Amelioration of insulin resistance and hypertension in a fructose-fed rat model with fish oil supplementation

Rare mono- and disaccharides as healthy alternative for traditional sugars and sweeteners?

Obesity and type 2 diabetes are major health problems affecting hundreds of millions of people. Caloric overfeeding with calorie-dense food ingredients like sugars may contribute to these chronic diseases. Sugar research has also identified mechanisms via which conventional sugars like sucrose and fructose can adversely influence metabolic health. To replace these sugars, numerous sugar replacers including artificial sweeteners and sugar alcohols have been developed. Rare sugars became new candidates to replace conventional sugars and their health effects are already reported in individual studies, but overviews and critical appraisals of their health effects are missing. This is the first paper to provide a detailed review of the metabolic health effects of rare sugars as a group. Especially allulose has a wide range of health effects. Tagatose and isomaltulose have several health effects as well, while other rare sugars mainly provide health benefits in mechanistic studies. Hardly any health claims have been approved for rare sugars due to a lack of evidence from human trials. Human trials with direct measures for disease risk factors are needed to allow a final appraisal of promising rare sugars. Mechanistic cell culture studies and animal models are required to enlarge our knowledge on understudied rare sugars.

Apigenin ameliorates vascular injury in rats with high fructose-induced metabolic disturbance by inhibiting PI3K/AKT/GLUT1

The abuse of fructose in daily diet may cause cardiovascular diseases that seriously threaten human health, and both safe and efficient solutions need to be developed. We investigated whether apigenin can prevent the harmful impact of excessive fructose on cardiovascular events. Based on the reduction of percentage of body fat and systolic pressure as well as the improvements in insulin resistance, lipid metabolism, and pathological injury to the thoracic aorta, we suggested that high levels of fructose cause vascular injury and metabolic disorders, which can be improved to some extent by using apigenin. Fundamentally, apigenin down-regulates levels of phosphoinositide 3-kinase (PI3K), protein kinase B (AKT), and glucose transporter 1 (GLUT1), which increase with high concentrations of fructose. Moreover, the inflammation and asymmetric dimethylarginine (ADMA) levels increased in fructose group, but they decreased when the rats were fed with apigenin. The results suggest that PI3K/AKT/GLUT1 may have potential for alleviating cardiovascular injury, and apigenin can be an excellent candidate for supplements to ameliorate cardiovascular diseases related to high fructose consumption.

(A) Chemical structure of apigenin. (B) Graph illuminating the experimental design for the time course of fructose and apigenin administration, and the timelines for the histological studies, biochemical analysis and western blot analysis.

Is There a Role for Bioactive Lipids in the Pathobiology of Diabetes Mellitus?

Inflammation, decreased levels of circulating endothelial nitric oxide (eNO) and brain-derived neurotrophic factor (BDNF), altered activity of hypothalamic neurotransmitters (including serotonin and vagal tone) and gut hormones, increased concentrations of free radicals, and imbalance in the levels of bioactive lipids and their pro- and anti-inflammatory metabolites have been suggested to play a role in diabetes mellitus (DM). Type 1 diabetes mellitus (type 1 DM) is due to autoimmune destruction of pancreatic β cells because of enhanced production of IL-6 and tumor necrosis factor-α (TNF-α) and other pro-inflammatory cytokines released by immunocytes infiltrating the pancreas in response to unknown exogenous and endogenous toxin(s). On the other hand, type 2 DM is due to increased peripheral insulin resistance secondary to enhanced production of IL-6 and TNF-α in response to high-fat and/or calorie-rich diet (rich in saturated and trans fats). Type 2 DM is also associated with significant alterations in the production and action of hypothalamic neurotransmitters, eNO, BDNF, free radicals, gut hormones, and vagus nerve activity. Thus, type 1 DM is because of excess production of pro-inflammatory cytokines close to β cells, whereas type 2 DM is due to excess of pro-inflammatory cytokines in the systemic circulation. Hence, methods designed to suppress excess production of pro-inflammatory cytokines may form a new approach to prevent both type 1 and type 2 DM. Roux-en-Y gastric bypass and similar surgeries ameliorate type 2 DM, partly by restoring to normal: gut hormones, hypothalamic neurotransmitters, eNO, vagal activity, gut microbiota, bioactive lipids, BDNF production in the gut and hypothalamus, concentrations of cytokines and free radicals that results in resetting glucose-stimulated insulin production by pancreatic β cells. Our recent studies suggested that bioactive lipids, such as arachidonic acid, eicosapentaneoic acid, and docosahexaenoic acid (which are unsaturated fatty acids) and their anti-inflammatory metabolites: lipoxin A4, resolvins, protectins, and maresins, may have antidiabetic actions. These bioactive lipids have anti-inflammatory actions, enhance eNO, BDNF production, restore hypothalamic dysfunction, enhance vagal tone, modulate production and action of ghrelin, leptin and adiponectin, and influence gut microbiota that may explain their antidiabetic action. These pieces of evidence suggest that methods designed to selectively deliver bioactive lipids to pancreatic β cells, gut, liver, and muscle may prevent type 1 and type 2 DM.

Curcumin inhibits hyperlipidemia and hepatic fat accumulation in high-fructose-fed male Wistar rats

CONTEXT

Curcumin, an active principal of Curcuma longa Linn. (Zingiberaceae), has potent antioxidant and anti-inflammatory properties.

OBJECTIVES

This study investigated the effects of curcumin on hyperlipidemia and hepatic steatosis in high-fructose-fed Wistar rats.

MATERIALS AND METHODS

Forty male Wistar rats were divided into four groups with 10 rats in each. Two groups were fed with standard rodent diet and the other two with 60% high-fructose diet for 10 weeks. Curcumin (200 mg/kg body weight) was administered along with the diets simultaneously to each of the aforementioned diet groups. After 10 weeks of experiment, blood samples were collected from tail vein. Liver, adipose and epididymal tissues were collected after sacrifice of the animals and stored for further analyses.

RESULTS

Administration of curcumin reduced body weight (280.6 ± 7.4 g), liver weight (2.5 ± 0.2 g/100 g BW), adipose weight (1.4 ± 0.3 g/100 g BW), plasma levels of TAG (86.1 ± 13.5 mg/dL), VLDL-C (17.2 ± 2.7 mg/dL), lipid ratios and increased HDL-C (28.4 ± 4.5 mg/dL) in fructose-fed rats. Curcumin supplementation significantly lowered TAG content and decreased the protein expression of LXR-α (43%) and SREBP1c (59%) in the liver. Furthermore, curcumin suppressed the expression of lipogenic enzymes, ACLY (95%), ACC (50%) and FAS (77%) in rats fed with high-fructose diet. No significant change was found in the expression of PPAR-α.

DISCUSSION AND CONCLUSION

Curcumin prevented the high-fructose induced hyperlipidemia and hepatic steatosis.

The combined action of omega-3 polyunsaturated fatty acids and grape proanthocyanidins on a rat model of diet-induced metabolic alterations

It has been suggested that food components such as ω-3 polyunsaturated fatty acids (ω-3 PUFAs) and (poly)phenols counteract diet-induced metabolic alterations by common or complementary mechanisms. To examine the effects of a combination of ω-3 PUFAs and (poly)phenols on such alterations, adult Wistar-Kyoto rats were fed an obesogenic high-fat high-sucrose diet supplemented, or not, for 24 weeks with: eicosapentaenoic acid (EPA)/docosahexaenoic acid (DHA) 1 : 1 (16.6 g kg(-1) feed); proanthocyanidin-rich grape seed extract (GSE, 0.8 g kg(-1) feed); or EPA/DHA 1 : 1 + GSE. Body weight, feed intake, and plasma glucose were evaluated every 6 weeks, while adipose tissue weight, insulin, glucagon, ghrelin, leptin, adiponectin, cholesterol, and triglycerides were evaluated at the end of the experiment. ω-3 PUFAs reduced plasma leptin and cholesterol levels, but did not modify diet-induced perigonadal fat or plasma insulin levels; while GSE increased plasma triglyceride levels. The combined action of ω-3 PUFAs and the proanthocyanidins reduced plasma insulin and leptin, as well as partially prevented perigonadal fat accumulation. While separate supplementation with ω-3 PUFAs or grape proanthocyanidins may not counteract all the key metabolic changes induced by a high-energy-dense diet, the combination of both supplements reverts altered insulin, leptin and triglyceride levels to normal.

