Leucine supplementation in rats induced a delay in muscle IR/PI3K signaling pathway associated with overall impaired glucose tolerance

Although activation of the mammalian target of rapamycin complex/p70 S6 kinase (S6K1) pathway by leucine is efficient to stimulate muscle protein synthesis, it can also exert inhibition on the early steps of insulin signaling leading to insulin resistance. We investigated the impact of 5-week leucine supplementation on insulin signaling and sensitivity in 4-month old rats fed a 15% protein diet supplemented (LEU) or not (C) with 4.5% leucine. An oral glucose tolerance test was performed in each rat at the end of the supplementation and glucose transport was measured in vitro using isolated epitrochlearis muscles incubated with 2-deoxy-d-[³H]-glucose under increasing insulin concentrations. Insulin signaling was assessed on gastrocnemius at the postabsorptive state or 30 and 60 min after gavage with a nutrient bolus. Tyrosine phosphorylation of IRβ, IRS1 and PI3 kinase activity were reduced in LEU group 30 min after feeding (−36%, −36% and −38% respectively, P<.05) whereas S6K1, S6rp and 4EBP1 phosphorylations were similar. Overall glucose tolerance was reduced in leucine-supplemented rats and was associated with accumulation of perirenal adipose tissue (+27%, P<.05). Conversely, in vitro insulin-response of muscle glucose transport tended to be improved in leucine-supplemented rats. In conclusion, dietary leucine supplementation in adult rats induced a delay in the postprandial stimulation in the early steps of muscle insulin signaling without muscle resistance on insulin-induced glucose uptake. However, it resulted in overall glucose intolerance linked to increased local adiposity. Further investigations are necessary to clearly define the beneficial and/or deleterious effects of chronic dietary leucine supplementation in healthy subjects.     

Insulin secretion after dietary supplementation with conjugated linoleic acids and n-3 polyunsaturated fatty acids in normal and insulin-resistant mice

Conjugated linoleic acids (CLAs) and n-3 polyunsaturated fatty acids (PUFAs) improve insulin sensitivity in insulin-resistant rodents. However, the effects of these fatty acids on insulin secretion are not known but are of importance to completely understand their influence on glucose homeostasis. We therefore examined islet function after dietary supplementation consisting of 1% CLAs in combination with 1% n-3 enriched PUFAs for 12 wk to mice on a normal diet and to insulin-resistant mice fed a high-fat diet (58% fat). In the mice fed a normal diet, CLA/PUFA supplementation resulted in insulin resistance associated with low plasma adiponectin levels and low body fat content. Intravenous and oral glucose tolerance tests revealed a marked increase in insulin secretion, which nevertheless was insufficient to counteract the insulin resistance, resulting in glucose intolerance. In freshly isolated islets from mice fed the normal diet, both basal and glucose-stimulated insulin secretion were adaptively augmented by CLA/PUFA, and at a high glucose concentration this was accompanied by elevated glucose oxidation. In contrast, in high-fat-fed mice, CLA/PUFA did not significantly affect insulin secretion, insulin resistance, or glucose tolerance. It is concluded that dietary supplementation of CLA/PUFA in mice fed the normal diet augments insulin secretion, partly because of increased islet glucose oxidation, but that this augmentation is insufficient to counterbalance the induction of insulin resistance, resulting in glucose intolerance. Furthermore, the high-fat diet partly prevents the deleterious effects of CLA/PUFA, but this dietary supplementation was not able to counteract high-fat-diet-induced insulin resistance.     

