25.22 microM. Fructose concentrations in many of these samples were below the limit of detection of the enzymatic method.

CONCLUSION

Derivatization of aldose and ketose monosaccharides to their respective O-methyloxime acetates for GC/MS analysis is a facile method for determination of serum/plasma glucose and fructose samples.

Inhibition of protein glycation by extracts of culinary herbs and spices

We tested whether polyphenolic substances in extracts of commercial culinary herbs and spices would inhibit fructose-mediated protein glycation. Extracts of 24 herbs and spices from a local supermarket were tested for the ability to inhibit glycation of albumin. Dry samples were ground and extracted with 10 volumes of 50% ethanol, and total phenolic content and ferric reducing antioxidant potential (FRAP) were measured. Aliquots were incubated in triplicate at pH 7.4 with 0.25 M fructose and 10 mg/mL fatty acid-free bovine albumin. Fluorescence at 370 nm/440 nm was used as an index of albumin glycation. In general, spice extracts inhibited glycation more than herb extracts, but inhibition was correlated with total phenolic content (R(2) = 0.89). The most potent inhibitors included extracts of cloves, ground Jamaican allspice, and cinnamon. Potent herbs tested included sage, marjoram, tarragon, and rosemary. Total phenolics were highly correlated with FRAP values (R(2) = 0.93). The concentration of phenolics that inhibited glycation by 50% was typically 4-12 microg/mL. Relative to total phenolic concentration, extracts of powdered ginger and bay leaf were less effective than expected, and black pepper was more effective. Prevention of protein glycation is an example of the antidiabetic potential for bioactive compounds in culinary herbs and spices.

Tagatose, a new antidiabetic and obesity control drug

A potentially important new drug for treating type 2 diabetes, tagatose, is now in phase 3 clinical trial. The history, development, additional health benefits, mechanisms of action and the potential for the drug are presented in context with a review of the rapidly growing epidemic of type 2 diabetes and treatments for it. An epimer of fructose, the natural hexose tagatose was originally developed by Spherix Incorporated (formerly Biospherics Inc.) as a low-calorie sugar substitute. Only 20% of orally ingested tagatose is fully metabolized, principally in the liver, following a metabolic pathway identical to that of fructose. Following a decade of studies, tagatose became generally recognized as safe for use in foods and beverages under US FDA regulation. The simple sugar is commercially produced by isomerization of galactose, which is prepared from lactose. Early human studies suggested tagatose as a potential antidiabetic drug through its beneficial effects on postprandial hyperglycaemia and hyperinsulinaemia. A subsequent 14-month trial confirmed its potential for treating type 2 diabetes, and tagatose showed promise for inducing weight loss and raising high-density lipoprotein cholesterol, both important to the control of diabetes and constituting benefits independent of the disease. Furthermore, tagatose was shown to be an antioxidant and a prebiotic, both properties cited in the maintenance and promotion of health. No current therapies for type 2 diabetes provide these multiple health benefits. The predominant side effects of tagatose are gastrointestinal disturbances associated with excessive consumption, generally accommodated within 1- to 2-week period. The health and use potentials for tagatose (branded Naturlose((R)) for this use) are given with respect to current type 2 diabetes drugs and markets. Under an FDA-affirmed protocol, Spherix is currently conducting a phase 3 trial to evaluate a placebo-subtracted treatment effect based on a decrease in HbA(1c) levels. Side effects, contraindications and possibly beneficial new findings will be carefully monitored. It is hoped that early results of the trial may become available by mid-2008. If a subsequent NDA is successful, tagatose may fill a major health need.

Daily treatment with sildenafil reverses endothelial dysfunction and oxidative stress in an animal model of insulin resistance

OBJECTIVES

Patients with insulin resistance exhibit endothelial dysfunction with decreased nitric oxide (NO) production and increased oxidative stress. We postulated that daily sildenafil improved endothelial function in fructose-fed rats.

METHODS AND RESULTS

Wistar rats were fed a standard or fructose-enriched diet (FFR) for 9 wk. From weeks 6-8, sildenafil was administered twice daily (sc, 20 m g/kg), followed by a 1-wk washout. Concentration-response curves (CRCs) to endothelium-dependent (acetylcholine [Ach] and A23187) and -independent (sodium nitroprusside [SNP]) relaxing agents were performed on isolated precontracted aortas and superior mesenteric arteries (SMAs). Vascular cyclic guanosine monophosphate (cGMP) content, urinary excretion of nitrates/nitrites (NOx) and 8-isoprostanes (IPT), and plasma levels of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) were evaluated. Relaxations to ACh were significantly reduced in aortas and SMAs of FFR. Sildenafil restored ACh-induced relaxations in aortas and provoked a significant leftward shift of the CRC to ACh in SMAs, whereas it did not modify the enhanced relaxations to SNP in FFR. IL-6, TNF-alpha, vascular cGMP, and urinary NOx levels were not modified by the fructose or sildenafil treatment. Urinary IPT levels were significantly elevated in FFR and normalized by sildenafil.