Fructose Metabolism and Relation to Atherosclerosis, Type 2 Diabetes, and Obesity

A high intake of sugars has been linked to diet-induced health problems. The fructose content in sugars consumed may also affect health, although the extent to which fructose has a particularly significant negative impact on health remains controversial. The aim of this narrative review is to describe the body's fructose management and to discuss the role of fructose as a risk factor for atherosclerosis, type 2 diabetes, and obesity. Despite some positive effects of fructose, such as high relative sweetness, high thermogenic effect, and low glycaemic index, a high intake of fructose, particularly when combined with glucose, can, to a larger extent than a similar glucose intake, lead to metabolic changes in the liver. Increased de novo lipogenesis (DNL), and thus altered blood lipid profile, seems to be the most prominent change. More studies with realistic consumption levels of fructose are needed, but current literature does not indicate that a normal consumption of fructose (approximately 50–60 g/day) increases the risk of atherosclerosis, type 2 diabetes, or obesity more than consumption of other sugars. However, a high intake of fructose, particularly if combined with a high energy intake in the form of glucose/starch, may have negative health effects via DNL.

Effect of n-3 PUFA supplementation at different EPA:DHA ratios on the spontaneously hypertensive obese rat model of the metabolic syndrome

The increasing incidence of the metabolic syndrome (MetS), a combination of risk factors before the onset of CVD and type 2 diabetes, encourages studies on the role of functional food components such as long-chain n-3 PUFA as preventive agents. In the present study, we explore the effect of EPA and DHA supplementation in different proportions on spontaneously hypertensive obese (SHROB) rats, a model for the MetS in a prediabetic state with mild oxidative stress. SHROB rats were randomised into four groups (n 7), each supplemented with EPA/DHA at ratios of 1:1, 2:1 and 1:2, or soyabean oil as the control for 13 weeks. The results showed that in all the proportions tested, EPA/DHA supplementation significantly lowered total and LDL-cholesterol concentrations, compared with those of the control group. EPA/DHA supplementation at the ratios of 1:1 and 2:1 significantly decreased inflammation (C-reactive protein levels) and lowered oxidative stress (decreased excretion of urinary isoprostanes), mainly at the ratio of 1:2. The activity of antioxidant enzymes increased in erythrocytes, abdominal fat and kidneys, with magnitudes depending on the EPA:DHA ratio. PUFA mixtures from fish affected different MetS markers of CVD risk factors in SHROB rats, depending on the ratios of EPA/DHA supplementation. The activation of endogenous defence systems may be related to the reduction of inflammation and oxidative stress.

The Effect of n-3 Fatty Acids on Glucose Homeostasis and Insulin Sensitivity

Type 2 diabetes (T2D) as well as cardiovascular disease (CVD) represent major complications of obesity and associated metabolic disorders (metabolic syndrome). This review focuses on the effects of long-chain n-3 polyunsaturated fatty acids (omega-3) on insulin sensitivity and glucose homeostasis, which are improved by omega-3 in many animal models of metabolic syndrome, but remain frequently unaffected in humans. Here we focus on: (i) mechanistic aspects of omega-3 action, reflecting also our experiments in dietary obese mice; and (ii) recent studies analysing omega-3’s effects in various categories of human subjects. Most animal experiments document beneficial effects of omega-3 on insulin sensitivity and glucose metabolism even under conditions of established obesity and insulin resistance. Besides positive results obtained in both cross-sectional and prospective cohort studies on healthy human populations, also some intervention studies in prediabetic subjects document amelioration of impaired glucose homeostasis by omega-3. However, the use of omega-3 to reduce a risk of new-onset diabetes in prediabetic subjects still remains to be further characterized. The results of a majority of clinical trials performed in T2D patients suggest that omega-3 have none or marginal effects on metabolic control, while effectively reducing hypertriglyceridemia in these patients. Despite most of the recent randomized clinical trials do not support the role of omega-3 in secondary prevention of CVD, this issue remains still controversial. Combined interventions using omega-3 and antidiabetic or hypolipidemic drugs should be further explored and considered for treatment of patients with T2D and other diseases.

Bekhit A. Anti-diabetic activity of red pitaya (Hylocereus polyrhizus) fruit

This study investigated the anti-diabetic activity of red pitaya (Hylocereus polyrhizus) fruit.

Cardioprotective and antihypertensive effects of Enicostemma littorale Blume extract in fructose-fed rats

We investigated the protective effects of Enicostemma littorale Blume (EL) extract on hypertension and insulin resistance along with its associated cardiovascular complications in high fructose (HF) fed rats. For this, rats were divided among 4 groups: (i) control, fed laboratory chow; (ii) fed with a high level of fructose; (iii) fed with a high level of fructose plus E. littorale extract; and (iv) fed with a high level of fructose plus rosiglitazone (Rg). EL and Rg treatments were given simultaneously with HF diet. The results show that untreated HF-fed rats showed altered oral glucose tolerance, increased fasting insulin, and increased fasting glucose. These rats also exhibited hypertriglyceridemia, moderate hypertension, platelet hyperaggregability, decreased prothrombin time, activated partial thromboplastin time, altered vascular reactivity, and increased serum levels of enzymes (creatine kinase, type muscle-brain (CK-MB), aspartate aminotransferase (SGOT), lactate dehydrogenase (LDH), and alanine aminotransferase (SGPT). This is the first demonstration of platelet hyperaggregation and prothrombotic alteration in HF-fed rats. HF-fed rats treated with EL showed improved insulin resistance, along with reduced hypertriglyceridemia, hypertension, platelet aggregability, blood coagulation, serum enzymes (CK-MB, SGOT, LDH and SGPT), and vascular reactivity. These effects of EL in HF-induced hypertensive rats might be associated with the suppression of hyperinsulinemia and hypertriglyceridemia, along with its antiatherogenic and antithrombogenic potential. These data indicate that the aqueous extract of EL has great therapeutic potential for the prevention and (or) management of insulin resistance and the associated hypertension.

The Relation between Fructose-Induced Metabolic Syndrome and Altered Renal Haemodynamic and Excretory Function in the Rat

This paper explores the possible relationships between dietary fructose and altered neurohumoral regulation of renal haemodynamic and excretory function in this model of metabolic syndrome. Fructose consumption induces hyperinsulinemia, hypertriglyceridaemia, insulin resistance, and hypertension. The pathogenesis of fructose-induced hypertension is dubious and involves numerous pathways acting both singly and together. In addition, hyperinsulinemia and hypertension contribute significantly to progressive renal disease in fructose-fed rats. Moreover, increased activity of the renin-angiotensin and sympathetic nervous systems leading to downregulation of receptors may be responsible for the blunted vascular sensitivity to angiotensin II and catecholamines, respectively. Various approaches have been suggested to prevent the development of fructose-induced hypertension and/or metabolic alteration. In this paper, we address the role played by the renin-angiotensin and sympathetic nervous systems in the haemodynamic alterations that occur due to prolonged consumption of fructose.