A long-term maternal diet intervention is necessary to avoid the obesogenic effect of maternal high-fat diet in the offspring

Although a pre-pregnancy dietary intervention is believed to be able to prevent offspring obesity, research evidence is absent. We hypothesize that a long period of pre-pregnancy maternal diet transition from a high-fat (HF) diet to a normal-fat (NF) diet effectively prevents offspring obesity, and this preventive effect is independent of maternal body weight change. In our study, female mice were either continued on an NF diet (NF group) or an HF diet (HF group) until weaning, or switched from an HF to an NF for 1 week (H1N group), 5 weeks (H5N group) or 9 weeks (H9N group) before pregnancy. After weaning, the offspring were given the HF diet for 12 weeks to promote obesity. The mothers, regardless of which group, did not display maternal body weight change and glucose intolerance either before pregnancy or after weaning. Compared to the HF group, the H1N and H5N, but not the H9N, offspring developed glucose intolerance earlier, with more severely imbalanced glucose homeostasis. These offspring also displayed hepatocyte degeneration and significant adipocyte hypertrophy associated with higher expression of lipogenesis genes. The molecular mechanistic study showed blunted insulin signaling, overactivated adipocyte Akt signaling and hepatic AMPK signaling with enhanced lipogenesis genes in the H1N and H5N versus the NF offspring. However, maternal H9N diets normalized glucose and lipid metabolism of the offspring via resensitized insulin signaling and normalized Akt and AMPK signaling. In summary, we showed that a long-term maternal diet intervention effectively released the intergenerational obesogenic effect of maternal HF diet independent of maternal weight management.     

Mice lacking adiponectin show decreased hepatic insulin sensitivity and reduced responsiveness to peroxisome proliferator-activated receptor gamma agonists

The adipose tissue-derived hormone adiponectin improves insulin sensitivity and its circulating levels are decreased in obesity-induced insulin resistance. Here, we report the generation of a mouse line with a genomic disruption of the adiponectin locus. We aimed to identify whether these mice develop insulin resistance and which are the primary target tissues affected in this model. Using euglycemic/insulin clamp studies, we demonstrate that these mice display severe hepatic but not peripheral insulin resistance. Furthermore, we wanted to test whether the lack of adiponectin magnifies the impairments of glucose homeostasis in the context of a dietary challenge. When exposed to high fat diet, adiponectin null mice rapidly develop glucose intolerance. Specific PPARgamma agonists such as thiazolidinediones (TZDs) improve insulin sensitivity by mechanisms largely unknown. Circulating adiponectin levels are significantly up-regulated in vivo upon activation of PPARgamma. Both TZDs and adiponectin have been shown to activate AMP-activated protein kinase (AMPK) in the same target tissues. We wanted to address whether the ability of TZDs to improve glucose tolerance is dependent on adiponectin and whether this improvement involved AMPK activation. We demonstrate that the ability of PPARgamma agonists to improve glucose tolerance in ob/ob mice lacking adiponectin is diminished. Adiponectin is required for the activation of AMPK upon TZD administration in both liver and muscle. In summary, adiponectin is an important contributor to PPARgamma-mediated improvements in glucose tolerance through mechanisms that involve the activation of the AMPK pathway.     

The Development of Diet-Induced Obesity and Glucose Intolerance in C57Bl/6 Mice on a High-Fat Diet Consists of Distinct Phases

High–fat (HF) diet-induced obesity and insulin insensitivity are associated with inflammation, particularly in white adipose tissue (WAT). However, insulin insensitivity is apparent within days of HF feeding when gains in adiposity and changes in markers of inflammation are relatively minor. To investigate further the effects of HF diet, C57Bl/6J mice were fed either a low (LF) or HF diet for 3 days to 16 weeks, or fed the HF-diet matched to the caloric intake of the LF diet (PF) for 3 days or 1 week, with the time course of glucose tolerance and inflammatory gene expression measured in liver, muscle and WAT. HF fed mice gained adiposity and liver lipid steadily over 16 weeks, but developed glucose intolerance, assessed by intraperitoneal glucose tolerance tests (IPGTT), in two phases. The first phase, after 3 days, resulted in a 50% increase in area under the curve (AUC) for HF and PF mice, which improved to 30% after 1 week and remained stable until 12 weeks. Between 12 and 16 weeks the difference in AUC increased to 60%, when gene markers of inflammation appeared in WAT and muscle but not in liver. Plasma proteomics were used to reveal an acute phase response at day 3. Data from PF mice reveals that glucose intolerance and the acute phase response are the result of the HF composition of the diet and increased caloric intake respectively. Thus, the initial increase in glucose intolerance due to a HF diet occurs concurrently with an acute phase response but these effects are caused by different properties of the diet. The second increase in glucose intolerance occurs between 12 - 16 weeks of HF diet and is correlated with WAT and muscle inflammation. Between these times glucose tolerance remains stable and markers of inflammation are undetectable.     