CONCLUSIONS

Endothelial dysfunction and oxidative stress associated with insulin resistance can be reversed by daily sildenafil, even 1 wk after treatment cessation.

Selective holographic detection of glucose using tertiary amines

Introducing tertiary amine monomers into holographic sensors containing phenylboronic acids gives greatly improved selectivity for glucose.

Crosslinking of phenylboronic acid receptors as a means of glucose selective holographic detection

A holographic sensor for the detection of glucose has been developed that is based on a hydrogel film containing phenylboronic acid receptors. Changes to the replay wavelength of the hologram were used to characterise the swelling and de-swelling behaviour of the hydrogel matrix upon receptor-ligand binding. The effect of introducing a fixed positive charge into the polymer matrix by modification of the hydrogel with a quaternary amine group (3-acrylamidopropyl)trimethylammonium chloride (ATMA), was investigated for a range of sugars and the alpha-hydroxy acid, lactate, at physiological pH. The quaternary amine-modified hydrogel matrix was found to contract in the presence of glucose, whereas, it was minimally responsive to other saccharides. The selectivity of the sensor for glucose compared to lactate was also significantly improved compared to the unmodified film. A crosslinking mechanism is proposed to explain the enhanced selectivity to glucose.

Simultaneous determination of glucose, 1,5-anhydrod-glucitol and related sugar alcohols in serum by high-performance liquid chromatography with benzoic acid derivatization

A new, simple and sensitive pre-column high-performance chromatographic method for the determination of diabetes marker d-glucose, 1,5-anhydro-d-glucitol and related compounds is reported. Sugars (d-glucose, d-galactose, d-mannose, sucrose and arabinose) were derivatized with benzoic acid (BA) at 80 degrees C for 60 min. l-Fucose, fructose, d-lactose, l-rhamnose, arabinose and ascorbic acid were not reacted. Sugar alcohols (xylitol, erythritol, mannitol, sorbitol myo-inositol) were also derivatized with BA at 80 degrees C for 60 min. The fluorescence derivatives were separated on a TSK amide 80 column (4.6 mm i.d. x 250 mm, 5 microm) with acetonitrile-50 mm acetate buffer (pH 5.6; 4:96, v/v) as the mobile phase. The detection wavelength of beizoic acid derivatives was lambda(ex) 275 nm and lambda(em) 315 nm. The detection limits of sugars were 10-80 microg/mL. The calibration graphs were linear up to 10 mg/mL. The relative standard deviations of 500 microg/mL sugars were 7.0-7.3%. The proposed method was compared with the enzymatic photometric glucose analysis method (Glucose B-Test II Wako). The correlation coefficient was 0.83 (n = 20) and y = 0.82x + 5.91, where y and x are concentrations in microg/mL obtained by the proposed pre-column HPLC and enzyme-photometric method, respectively. The detection limits of sugar alcohols were 100-1000 ng/mL. The calibration graphs were linear to 50 microg/mL and relative standard deviations of 10 microg/mL were 7.2-8.2%. The 1,5-AG data by the proposed method was also compared with the enzymatic photometric 1,5-AG analysis method (Rana AG 1,5-AG determination kit, Nihon Kayaku) and good correlation (r = 0.91, n = 20) was also obtained. The proposed method was applied to the simultaneous determination of d-glucose, 1,5-AG and related sugar alcohols in serum from healthy males.

SLC5A9/SGLT4, a new Na+-dependent glucose transporter, is an essential transporter for mannose, 1,5-anhydro-D-glucitol, and fructose

We isolated a cDNA clone of SLC5A9/SGLT4 from human small intestinal full-length cDNA libraries, and functionally characterized it in vitro. The messenger RNA encoding SGLT4 was mainly expressed in the small intestine and kidney, among the human tissues tested. COS-7 cells transiently expressing SGLT4 exhibited Na(+)-dependent alpha-methyl-D-glucopyranoside (AMG) transport activity with an apparent K(m) of 2.6 mM, suggesting that SGLT4 is a low affinity-type transporter. The rank order of naturally occurring sugar analogs for the inhibition of AMG transport was: D-mannose (Man) >> D-glucose (Glc) > D-fructose (Fru) = 1,5-anhydro-D-glucitol (1,5AG) > D-galactose (Gal). Recognition of Man as a substrate was confirmed by direct uptake of Man into the cell. COS-7 cells expressing a putative murine SGLT4 ortholog showed similar Na(+)-dependent AMG transport activity and a similar deduced substrate specificity. These results suggest that SGLT4 would have unique physiological functions (i.e., absorption and/or reabsorption of Man, 1,5AG, and Fru, in addition to Glc).