Effects of lifestyle interventions on inflammatory markers in the metabolic syndrome

Metabolic syndrome (MetS) is a clustering of cardiovascular risk factors which places individuals at increased risk for cardiovascular morbidity and mortality. In addition to obesity and insulin resistance, inflammation is emerging as a potential etiologic factor of the syndrome. One hypothesis suggests that obesity contributes to insulin resistance through increased production of adipose-derived inflammatory cytokines. Currently, lifestyle change is the first line of treatment for MetS. Only recently, however, have studies begun exploring the effect of lifestyle interventions on the mediation of inflammation in individuals with MetS. This review summarizes the strongest evidence (i.e. randomized controlled trial data) for a role of lifestyle interventions (diet and/or exercise) on the improvement of inflammatory biomarkers in people with MetS. Of six studies assessed, lifestyle interventions were consistently successful at improving the inflammatory and metabolic profiles. Interestingly, improvements in the inflammatory profile were found to be largely independent of obesity. Data currently suggest that alterations in dietary composition may be the most effective lifestyle change, although there is a need for more research in this area.

N-3 polyunsaturated fatty acids regulate lipid metabolism through several inflammation mediators: mechanisms and implications for obesity prevention

Obesity is a growing problem that threatens the health and welfare of a large proportion of the human population. The n-3 polyunsaturated fatty acids (PUFA) are dietary factors that have potential to facilitate reduction in body fat deposition and improve obesity-induced metabolic syndromes. The n-3 PUFA up-regulate several inflammation molecules including serum amyloid A (SAA), tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in hepatocytes and adipocytes. Actions of these inflammation mediators resemble those of n-3 PUFA in the modulation of many lipid metabolism-related genes. For instance, they both suppress expressions of perilipin, sterol regulatory element binding protein-1 (SREBP-1) and lipoprotein lipase (LPL) to induce lipolysis and reduce lipogenesis. This review will connect these direct or indirect regulating pathways between n-3 PUFA, inflammation mediators, lipid metabolism-related genes and body fat reduction. A thorough knowledge of these regulatory mechanisms will lead us to better utilization of n-3 PUFA to reduce lipid deposition in the liver and other tissues, therefore presenting an opportunity for developing new strategies to treat obesity.

Obesity: genes, brain, gut, and environment

Obesity, which is assuming alarming proportions, has been attributed to genetic factors, hypothalamic dysfunction, and intestinal gut bacteria and an increase in the consumption of energy-dense food. Obesity predisposes to the development of type 2 diabetes mellitus, hypertension, coronary heart disease, and certain forms of cancer. Recent studies have shown that the intestinal bacteria in obese humans and mice differ from those in lean that could trigger a low-grade systemic inflammation. Consumption of a calorie-dense diet that initiates and perpetuates obesity could be due to failure of homeostatic mechanisms that regulate appetite, food consumption, and energy balance. Hypothalamic factors that regulate energy needs of the body, control appetite and satiety, and gut bacteria that participate in food digestion play a critical role in the onset of obesity. Incretins, cholecystokinin, brain-derived neurotrophic factor, leptin, long-chain fatty acid coenzyme A, endocannabinoids and vagal neurotransmitter acetylcholine play a role in the regulation of energy intake, glucose homeostasis, insulin secretion, and pathobiology of obesity and type 2 diabetes mellitus. Thus, there is a cross-talk among the gut, liver, pancreas, adipose tissue, and hypothalamus. Based on these evidences, it is clear that management of obesity needs a multifactorial approach.

Eicosapentaenoic acid improves imbalance between vasodilator and vasoconstrictor actions of endothelium-derived factors in mesenteric arteries from rats at chronic stage of type 2 diabetes

Accumulating evidence demonstrates that dietary intake of n-3 polyunsaturated fatty acids (PUFAs) is associated with a reduced incidence of several cardiovascular diseases that involve endothelial dysfunction. However, the molecular mechanism remains unclear. We previously reported that mesenteric arteries from type 2 diabetic Otsuka Long-Evans Tokushima fatty (OLETF) rats exhibit endothelial dysfunction, leading to an imbalance between endothelium-derived vasodilators [namely, nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)] and vasoconstrictors [endothelium-derived contracting factors (EDCFs)] [namely cyclooxygenase (COX)-derived prostanoids] (Am J Physiol Heart Circ Physiol 293:H1480-H1490, 2007). We hypothesized that treating OLETF rats with eicosapentaenoic acid (EPA), a major n-3 PUFA, may improve endothelial dysfunction by correcting this imbalance. In OLETF rats [compared with age-matched control Long-Evans Tokushima Otsuka (LETO) rats]: 1) acetylcholine (ACh)-induced (endothelium-dependent) relaxation was impaired, 2) NO- and EDHF-mediated relaxations and nitrite production were reduced, and 3) ACh-induced EDCF-mediated contraction, production of prostanoids, and the protein expressions of COX-1 and COX-2 were all increased. When OLETF rats received chronic EPA treatment long-term (300 mg/kg/day p.o. for 4 weeks), their isolated mesenteric arteries exhibited: 1) improvements in ACh-induced NO- and EDHF-mediated relaxations and COX-mediated contraction, 2) reduced EDCF- and arachidonic acid-induced contractions, 3) normalized NO metabolism, 4) suppressed production of prostanoids, 5) reduced COX-2 expression, and 6) reduced phosphoextracellular signal-regulated kinase (ERK) expression. Moreover, EPA treatment reduced both ERK2 and nuclear factor (NF)-kappaB activities in isolated OLETF aortas. We propose that EPA ameliorates endothelial dysfunction in OLETF rats by correcting the imbalance between endothelium-derived factors, at least partly, by inhibiting ERK, decreasing NF-kappaB activation, and reducing COX-2 expression.

Fructose, but not dextrose, accelerates the progression of chronic kidney disease

The metabolic syndrome has recently been recognized as a risk factor for kidney disease, but the mechanisms mediating this risk remain unclear. High fructose consumption by animals produces a model of the metabolic syndrome with hypertension, hyperlipidemia, and insulin resistance. The present study was conducted to test the hypothesis that consumption of a high-fructose diet could accelerate the progression of chronic kidney disease. Three groups of 14 male Sprague-Dawley rats were pair fed a specialized diet containing 60% fructose (FRU) or 60% dextrose (DEX) or standard rat chow (CON). After the animals were fed their assigned diet for 6 wk, five-sixths nephrectomy was performed, and the assigned diet was continued for 11 wk. Proteinuria was significantly increased and creatinine clearance was decreased in the FRU group compared with the CON and DEX groups, and blood urea nitrogen was higher in the FRU group than in the CON and DEX groups. Kidneys from the FRU group were markedly larger than kidneys from the CON and DEX groups. Glomerular sclerosis, tubular atrophy, tubular dilatation, and cellular infiltration appeared markedly worse in kidneys from the FRU group than in kidneys from the DEX and CON groups. Monocyte chemoattractant protein-1 (MCP-1) was measured in renal tissue homogenate and found to be increased in the FRU group. In vitro studies were conducted to determine the mechanism for increased renal MCP-1, and fructose stimulation of proximal tubular cells resulted in production of MCP-1. In conclusion, consumption of a high-fructose diet greatly accelerates progression of chronic kidney disease in the rat remnant kidney model.

Is metabolic syndrome X a disorder of the brain with the initiation of low-grade systemic inflammatory events during the perinatal period?