Antioxidant alpha-tocopherol ameliorates glycemic control of GK rats, a model of type 2 diabetes

We have shown recently that oxidative stress by chronic hyperglycemia damages the pancreatic beta-cells of GK rats, a model of non-obese type 2 diabetes, which may worsen diabetic condition and suggested the administration of antioxidants as a supportive therapy. To determine if natural antioxidant alpha-tocopherol (vitamin E) has beneficial effects on the glycemic control of type 2 diabetes, GK rats were fed a diet containing 0, 20 or 500 mg/kg diet alpha-tocopherol. Intraperitoneal glucose tolerance test revealed a significant increment of insulin secretion at 30 min and a significant decrement of blood glucose levels at 30 and 120 min after glucose loading in the GK rats fed with high alpha-tocopherol diet. The levels of glycated hemoglobin A1c, an indicator of glycemic control, were also reduced. Vitamin E supplementation clearly ameliorated diabetic control of GK rats, suggesting the importance of not only dietary supplementation of natural antioxidants but also other antioxidative intervention as a supportive therapy of type 2 diabetic patients.     

Sex-dependent differences in rat hepatic lipid accumulation and insulin sensitivity in response to diet-induced obesity

Ectopic deposition of lipids in liver and other extrahepatic tissues alters their function and occurs once adipose tissue fat storage capacity is exceeded. We investigated sexual dimorphism in the effects of dietary obesity on the liver insulin signaling pathway, as well as its connection to differences in hepatic fat accumulation. Ten-week-old Wistar rats of both sexes were fed a standard diet or a high-fat diet for 26 weeks. Insulin, adipokine levels, and glucose tolerance were measured. Lipid content, PPARα mRNA expression and protein levels of insulin receptor subunit β (IRβ), IR substrate 2 (IRS-2), Ser/Thr kinase A (Akt), and pyruvate dehydrogenase kinase isozyme 4 (PDK4) were measured in liver. In control rats, serum parameters and hepatic levels of IRβ, IRS-2, and Akt proteins pointed to a profile of better insulin sensitivity in females. In response to dietary treatment, female rats exhibited a greater increase in body mass and adiposity and lower liver fat accumulation than males, but maintained better glucose tolerance. The reduced insulin signaling capacity in the liver of obese female rats seems to prevent lipid accumulation and probably lipotoxicity-associated hepatic disorders.     

Liquid fructose down-regulates liver insulin receptor substrate 2 and gluconeogenic enzymes by modifying nutrient sensing factors in rats

High consumption of fructose-sweetened beverages has been linked to a high prevalence of chronic metabolic diseases. We have previously shown that a short course of fructose supplementation as a liquid solution induces glucose intolerance in female rats. In the present work, we characterized the fructose-driven changes in the liver and the molecular pathways involved. To this end, female rats were supplemented or not with liquid fructose (10%, w/v) for 7 or 14 days. Glucose and pyruvate tolerance tests were performed, and the expression of genes related to insulin signaling, gluconeogenesis and nutrient sensing pathways was evaluated.Fructose-supplemented rats showed increased plasma glucose excursions in glucose and pyruvate tolerance tests and reduced hepatic expression of several genes related to insulin signaling, including insulin receptor substrate 2 (IRS-2). However, the expression of key gluconeogenic enzymes, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, was reduced. These effects were caused by an inactivation of hepatic forkhead box O1 (FoxO1) due to an increase in its acetylation state driven by a reduced expression and activity of sirtuin 1 (SIRT1). Further contributing to FoxO1 inactivation, fructose consumption elevated liver expression of the spliced form of X-box-binding-protein-1 as a consequence of an increase in the activity of the mammalian target of rapamycin 1 and protein 38-mitogen activated protein kinase (p38-MAPK). Liquid fructose affects both insulin signaling (IRS-2 and FoxO1) and nutrient sensing pathways (p38-MAPK, mTOR and SIRT1), thus disrupting hepatic insulin signaling without increasing the expression of key gluconeogenic enzymes.     