An imbalance between pro- and anti-inflammatory molecules occurs in metabolic syndrome X. High-energy diet, saturated fats and trans-fats during perinatal period could suppress Delta(6) and Delta(5) desaturases both in the maternal and fetal tissues, resulting in a decrease in the concentrations of long-chain polyunsaturated fatty acids (LCPUFAs): arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) that have a negative feedback control on inflammation. EPA, DHA and AA augment endothelial nitric oxide synthesis, potentiate insulin action both in the peripheral tissues and brain and alter leptin production. LCPUFAs are essential for brain growth and development and synaptogenesis and modulate the action of several neurotransmitters and hypothalamic peptides. This suggests that metabolic syndrome X could be a disorder of the brain due to suboptimal LCPUFAs during perinatal period that triggers low-grade systemic inflammation, implying that perinatal strategies are needed to prevent its development.

Dietary n-3 fatty acids affect mRNA level of brown adipose tissue uncoupling protein 1, and white adipose tissue leptin and glucose transporter 4 in the rat

We examined the effect of dietary fats rich in n-3 polyunsaturated fatty acids (PUFA) on mRNA levels in white and brown adipose tissues in rats. Four groups of rats were fed on a low-fat diet (20 g safflower oil/kg) or a high-fat diet (200 g/kg) containing safflower oil, which is rich in n-6 PUFA (linoleic acid), or perilla (α-linolenic acid) or fish oil (eicosapentaenoic and docosahexaenoic acids), both of which are rich in n-3 PUFA, for 21 d. Energy intake was higher in rats fed on a high-safflower-oil diet than in those fed on low-fat or high-fish-oil diet, but no other significant differences were detected among the groups. Perirenal white adipose tissue weight was higher and epididymal white adipose tissue weight tended to be higher in rats fed on a high-safflower-oil diet than in those fed on a low-fat diet. However, high-fat diets rich in n-3 PUFA, compared to a low-fat diet, did not increase the white adipose tissue mass. High-fat diets relative to a low-fat diet increased brown adipose tissue uncoupling protein 1 mRNA level. The increases were greater with fats rich in n-3 PUFA than with n-6 PUFA. A high-safflower-oil diet, compared to a low-fat diet, doubled the leptin mRNA level in white adipose tissue. However, high-fat diets rich in n-3 PUFA failed to increase it. Compared to a low-fat diet, high-fat diets down-regulated the glucose transporter 4 mRNA level in white adipose tissue. However, the decreases were attenuated with high-fat diets rich in n-3 PUFA. It is suggested that the alterations in gene expression in adipose tissue contribute to the physiological activities of n-3 PUFA in preventing body fat accumulation and in regulating glucose metabolism in rats.

Preventive Effect of Diabegon, a Polyherbal Preparation, During Progression of Diabetes Induced by High-Fructose Feeding in Rats

In the present study, the polyherbal preparation diabegon, containing 18 plant extracts with hypoglycemic activity, was evaluated for its preventive effect during progression of type 2 diabetes in high-fructose-diet-fed rats. Oral administration of diabegon (100 mg/kg body weight) delayed development of glucose intolerance for 4 weeks in comparison with the diabetic control group, and the effect of diabegon was compared to that of the standard insulin sensitizer drug rosiglitazone. Diabegon treatment also ameliorated the elevation of glycosylated haemoglobin, liver glycogen content, plasma insulin, homeostasis model assessment, free fatty acids, triglycerides, total cholesterol, LDL-cholesterol, and VLDL-cholesterol, whereas it increased HDL-cholesterol after 56 days of treatment (P<0.05). The mechanism of action by which diabegon attenuates insulin resistance and dyslipidemia may be through induction of peroxisome proliferator-activated receptor-gamma and lipoprotein lipase activity in peripheral tissues (muscles). Moreover, diabegon administration for 56 days also produced no alteration in liver and kidney function tests, which seems to indicate its non-toxicity during treatment. Our present results suggest that diabegon may be included in diabetes mellitus treatment regimens, as a drug with good antidiabetic actions but no toxic manifestations.

Modulation of Cutaneous Fatty Acid-binding Protein mRNA Expression in Rat Adipose Tissues by Hereditary Obesity and Dietary Fats

Cutaneous fatty acid-binding protein (C-FABP) is a member of the intracellular lipid-binding protein multigene family expressed in various tissues. A high level of C-FABP mRNA in adipose tissue has been reported, but its physiological significance in regulating adipose tissue function is not clear. To obtain insights into the role of C-FABP in adipose tissue, we studied the obesity-related and dietary fat-related changes of C-FABP mRNA expression in adipose tissues. C-FABP mRNA levels in interscapular brown adipose tissue, and epididymal and perirenal white adipose tissues were higher in Zucker fatty rats than in lean controls despite that the difference in brown adipose tissue was not significant. Fish oil compared to palm and safflower oils significantly reduced the mRNA level of C-FABP in brown adipose tissue and epididymal and perirenal white adipose tissues in Sprague-Dawley rats except for one occasion. Our study demonstrated that C-FABP is a protein whose mRNA expression is easily modified by hereditary obesity and the type of dietary fat. Therefore, C-FABP may play a significant role in regulating adipocyte function in response to changes in nutritional conditions.

Potential impact of carbohydrate and fat intake on pathological left ventricular hypertrophy

Currently, a high carbohydrate/low fat diet is recommended for patients with hypertension; however, the potentially important role that the composition of dietary fat and carbohydrate plays in hypertension and the development of pathological left ventricular hypertrophy (LVH) has not been well characterized. Recent studies demonstrate that LVH can also be triggered by activation of insulin signaling pathways, altered adipokine levels, or the activity of peroxisome proliferator-activated receptors (PPARs), suggesting that metabolic alterations play a role in the pathophysiology of LVH. Hypertensive patients with high plasma insulin or metabolic syndrome have a greater occurrence of LVH, which could be due to insulin activation of the serine-threonine kinase Akt and its downstream targets in the heart, resulting in cellular hypertrophy. PPARs also activate cardiac gene expression and growth and are stimulated by fatty acids and consumption of a high fat diet. Dietary intake of fats and carbohydrate and the resultant effects of plasma insulin, adipokine, and lipid concentrations may affect cardiomyocyte size and function, particularly in the setting of chronic hypertension. This review discusses potential mechanisms by which dietary carbohydrates and fats ca affect cardiac growth, metabolism, and function, mainly in the context of pressure overload-induced LVH.

Essential fatty acids: biochemistry, physiology and pathology

Essential fatty acids (EFAs), linoleic acid (LA), and alpha-linolenic acid (ALA) are essential for humans, and are freely available in the diet. Hence, EFA deficiency is extremely rare in humans. To derive the full benefits of EFAs, they need to be metabolized to their respective long-chain metabolites, i.e., dihomo-gamma-linolenic acid (DGLA), and arachidonic acid (AA) from LA; and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from ALA. Some of these long-chain metabolites not only form precursors to respective prostaglandins (PGs), thromboxanes (TXs), and leukotrienes (LTs), but also give rise to lipoxins (LXs) and resolvins that have potent anti-inflammatory actions. Furthermore, EFAs and their metabolites may function as endogenous angiotensin-converting enzyme and 3-hdroxy-3-methylglutaryl coenzyme A reductase inhibitors, nitric oxide (NO) enhancers, anti-hypertensives, and anti-atherosclerotic molecules. Recent studies revealed that EFAs react with NO to yield respective nitroalkene derivatives that exert cell-signaling actions via ligation and activation of peroxisome proliferator-activated receptors. The metabolism of EFAs is altered in several diseases such as obesity, hypertension, diabetes mellitus, coronary heart disease, schizophrenia, Alzheimer's disease, atherosclerosis, and cancer. Thus, EFAs and their derivatives have varied biological actions and seem to be involved in several physiological and pathological processes.

Effect of Skim Milk and Dahi (Yogurt) on Blood Glucose, Insulin, and Lipid Profile in Rats Fed with High Fructose Diet

In the present study, the effect of skim milk and the fermented milk product named dahi (yogurt) on plasma glucose, insulin, and lipid levels as well as on liver glycogen and lipid contents in rats fed with high fructose diet has been investigated. Rats were fed with high fructose diet (21%) supplemented with skim milk, dahi (10 g/day each), or no milk product (control group) for 6 weeks. After 6 weeks of high fructose diet administration, the plasma glucose became significantly higher in control animals (246 mg/dL), whereas it was lower in skim milk (178 mg/dL)- and dahi (143 mg/dL)-fed rats. The glucose tolerance became impaired at the third week of feeding of high fructose diet in control animals, whereas in skim milk- and dahi-fed animals achievement of glucose intolerance was delayed until the fourth and fifth week, respectively. Blood glycosylated hemoglobin and plasma insulin were significantly lower in skim milk (10% and 34%, respectively)- and dahi (17%, and 48%, respectively)-fed animals than those of the control group. Plasma total cholesterol, triglycerides, low-density lipoprotein-cholesterol, and very-low-density lipoprotein-cholesterol and blood free fatty acids were significantly lower in skim milk (13%, 14%, 14%, 19%, and 14%, respectively)- and dahi (22%, 33%, 30%, 33%, and 29%, respectively)-fed animals as compared with control animals. Moreover, the total cholesterol, triglyceride, and glycogen contents in liver tissues were also lower in skim milk (55%, 50%, and 36%, respectively)- and dahi (64%, 27%, and 4%, respectively)-fed animals as compared with control animals. In contrast, high-density lipoprotein-cholesterol in plasma was higher in skim milk (14%)- and dahi (29%)-fed animals as compared with control animals. These results indicate that skim milk and its fermented milk product, dahi, delay the progression of fructose-induced diabetes and dyslipidemia in rats and that these may be useful as antidiabetic food supplements that can be included in daily meals of the diabetic as well as normal population.

Plasma leptin response to oral glucose tolerance and fasting/re-feeding tests in rats with fructose-induced metabolic derangements

The aim of this study was to compare the postprandial leptin response in rats with and without metabolic syndrome induced by a fructose-enriched diet. The effect of aging and the association between variations in metabolic substrates was also evaluated. Oral glucose tolerance test (OGTT) and fasting/re-feeding test were used to evaluate the responses of leptin and to explore the dynamic relationship between endogenous leptin and metabolic substrates, including glucose, insulin and triglycerides (TG). At the 7th week, plasma leptin was unchanged in control rats after oral glucose loading. However, plasma leptin levels increased in fructose-fed rats with insulin resistant OGTT curves. At the 11th month, plasma leptin level was reduced during starvation and returned to the level prior to starvation during re-feeding in control rats. In contrast, the starvation-induced reduction in leptin showed a potentially larger rebound effect during re-feeding in fructose-fed rats. Analysis of covariance demonstrated that there alone was no interactive effect of dietary manipulation between leptin and TG, suggesting that fructose diet-induced insulin resistance-related metabolic syndrome may concomitantly elevate leptin and TG. Furthermore, multiple regression analysis suggests TG was the primary correlative determinant of endogenous leptin concentration. Our data showed that there are different patterns of leptin response to OGTT and fasting/re-feeding tests in rats with and without metabolic syndrome. The results suggest that these effects may be related to a TG-mediated impairment of leptin function and a protective mechanism to reduce lipid-induced tissue damage in patients with metabolic syndrome.

Effect of fructose overfeeding and fish oil administration on hepatic de novo lipogenesis and insulin sensitivity in healthy men

High-fructose diet stimulates hepatic de novo lipogenesis (DNL) and causes hypertriglyceridemia and insulin resistance in rodents. Fructose-induced insulin resistance may be secondary to alterations of lipid metabolism. In contrast, fish oil supplementation decreases triglycerides and may improve insulin resistance. Therefore, we studied the effect of high-fructose diet and fish oil on DNL and VLDL triglycerides and their impact on insulin resistance. Seven normal men were studied on four occasions: after fish oil (7.2 g/day) for 28 days; a 6-day high-fructose diet (corresponding to an extra 25% of total calories); fish oil plus high-fructose diet; and control conditions. Following each condition, fasting fractional DNL and endogenous glucose production (EGP) were evaluated using [1-13C]sodium acetate and 6,6-2H2 glucose and a two-step hyperinsulinemic-euglycemic clamp was performed to assess insulin sensitivity. High-fructose diet significantly increased fasting glycemia (7 +/- 2%), triglycerides (79 +/- 22%), fractional DNL (sixfold), and EGP (14 +/- 3%, all P < 0.05). It also impaired insulin-induced suppression of adipose tissue lipolysis and EGP (P < 0.05) but had no effect on whole- body insulin-mediated glucose disposal. Fish oil significantly decreased triglycerides (37%, P < 0.05) after high-fructose diet compared with high-fructose diet without fish oil and tended to reduce DNL but had no other significant effect. In conclusion, high-fructose diet induced dyslipidemia and hepatic and adipose tissue insulin resistance. Fish oil reversed dyslipidemia but not insulin resistance.

Pathophysiology of metabolic syndrome X and its links to the perinatal period

It is proposed that metabolic syndrome X is initiated in the perinatal period as a low-grade systemic inflammatory condition. Increased consumption of energy-dense diets by pregnant women and lactating mothers suppresses the activities of Delta-6 and Delta-5 desaturases not only in maternal tissues but also in fetal liver and the placenta, resulting in decreased plasma and tissue concentrations of long-chain polyunsaturated fatty acids omega-6 arachidonic acid (AA), omega-3 eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). EPA, DHA, and AA have negative feedback control on tumor necrosis factor-alpha and IL-6 synthesis. Hence, EPA, DHA, and AA deficiencies induced by an energy-dense diet increase generation of tumor necrosis factor-alpha and interleukin-6, markers of inflammation that in turn decrease production of endothelial nitric oxide and adiponectin to induce insulin resistance in maternal and fetal tissues. Increased concentrations of tumor necrosis factor-alpha and interleukin-6 enhance expression and activity of 11beta-hydroxysteroid dehydrogenase type 1 enzyme, which produces abdominal obesity, insulin resistance, hyperlipidemia, hyperphagia, and hyperleptinemia, characteristic features of metabolic syndrome X. Continued consumption of an energy-dense diet in childhood aggravates these molecular events. This implies that supplementation of long-chain polyunsaturated fatty acids (especially AA, EPA, and DHA in appropriate ratios) from the perinatal period through adulthood could prevent, arrest, or postpone development of metabolic syndrome X.

Dietary fat type alters glucose metabolism in isolated rat hepatocytes

Dietary fat type can influence the regulation of carbohydrate metabolism in multiple tissue types. The influence of feeding high-fat (40% of kilocalories) diets containing either menhaden oil (MO) or coconut oil (CO) on hepatic glycogenolytic and gluconeogenic capacities was studied in isolated rat hepatocytes. Estimates of both glycogenolytic and gluconeogenic capacities were performed on hepatocytes isolated from fed and fasted animals, respectively. In MO-fed animals, both basal and hormone-stimulated rates of glucose production were significantly greater than those in CO-fed animals. However, both groups displayed a similar maximal increase in glucose production above basal for glucagon and epinephrine (2.3- and 1.9-fold, respectively). Basal rates of adenosine 3',5'-cyclic phosphate (cAMP) production were not different between groups whereas glucagon-stimulated cAMP production was increased twofold in the MO-fed group. In both MO and CO groups, the addition of 10 nM insulin reduced glucose production in fed animals to similar absolute rates. In animals fasted for 24 hours, gluconeogenic capacity was estimated using 10 mM pyruvate, lactate, or glycerol. Glucose production from all substrates was significantly greater in CO-fed animals. In addition to increased gluconeogenic rates, maximal phosphoenolpyruvate carboxykinase (PEPCK) activity was increased in the CO-fed group. Insulin reduced glucose production in both dietary groups, but the absolute rate of glucose production was 28% greater in the CO-fed group relative to the MO-fed group. In summary, dietary fat type can markedly influence the regulation of hepatic glucose metabolism in multiple metabolic pathways. MO feeding promoted glycogenolysis and sensitivity to insulin whereas CO feeding favored gluconeogenesis and reduced insulin sensitivity.