L-Arginine improves multiple physiological parameters in mice exposed to diet-induced metabolic disturbances

L-Arginine (L-Arg) is a conditionally essential amino acid and a natural constituent of dietary proteins. Studies in obese rats and type 2 diabetic humans have indicated that dietary supplementation with L-Arg can diminish gain in white adipose tissue (WAT) and improve insulin sensitivity. However, the effects of L-Arg on glucose homeostasis, body composition and energy metabolism remain unclear. In addition, no studies have, to our knowledge, examined whether L-Arg has beneficial effects as a dietary supplement in the mouse model. In the present study, we investigated the effects of L-Arg supplementation to male C57BL/6 mice on an array of physiological parameters. L-Arg supplemented mice were maintained on a low-protein diet and body composition, appetite regulation, glucose tolerance, insulin sensitivity and energy expenditure were evaluated. A significant reduction in epididymal WAT was observed in L-Arg supplemented mice compared with mice fed an isocaloric control diet. Surprisingly, the L-Arg supplemented animals were hyperphagic corresponding to a highly significant decrease in feed efficiency, as body weight developed in a similar pattern in both experimental groups. Glucose homeostasis experiments revealed a major effect of L-Arg supplementation on glucose tolerance and insulin sensitivity, interestingly, independent of a parallel regulation in whole-body adiposity. Increased L-Arg ingestion also raised energy expenditure; however, no concurrent effect on locomotor activity, substrate metabolism or expression of uncoupling proteins (UCP1 and UCP2) in adipose tissues was displayed. In conclusion, dietary L-Arg supplementation substantially affects an array of metabolic-associated parameters including a reduction in WAT, hyperphagia, improved insulin sensitivity and increased energy expenditure in mice fed a low-protein diet.     

Antidiabetic effect of enterolactone in cultured muscle cells and in type 2 diabetic model db/db mice

Enterolactone (ENL) is formed by the conversion of dietary precursors like strawberry lignans via the gut microbiota. Urinary concentrations of lignan metabolites are reported to be significantly associated with a lower risk of Type 2 diabetes (T2D). In the present study, antidiabetic effect of ENL and its modes of action were studied in vitro and in vivo employing a rat skeletal muscle-derived cell line, L6 myocytes in culture, and T2D model db/db mice. ENL dose-dependently increased glucose uptake in L6 myotubes under insulin absent condition. This increase by ENL was canceled by compound C, an inhibitor of 5′-adenosine monophosphate-activated protein kinase (APMK). Activation (=phosphorylation) of AMPK and translocation of glucose transporter 4 (GLUT4) to plasma membrane in L6 myotubes were demonstrated by Western blotting analyses. Promotion by ENL of GLUT4 translocation to plasma membrane was also visually demonstrated by immunocytochemistry in L6 myoblasts that were transfected with glut4 cDNA-coding vector. T2D model db/db mice were fed the basal 20 % casein diet (20C) or 20C supplemented with ENL (0.001 or 0.01 %) for 6 weeks. Fasting blood glucose (FBG) levels were measured every week and intraperitoneal glucose tolerance test (IPGTT) was conducted. ENL at a higher dose (0.01 % in 20C) suppressed the increases in FBG levels. ENL was also demonstrated to improve the index of insulin resistance (HOMA-IR) and glucose intolerance by IPGTT in db/db mice. From these results, ENL is suggested to be an antidiabetic chemical entity converted from dietary lignans by gut microbiota.     