Chronic N-acetylcysteine prevents fructose-induced insulin resistance and hypertension in rats

We examined if administration of an antioxidant compound protects against the development of insulin resistance and hypertension. Male rats were assigned randomly into four groups, and treated for 12 weeks with normal chow, normal chow plus N-acetylcysteine (1.5 g/day/kg), fructose (60% of diet), and fructose plus N-acetylcysteine. After 10 weeks, plasma triglyceride and 15-F2t-isoprostane, and insulin sensitivity were measured, and after 12 weeks, pressor response to methoxamine (15-60 microg/kg min) was assessed. Relative to normal chow-fed controls, the fructose-fed rats had increased blood pressure, plasma insulin, triglyceride and 15-F2t-isoprostane, and decreased insulin sensitivity; these changes were inhibited by N-acetylcysteine. Maximal pressor response to methoxamine was attenuated in the fructose-fed rats given N-acetylcysteine relative to the other three groups. Therefore, chronic treatment with N-acetylcysteine increases insulin sensitivity and prevents the blood pressure increase associated with fructose feeding in rats, the mechanism may involve the decrease of oxidative stress and alpha-adrenoceptor-mediated vasoconstriction.

Insulin infusion induces endothelin-1-dependent hypertension in rats

We previously showed that chronic insulin infusion induces insulin resistance, hyperendothelinemia, and hypertension in rats (C. C. Juan, V. S. Fang, C. F. Kwok, J. C. Perng, Y. C. Chou, and L. T. Ho. Metabolism 48: 465-471, 1999). Endothelin-1 (ET-1), a potent vasoconstrictor, is suggested to play an important role in maintaining vascular tone and regulating blood pressure, and insulin increases ET-1 production in vivo and in vitro. In the present study, BQ-610, a selective endothelin A receptor antagonist, was used to examine the role of ET-1 in insulin-induced hypertension in rats. BQ-610 (0.7 mg/ml; 0.5 ml/kg body wt) or normal saline was given intraperitoneally two times daily for 25 days to groups of rats infused with either saline or insulin (2 U/day via sc-implanted osmotic pumps), and changes in plasma levels of insulin, glucose, and ET-1 and the systolic blood pressure were measured over the experimental period, whereas changes in insulin sensitivity were examined at the end of the experimental period. Plasma insulin and ET-1 levels were measured by RIA, plasma glucose levels using a glucose analyzer, systolic blood pressure by the tail-cuff method, and insulin sensitivity by an oral glucose tolerance test. Our studies showed that insulin infusion caused sustained hyperinsulinemia in both saline- and BQ-610-injected rats over the infusion period. After pump implantation (2 wk), the systolic blood pressure was significantly higher in insulin-infused rats than in saline-infused rats in the saline-injected group (133 +/- 3.1 vs. 113 +/- 1.1 mmHg, P < 0.05) but not in the BQ-610-injected group (117 +/- 1.2 vs. 117 +/- 1.8 mmHg). Plasma ET-1 levels in both sets of insulin-infused rats were higher than in saline-infused controls (2.5 +/- 0.6 and 2.5 +/- 0.8 vs. 1.8 +/- 0.4 and 1.7 +/- 0.3 pmol/l, P < 0.05). Oral glucose tolerance tests showed that BQ-610 treatment did not prevent the insulin resistance caused by chronic insulin infusion. No significant changes were found in insulin sensitivity and blood pressure in saline-infused rats treated with BQ-610. In a separate experiment, insulin infusion induced the increase in arterial ET-1 content, hypertension, and subsequent plasma ET-1 elevation in rats. These results suggest that, in the insulin infusion rat model, ET-1 plays a mediating role in the development of hypertension, but not of insulin resistance.

Iatrogenic lipodystrophy in HIV patients – the need for very-low-fat diets

In HIV patients, chronic treatment with protease inhibitors often precipitates a peripheral lipodystrophy associated with insulin resistance syndrome and premature coronary disease. In vitro studies demonstrate that these drugs can compromise the ability of adipocytes to store triglycerides; in vivo, peripheral subcutaneous adipocytes appear to be most affected, such that body fat often redistributes to visceral or truncal adipose stores. Dysfunction of peripheral subcutaneous adipocytes - ordinarily quite efficient for storing fat - can be expected to give rise to an excessive flux of free fatty acids (FFAs) following fatty meals; chronic overexposure of tissues to FFAs is a likely explanation for the insulin resistance syndrome associated with lipodystrophy. These considerations suggest that a very-low-fat diet - less than 15% fat calories - may ameliorate the cardiovascular risk associated with lipodystrophy; such diets are known to have a favorable effect on the insulin sensitivity of healthy subjects. Very-low-fat whole-food vegan diets are particularly recommendable in this context, as they may help to shrink visceral fat depots while markedly reducing LDL cholesterol. Appropriate adjunctive measures may include aerobic exercise training - beneficial both for insulin sensitivity and weight control - as well as administration of statins or policosanol, and of fibrates or fish oil, to decrease LDL and triglycerides, respectively. Despite perceptions to the contrary, very-low-fat diets can meet with good compliance in well-motivated subjects given appropriate instruction.

Can perinatal supplementation of long-chain polyunsaturated fatty acids prevent diabetes mellitus?

It is suggested that the negative correlation between breast-feeding and insulin resistance and diabetes mellitus can be related to the presence of significant amounts of long-chain polyunsaturated fatty acids in the human breast milk. Based on this, it is proposed that provision of adequate amounts of long chain polyunsaturated fatty acids during the critical periods of brain growth and development can prevent or postpone the development diabetes mellitus.

Is metabolic syndrome X an inflammatory condition?

It is suggested that metabolic syndrome X is a low-grade systemic inflammatory condition.

Is type 2 diabetes mellitus a disorder of the brain?

I propose that type 2 diabetes mellitus is due to damage to neurons in the ventromedial hypothalamus or to a defect in the action or properties of insulin or insulin receptors in the brain. These neuronal abnormalities are probably secondary to a marginal deficiency of long-chain polyunsaturated fatty acids during the critical periods of brain growth and development. Hence, supplementation of adequate amounts of long-chain polyunsaturated fatty acids during the third trimester of pregnancy to 2 y postterm can prevent or postpone the development of diabetes mellitus.