Glucose tolerance, lipids, and GLP-1 secretion in JCR:LA-cp rats fed a high protein fiber diet

Background: We have shown that individually, dietary fiber and protein increase secretion of the anorexigenic and insulinotropic hormone, glucagon‐like peptide‐1 (GLP‐1). Objective: Our objective was to combine, in one diet, high levels of fiber and protein to maximize GLP‐1 secretion, improve glucose tolerance, and reduce weight gain. Methods and Procedures: Lean (+/?) and obese (cp/cp) male James C Russell corpulent (JCR:LA‐cp) rats lacking a functional leptin receptor were fed one of four experimental diets (control, high protein (HP), high fiber (HF, prebiotic fiber inulin), or combination (CB)) for 3 weeks. An oral glucose tolerance test (OGTT) was performed to evaluate plasma GLP‐1, insulin and glucose. Plasma lipids and intestinal proglucagon mRNA expression were determined. Results: Energy intake was lower with the HF diet in lean and obese rats. Weight gain did not differ between diets. Higher colonic proglucagon mRNA in lean rats fed a CB diet was associated with higher GLP‐1 secretion during OGTT. The HP diet significantly reduced plasma glucose area under the curve (AUC) during OGTT in obese rats, which reflected both an increased GLP‐1 AUC and higher fasting insulin. Diets containing inulin resulted in the lowest plasma triglyceride and total cholesterol levels. Discussion: Overall, combining HP with HF in the diet increased GLP‐1 secretion in response to oral glucose, but did not improve glucose tolerance or lipid profiles more than the HF diet alone did. We also suggest that glycemic and insulinemic response to prebiotics differ among rat models and future research work should examine their role in improving glucose tolerance in diet‐induced vs. genetic obesity with overt hyperleptinemia.     

The role of vitamin E or clay in growing Japanese quail fed diets polluted by cadmium at various levels

This study was conducted to verify whether vitamin (Vit) E or natural clay as feed additives has the potential to modulate the deleterious effects resulting from exposure to cadmium (Cd) in growing Japanese quail. 648 Japanese quail chicks (1 week old) were used to evaluate the effects of dietary Cd (0, 40, 80 and 120 mg/kg diet) and two levels of Vit E (0, 250 mg/kg diet) or two levels of natural clay (0 and 100 mg/kg diet) to study the influences of Cd, Vit E, clay or their different combinations on growth performance, carcass traits, some blood biochemical components and Cd residues in muscles and liver. Live BW and weight gain of quails were linearly decreased with increasing dietary Cd levels. Moreover, feed conversion was significantly worsened with increasing Cd level. Mortality percentage was linearly increased as dietary Cd level increased up to 120 mg/kg diet. Carcass percentage was linearly decreased as dietary Cd level increased. While, giblets percentage were linearly and quadratically differed as dietary Cd level increased. Cd caused significant changes in total plasma protein, albumin, globulin, A/G ratio, creatinine, urea-N and uric acid concentrations as well as ALT, AST and ALP activities. Increasing dietary Cd level was associated with its increase in the muscles and liver. Dietary supplementation with 250 mg of Vit E/kg diet or 100 mg clay/kg improved live BW, BW gain and feed conversion when compared with the un-supplemented diet. Quails fed diet contained 250 mg Vit E/kg and those fed 100 mg clay/kg had the highest percentages of carcass and dressing than those fed the un-supplemented diet. Blood plasma biochemical components studied were better when birds received 250 mg of Vit E/kg diet and those received 100 mg clay/kg. Cd residues in the muscles and liver were significantly less in the birds had 250 mg of Vit E/kg or those received 100 mg clay/kg diet than those un-supplemented with Vit E. Growth performance traits and blood plasma biochemical components studied were significantly affected linearly by the interactions among Cd and each of Vit E and clay levels. In conclusion, the present results indicate that the deleterious effects induced by Cd plays a role in decreasing the performance of Japanese quail and that dietary supplementation with natural clay or Vit E may be useful in partly alleviating the adverse effects of Cd.     

Indomethacin Treatment Prevents High Fat Diet-induced Obesity and Insulin Resistance but Not Glucose Intolerance in C57BL/6J Mice

Chronic low grade inflammation is closely linked to obesity-associated insulin resistance. To examine how administration of the anti-inflammatory compound indomethacin, a general cyclooxygenase inhibitor, affected obesity development and insulin sensitivity, we fed obesity-prone male C57BL/6J mice a high fat/high sucrose (HF/HS) diet or a regular diet supplemented or not with indomethacin (±INDO) for 7 weeks. Development of obesity, insulin resistance, and glucose intolerance was monitored, and the effect of indomethacin on glucose-stimulated insulin secretion (GSIS) was measured in vivo and in vitro using MIN6 β-cells. We found that supplementation with indomethacin prevented HF/HS-induced obesity and diet-induced changes in systemic insulin sensitivity. Thus, HF/HS+INDO-fed mice remained insulin-sensitive. However, mice fed HF/HS+INDO exhibited pronounced glucose intolerance. Hepatic glucose output was significantly increased. Indomethacin had no effect on adipose tissue mass, glucose tolerance, or GSIS when included in a regular diet. Indomethacin administration to obese mice did not reduce adipose tissue mass, and the compensatory increase in GSIS observed in obese mice was not affected by treatment with indomethacin. We demonstrate that indomethacin did not inhibit GSIS per se, but activation of GPR40 in the presence of indomethacin inhibited glucose-dependent insulin secretion in MIN6 cells. We conclude that constitutive high hepatic glucose output combined with impaired GSIS in response to activation of GPR40-dependent signaling in the HF/HS+INDO-fed mice contributed to the impaired glucose clearance during a glucose challenge and that the resulting lower levels of plasma insulin prevented the obesogenic action of the HF/HS diet.     

Two triterpeniods from Cyclocarya paliurus (Batal) Iljinsk (Juglandaceae) promote glucose uptake in 3T3-L1 adipocytes: The relationship to AMPK activation

PurposeThe current study investigated the efficacy of Cyclocarya paliurus chloroform extract (CPEC) and its two specific triterpenoids (cyclocaric acid B and cyclocarioside H) on the regulation of glucose disposal and the underlying mechanisms in 3T3-L1 adipocytes.MethodsMice and adipocytes were stimulated by macrophages-derived conditioned medium (Mac-CM) to induce insulin resistance. CPEC was evaluated in mice for its ability by oral glucose tolerance test (OGTT) and insulin tolerance test (ITT). To investigate the hypoglycemic mechanisms of CPEC and its two triterpenoids, glucose uptake, AMP-activated protein kinase (AMPK) activation, inhibitor of NF-κB kinase β (IKKβ) phosphorylation and insulin signaling transduction were detected in 3T3-L1 adipocytes using 2-NBDG uptake assay and Western blot analysis.ResultsMac-CM, an inflammatory stimulus which induced the glucose and insulin intolerance, increased phosphorylation of IKKβ, reduced glucose uptake and impaired insulin sensitivity. CPEC and two triterpenoids improved glucose consumption and increased AMPK phosphorylation under basal and inflammatory conditions. Moreover, CPEC and its two triterpenoids not only enhanced glucose uptake in an insulin-independent manner, but also restored insulin-mediated protein kinase B (Akt) phosphorylation by reducing the activation of IKKβ and regulating insulin receptor substrate-1 (IRS-1) serine/tyrosine phosphorylation. These beneficial effects were attenuated by AMPK inhibitor compound C, implying that the effects may be associated with AMPK activation.ConclusionsCPEC and its two triterpenoids promoted glucose uptake in the absence of insulin, as well as ameliorated IRS-1/PI3K/Akt pathway by inhibiting inflammation. These effects were related to the regulation of AMPK activity.     

Effects of early protein restriction and adult obesity on rat pancreatic hormone content and glucose tolerance.