The lipids that matter from infant nutrition to insulin resistance

Breast-fed infants showed decreased incidence of obesity, hypertension, diabetes mellitus, and coronary heart disease in later life and higher cognitive function. Breast milk is rich in long-chain polyunsaturated fatty acids (LCPUFAs) and brain preferentially accumulates LCPUFAs during the last trimester of pregnancy and the first few months of life. Breast-fed infants showed significantly lower plasma glucose levels and higher percentage of docosahexaenoic acid and total percentages of LCPUFAs in their skeletal muscle biopsies compared with formula fed. LCPUFAs suppress the production of pro-inflammatory cytokines, regulate the function of several neurotransmitters, enhance the number of insulin receptors in the brain and other tissues, and decrease insulin resistance. LCPUFAs may enhance the production of bone morphogenetic proteins (BMPs), which participate in neurogenesis. It is proposed that the beneficial effects of breast feeding in later life can be attributed to its rich LCPUFA content. It is likely that inadequate breast feeding results in marginal deficiency of LCPUFAs during the critical stages of development, which can lead to insulin resistance. Hence, promoting prolonged breast feeding and/or supplementing LCPUFAs during the critical stages of development may be beneficial in preventing insulin resistance.

Suppressed gene expression of adipocyte resistin in an insulin-resistant rat model probably by elevated free fatty acids

Resistin, the peptide specifically secreted from adipocytes, is a hormone antagonistic to insulin action and, thus, may serve as a link between human obesity due to adiposity and insulin resistance associated with type 2 diabetes. To test this hypothesis, we studied the gene expression of resistin in adipocytes isolated from rats fed with a fructose diet which induced insulin resistance. Compared to the control rats (C) on a normal chow diet, the fructose-fed rats (F) developed hyperinsulinemia, glucose intolerance, hypertriglyceridemia and hypertension, a profile reminiscent of the syndrome X of patients with non-insulin-dependent diabetes mellitus (NIDDM). The F rats had significantly elevated plasma free fatty acids (FFA), enlarged epididymal fat pads, and increased adipocyte size compared with the C rats. We examined the glucose transport and the relative quantity of resistin mRNA produced in the adipocytes of these two groups of rats. Compared to the C rats, the F rats had a clearly reduced insulin-stimulated glucose transport. The gene expression of resistin and other adipocyte peptides was measured on the mRNA by semiquantitative RT-PCR; the validity of this technique was established in advance with a rat-fasting and then refeeding experiment. The F rats showed a decreased expression of the resistin gene, whereas gene expression of leptin and angiotensinogen in contrast increased. Free fatty acids were found to suppress the expression of resistin gene in normal rat adipocytes. These results demonstrate that an insulin-resistant instance in the fructose diet rat model exists with the decreased gene expression of resistin.

Is obesity an inflammatory condition?

Obesity may be a low-grade systemic inflammatory disease. Overweight and obese children and adults have elevated serum levels of C-reactive protein, interleukin-6, tumor necrosis factor-alpha, and leptin, which are known markers of inflammation and closely associated with cardiovascular risk factors and cardiovascular and non-cardiovascular causes of death. This may explain the increased risk of diabetes, heart disease, and many other chronic diseases in the obese. The complex interaction between several neurotransmitters such as dopamine, serotonin, neuropeptide Y, leptin, acetylcholine, melanin-concentrating hormone, ghrelin, nitric oxide, and cytokines and insulin and insulin receptors in the brain ultimately determines and regulates food intake. Breast-feeding of more than 12 mo is associated with decreased incidence of obesity. Breast milk is a rich source of long-chain polyunsaturated fatty acids (LCPUFAs) and brain is especially rich in these fatty acids. LCPUFAs inhibit the production of proinflammatory cytokines and enhance the number of insulin receptors in various tissues and the actions of insulin and several neurotransmitters. LCPUFAs may enhance the production of bone morphogenetic proteins, which participate in neurogenesis, so these fatty acids might play an important role in brain development and function. It is proposed that obesity is a result of inadequate breast feeding, which results in marginal deficiency of LCPUFAs during the critical stages of brain development. This results in an imbalance in the structure, function, and feedback loops among various neurotransmitters and their receptors, which ultimately leads to a decrease in the number of dopamine and insulin receptors in the brain. Hence, promoting prolonged breast feeding may decrease the prevalence of obesity. Exercise enhances parasympathetic tone, promotes antiinflammation, and augments brain acetylcholine and dopamine levels, events that suppress appetite. Acetylcholine and insulin inhibit the production of proinflammatory cytokines and provide a negative feedback loop for postprandial inhibition of food intake, in part, by regulating leptin action. Statins, peroxisome proliferator-activated receptor-gamma binding agents, non-steroidal antiinflammatory drugs, and infant formulas supplemented with LCPUFAs, and LCPUFAs themselves, which suppress inflammation, may be beneficial in obesity.

Dietary mold oil rich in gamma linolenic acid increases insulin-dependent glucose utilization in isolated rat adipocytes

Effects of dietary fats differing in fatty acid composition on insulin-stimulated glucose metabolism in adipocytes isolated from rat white adipose tissue were compared. Rats were fed experimental diets containing various fats differing in fatty acid composition for 7 days. In the first experiment, rats were fed palm oil mainly consisting of palmitic (45.3%) and oleic acids (39.1%) or safflower oil rich in linoleic acid (71.6%). In the second trial, rats were fed palm oil, or a fat mixture rich in linoleic acid or mold oil rich in gamma-linolenic acid. Contents of fatty acids except for linoleic and gamma-linolenic acid were comparable between the fat mixture and mold oil. The former was devoid of gamma-linolenic acid and contained 42.0% linoleic acid, while the latter contained 25.9% gamma-linolenic and 15.7% linoleic acids. In the first experiment, the insulin-dependent increase in glucose oxidation and incorporation into lipids was higher in rats fed safflower oil compared to those fed palm oil. In the second experiment, the insulin-dependent increase in glucose oxidation and incorporation into lipids was higher in rats fed the fat mixture and mold oil than in those fed palm oil. However, the extent of the increase in these parameters was much greater in rats fed mold oil than in those fed the fat mixture. Therefore, dietary gamma-linolenic acid compared to linoleic acid increases glucose metabolism in response to insulin stimuli in isolated rat adipocytes.

A high-fructose diet induces insulin resistance but not blood pressure changes in normotensive rats

Rats fed a high-fructose diet represent an animal model for insulin resistance and hypertension. We recently showed that a high-fructose diet containing vegetable oil but a normal sodium/potassium ratio induced mild insulin resistance with decreased insulin receptor substrate-1 tyrosine phosphorylation in the liver and muscle of normal rats. In the present study, we examined the mean blood pressure, serum lipid levels and insulin sensitivity by estimating in vivo insulin activity using the 15-min intravenous insulin tolerance test (ITT, 0.5 ml of 6 microg insulin, iv) followed by calculation of the rate constant for plasma glucose disappearance (Kitt) in male Wistar-Hannover rats (110-130 g) randomly divided into four diet groups: control, 1:3 sodium/potassium ratio (R Na:K) diet (C 1:3 R Na:K); control, 1:1 sodium/potassium ratio diet (CNa 1:1 R Na:K); high-fructose, 1:3 sodium/potassium ratio diet (F 1:3 R Na:K), and high-fructose, 1:1 sodium/potassium ratio diet (FNa 1:1 R Na:K) for 28 days. The change in R Na:K for the control and high-fructose diets had no effect on insulin sensitivity measured by ITT. In contrast, the 1:1 R Na:K increased blood pressure in rats receiving the control and high-fructose diets from 117 +/- 3 and 118 +/- 3 mmHg to 141 +/- 4 and 132 +/- 4 mmHg (P < 0.05), respectively. Triacylglycerol levels were higher in both groups treated with a high-fructose diet when compared to controls (C 1:3 R Na:K: 1.2 +/- 0.1 mmol/l vs F 1:3 R Na:K: 2.3 +/- 0.4 mmol/l and CNa 1:1 R Na:K: 1.2 +/- 0.2 mmol/l vs FNa 1:1 R Na:K: 2.6 +/- 0.4 mmol/l, P < 0.05). These data suggest that fructose alone does not induce hyperinsulinemia or hypertension in rats fed a normal R Na:K diet, whereas an elevation of sodium in the diet may contribute to the elevated blood pressure in this animal model.