Rats were fed a diet containing either 20% ("control") or 8% ("reduced-protein") protein throughout pregnancy and lactation. Their female offspring were weaned onto the same respective diets. At 63 days of age one set of control and reduced-protein rats (n = 16 per group) underwent intraperitoneal glucose tolerance tests and one week later were killed and their pancreatic hormones extracted and measured. The reduced protein rats had better glucose tolerance (p < 0.001) and lower pancreatic insulin (p < 0.01) and amylin (p < 0.01) contents. Further sets of control and reduced-protein rats were then fed either chow or a cafeteria-style diet (n = 16 in each of the four groups). These rats underwent intraperitoneal glucose tolerance tests at 133 days of age, which showed the cafeteria-fed animals to have a worse glucose tolerance than the chow-fed animals irrespective of previous diet exposure (p < 0.0001). One week later reduced-protein rats still had lower pancreatic insulin contents (p < 0.05) (and a trend for lower amylin contents), but also had increased pancreatic glucagon contents (p < 0.05). There were no detectable differences in pancreatic somatostatin-like immunoreactivity or pancreatic polypeptide contents. These results are consistent with pancreatic beta- and alpha-cells being selectively susceptible to effects associated with early dietary protein restriction.     

Xanthohumol and 8-prenylnaringenin ameliorate diabetic-related metabolic dysfunctions in mice

Type 2 diabetes mellitus (T2DM) is a chronic disease characterized by metabolic disturbances in specific tissues. The present work aimed to analyze the effects of xanthohumol (XN) and 8-prenylnaringenin (8PN), two beer-derived polyphenols, in liver and skeletal muscle lipid and glycolytic metabolism in T2DM mice model. Thirty C57Bl/6 mice were randomly divided into five groups: standard diet (control), high-fat diet (DM), high-fat diet plus ethanol (DM-Ethanol), high-fat diet plus 10 mg/L XN (DM-XN) and high-fat diet plus 10 mg/L 8PN (DM-8PN) during 20 weeks. Fasting blood glucose and insulin tolerance tests were performed 1 week before sacrifice. At the end of the study, blood, liver and skeletal muscle were collected. Both XN and 8PN treatments prevented body weight gain; decreased glycemia, triglyceride, cholesterol and alkaline phosphatase levels; and improved insulin sensitivity. Polyphenols promoted hepatic and skeletal muscle AMP-activated protein kinase (AMPK) activation, diminishing the expression of target lipogenic enzymes (sterol regulatory element binding protein-1c and fatty acid synthase) and acetyl-CoA carboxylase activity. Moreover, both XN and 8PN treatments decreased VEGFR-1/VEGFB pathway, involved in fatty acid uptake, and increased AS160 expression, involved in GLUT4 membrane translocation. Presented data demonstrated that both XN and 8PN treatment resulted in AMPK signaling pathway activation, thus suppressing lipogenesis. Their consumption prevented body weight gain and improved plasma lipid profile, with significant improvement of insulin resistance and glucose tolerance. XN- or 8PN-enriched diet could ameliorate diabetic-associated metabolic disturbances by regulating glucose and lipid pathways.     

Dietary citrate acutely induces insulin resistance and markers of liver inflammation in mice

Citrate is widely used as a food additive being part of virtually all processed foods. Although considered inert by most of the regulatory agencies in the world, plasma citrate has been proposed to play immunometabolic functions in multiple tissues through altering a plethora of cellular pathways. Here, we used a short-term alimentary intervention (24 hours) with standard chow supplemented with citrate in amount corresponding to that found in processed foods to evaluate its effects on glucose homeostasis and liver physiology in C57BL/6J mice. Animals supplemented with dietary citrate showed glucose intolerance and insulin resistance as revealed by glucose and insulin tolerance tests. Moreover, animals supplemented with citrate in their food displayed fed and fasted hyperinsulinemia and enhanced insulin secretion during an oral glucose tolerance test. Citrate treatment also amplified glucose-induced insulin secretion in vitro in INS1-E cells. Citrate supplemented animals had increased liver PKCα activity and altered phosphorylation at serine or threonine residues of components of insulin signaling including IRS-1, Akt, GSK-3 and FoxO1. Furthermore, citrate supplementation enhanced the hepatic expression of lipogenic genes suggesting increased de novo lipogenesis, a finding that was reproduced after citrate treatment of hepatic FAO cells. Finally, liver inflammation markers were higher in citrate supplemented animals. Overall, the results demonstrate that dietary citrate supplementation in mice causes hyperinsulinemia and insulin resistance both in vivo and in vitro, and therefore call for a note of caution on the use of citrate as a food additive given its potential role in metabolic dysregulation.     

Prevention of high-fat diet-induced muscular lipid accumulation in rats by �� lipoic acid is not mediated by AMPK activation

Skeletal muscle triglyceride accumulation is associated with insulin resistance in obesity. Recently, it has been suggested that α lipoic acid (ALA) improves insulin sensitivity by lowering triglyceride accumulation in nonadipose tissues via activation of skeletal muscle AMP-activated protein kinase (AMPK). We examined whether chronic ALA supplementation prevents muscular lipid accumulation that is associated with high-fat diets via activation of AMPK. In addition, we tested if ALA supplementation was able to improve insulin sensitivity in rats fed low- and high-fat diets (LFD, HFD). Supplementing male Wistar rats with 0.5% ALA for 8 weeks significantly reduced body weight, both on LFD and HFD (−24% LFD+ALA vs. LFD, P < 0.01, and −29% HFD+ALA vs. HFD, P < 0.001). Oil red O lipid staining revealed a 3-fold higher lipid content in skeletal muscle after HFD compared with LFD and ALA-supplemented groups (P < 0.05). ALA improved whole body glucose tolerance (∼20% lower total area under the curve (AUC) in ALA supplemented groups vs. controls, P < 0.05). These effects were not mediated by increased muscular AMPK activation or ALA-induced improvement of muscular insulin sensitivity. To conclude, the prevention of HFD-induced muscular lipid accumulation and the improved whole body glucose tolerance are likely secondary effects due to the anorexic nature of ALA.     

Effect of Dietary Vitamin C and Catalase Inhibition on Antioxidants and Molecular Markers of Oxidative Damage in Guinea Pigs

Guinea pigs were fed for five weeks with two diets with different levels of vitamin C, low (33 mg of Vit C/Kg diet) and high (13,200 mg of Vit C/Kg of diet). Catalase was inhibited with 3-amino-1,2,4-triazole (AT) in half of the animals from each dietary group. AT caused an almost complete depletion of liver catalase activity (90%) in both dietary groups. Vitamin C supplementation increased total glutathione peroxidase activity and tissue vitamin C level and decreased levels of protein carbonyls and malondialdehyde (MDA) in both treated and non-treated animals. This vitamin C supplementation did not change any of the other antioxidant defences studied. Our results show that dietary vitamin C supplementation increases global antioxidant capacity and decreases endogenous oxidative damage in the guinea pig liver under normal non-stressful conditions. This supports the protective value of dietary antioxidant supplementation.     

Maternal high-fat diet consumption results in fetal malprogramming predisposing to the onset of metabolic syndrome-like phenotype in adulthood

Chronic consumption of a high-fat (HF) diet by female rats in their postweaning period resulted in significant increases in body weight and plasma levels of insulin, glucose, and triglycerides during pregnancy compared with female rats consuming a standard rodent laboratory chow (LC). On gestational day 21, plasma insulin levels and the insulin secretory response of islets to various secretogogues were significantly increased in HF fetuses. The HF male progeny weaned onto LC (HF/LC) demonstrated increases in body weight from postnatal day 60 onward. In adulthood, HF/LC male rats were significantly heavier than controls, had increased plasma levels of insulin, glucose, free fatty acids, and triglycerides, and demonstrated glucose intolerance. HF/LC male islets secreted increased amounts of insulin in response to low glucose concentrations, but their response to a high glucose concentration was similar to that of LC/LC islets. In another set of experiments, when the male progeny of HF female rats were weaned onto a high-sucrose diet (HF/HSu), their metabolic profile was further worsened. These results indicate that chronic consumption of a HF diet by female rats malprograms the male progeny for glucose intolerance and development of increased body weight in adulthood. The long-term high-fat feeding to female rats employed in this study bears resemblance to the dietary habits in Western societies. The results of this study implicate dietary practices of women in the etiology of the present epidemic of human obesity and related disorders.