The effect of dietary polyunsaturated fatty acid on insulin sensitivity and lipid metabolism in Otsuka Long-Evans Tokushima Fatty (OLETF) rats

The effects of insulin sensitivity and lipid metabolism of dietary lard, eicosapentaenoic acid-rich oil (EPA oil) or arachidonic acid oil (AA oil) in Otsuka Long-Evans Tokushima Fatty (OLETF) rats were examined. Blood glucose was not different in each group at 30, 60, 120 min on an oral glucose tolerance test. Fasting blood glucose levels were lower in lard and AA oil groups than in controls. Hepatic triglyceride concentration and liver histochemistry revealed that the fat content was higher in the lard group and the AA oil group than in controls. The EPA oil group showed TG levels as high as the control group. Serum total cholesterol in the EPA oil group was lower, while the level in the AA oil group was higher than in the lard and control groups. HDL cholesterol was 1.5-fold higher in the AA oil group than in controls. Dietary EPA oil or AA oil supplementation showed different effects on lipid metabolism in this model.

Combined use of insulin and endothelin-1 causes decrease of protein expression of beta-subunit of insulin receptor, insulin receptor substrate-1, and insulin-stimulated glucose uptake in rat adipocytes

Previously, we reported that insulin-stimulated glucose uptake (ISGU) can be inhibited by endothelin (ET-1). However, the mechanism by which ET-1 impairs ISGU in adipocytes remains unclear. This study investigated the effects of ET-1 on insulin action in rat adipocytes in order to elucidate the molecular mechanism of action of ET-1 on ISGU. The results show that ISGU was increased fivefold after 3-h treatment with 1 nM insulin. Treatment with 100 nM ET-1 had no effect on basal glucose uptake. However, ET-1 inhibited approximately 25% of ISGU and 20% of insulin binding after 3-h treatment in the presence of 1 nM insulin. Expression of the beta-subunit of the insulin receptor (IRbeta) and the insulin receptor substrate-1 (IRS-1) in adipocytes was not significantly affected by 1 nM insulin or by 100 nM ET-1, even after 3-h treatment. However, expressions of IRbeta and IRS-1 were dramatically decreased in a dose- and time-dependent manner when adipocytes were treated with both insulin and ET-1. Approximately 50% of IRbeta and 65% of IRS-1 expression levels were suppressed when adipocytes were simultaneously treated with both 1 nM insulin and 100 nM ET-1 for 3 h. These results suggest that the inhibitory effect of ET-1 on ISGU may be mediated via the insulin receptor and suppression of IRbeta/IRS-1 expression.

Syndrome X: medical nutrition therapy

A significant number of Americans are at risk for developing a condition of insulin resistance termed Syndrome X. Dyslipidemia, resistance to insulin, obesity, and blood pressure elevation--the deadly quartet--describe Syndrome X, which increases atherogenic risk and contributes to coronary artery disease. Lifestyle factors such as overeating and physical inactivity play a pivotal role in Syndrome X. This deadly duet has been aptly coined "hyperactive fork" and "hypoactive foot," respectively. In addition, emerging evidence suggests that certain nutrients may help protect against Syndrome X. This review provides a brief discussion of diet and lifestyle factors related to Syndrome X.

Modulation of triglyceride metabolism by glucocorticoids in diet-induced obesity

The involvement of glucocorticoids (GC) in the development of diet-induced obesity and in the concomitant adaptations of triglyceride (TG)-rich lipoprotein metabolism were examined. Rats were fed either rodent chow, which maintains a low lipid flux, or a diet high in sucrose and fat (HSF) that increases lipid flux, leading to metabolic perturbations similar to those that define the plurimetabolic syndrome in humans. The GC status was manipulated through adrenalectomy (ADX) and corticosterone (Cort) replacement. Compared with chow, the HSF diet increased energy intake (17%) and whole body (8%) and adipose tissue (80%) weights. The HSF diet also increased the acute postprandial rise in plasma insulin (4-fold) and TG (3-fold), fasting liver TG content (3-fold), triglyceridemia (54%), and adipose tissue lipoprotein lipase (LPL) activity (2-fold). ADX decreased energy intake and whole body and adipose tissue weights in both dietary cohorts, but more so in HSF-fed than in chow-fed animals. These ADX-induced effects were totally prevented by Cort replacement in rats fed chow, but only partially so in those fed the HSF diet in proportion to the degree of restoration of energy intake. In the chow-fed cohort, the above indexes of TG metabolism remained unaffected by the Cort status, whereas in the HSF-fed cohort, these variables were decreased by ADX to levels of chow-fed animals. Cort replacement in the HSF-fed animals restored indexes of TG metabolism to intact levels and reestablished the diet-related differences observed in intact animals. These findings indicate that GC modulate fasting TG metabolism only minimally when a diet that maintains a low lipid flux is fed. In contrast, their presence is a necessary condition for the development of diet-induced obesity and the concomitant alterations in insulin sensitivity and TG-rich lipoprotein metabolism.

Insulin-induced endothelin release and vasoreactivity in hypertriglyceridemic and hypertensive rats

Insulin-elicited endothelin release in hypertriglyceridemic, hypertensive, hyperinsulinemic (HTG) rats was shown. Weanling male Wistar rats were given 30% sucrose in their drinking water for 20-24 wk. In vitro contractions of aorta and femoral arteries were elicited with 40 mM KCl. Endothelin release induced with KCl plus 50 microU/ml insulin resulted in increases in contractile responses: 41 +/- 5.9 and 57 +/- 6% for control and 65.5 +/- 6 and 95 +/- 9% for HTG aortas and femoral arteries, respectively. The endothelin ET(B)-receptor blocker BQ-788 decreased responses to KCl + insulin by 39 +/- 8 and 53 +/- 5% in control and 48 +/- 13 and 79 +/- 3.5% in HTG aortas and femoral arteries, respectively. The ET(A)-receptor antagonist PD-151242 inhibited these responses by 12 +/- 10 and 1 +/- 9% in control and by 51.5 +/- 9 and 58.5 +/- 1% in HTG aortas and femoral arteries, respectively. These results suggest that endothelin may contribute to the hypertension in this model.

Evidence that endothelin-1 (ET-1) inhibits insulin-stimulated glucose uptake in rat adipocytes mainly through ETA receptors

The specificity of endothelin (ET) receptors involved in the inhibition of insulin-stimulated glucose uptake (ISGU) in rat adipocytes was investigated. Adipocytes were isolated from the epididymal fat pads of Sprague-Dawley rats. To determine receptor subtypes, we used three ET isopeptides, ET-1 and ET-2, both of which are nonselective agonists, and ET-3, a selective agonist for ETC receptors, to displace [125I]ET-1 binding from the fat cells. The efficiency of displacement was ET-1 > ET-2 >> ET-3, indicating that the primary receptors involved belonged to the ETA subtype. At an equal concentration of 1 micromol/L, BQ-610, a selective ETA antagonist, displaced [125I]ET-1 from binding to fat cells, whereas IRL-1038, a selective ETB antagonist, did not. Using [3H]2-deoxy-D-1-glucose ([3H]2-DG) as a tracer in studies of glucose uptake, we found that equimolar BQ-610 completely reversed the inhibitory effect of ET-1 on ISGU, whereas IRL-1038 was ineffective. Northern blot analysis of adipocyte receptors showed abundant mRNA for ETA, but no ETB subtype. These results clearly demonstrate that ETA is the predominant receptor in rat adipocytes.