Bethany P. Cummings

Endocrine and metabolic effects of consuming fructose- and glucose-sweetened beverages with meals in obese men and women: influence of insulin resistance on plasma triglyceride responses.

CONTEXT Compared with glucose-sweetened beverages, consumption of fructose-sweetened beverages with meals elevates postprandial plasma triglycerides and lowers 24-h insulin and leptin profiles in normal-weight women. The effects of fructose, compared with glucose, ingestion on metabolic profiles in obese subjects has not been studied. OBJECTIVE The objective of the study was to compare the effects of fructose- and glucose-sweetened beverages consumed with meals on hormones and metabolic substrates in obese subjects. DESIGN AND SETTING The study had a within-subject design conducted in the clinical and translational research center. PARTICIPANTS Participants included 17 obese men (n = 9) and women (n = 8), with a body mass index greater than 30 kg/m(2). INTERVENTIONS Subjects were studied under two conditions involving ingestion of mixed nutrient meals with either glucose-sweetened beverages or fructose-sweetened beverages. The beverages provided 30% of total kilocalories. Blood samples were collected over 24 h. MAIN OUTCOME MEASURES Area under the curve (24 h AUC) for glucose, lactate, insulin, leptin, ghrelin, uric acid, triglycerides (TGs), and free fatty acids was measured. RESULTS Compared with glucose-sweetened beverages, fructose consumption was associated with lower AUCs for insulin (1052.6 +/- 135.1 vs. 549.2 +/- 79.7 muU/ml per 23 h, P < 0.001) and leptin (151.9 +/- 22.7 vs. 107.0 +/- 15.0 ng/ml per 24 h, P < 0.03) and increased AUC for TG (242.3 +/- 96.8 vs. 704.3 +/- 124.4 mg/dl per 24 h, P < 0.0001). Insulin-resistant subjects exhibited larger 24-h TG profiles (P < 0.03). CONCLUSIONS In obese subjects, consumption of fructose-sweetened beverages with meals was associated with less insulin secretion, blunted diurnal leptin profiles, and increased postprandial TG concentrations compared with glucose consumption. Increases of TGs were augmented in obese subjects with insulin resistance, suggesting that fructose consumption may exacerbate an already adverse metabolic profile present in many obese subjects.

Fructose‐Fed Rhesus Monkeys: A Nonhuman Primate Model of Insulin Resistance, Metabolic Syndrome, and Type 2 Diabetes

The incidence of insulin resistance has increased dramatically over the past several years, and we and others have proposed that this increase may at least in part be attributable to increased dietary fructose consumption. However, a major limitation to the study of diet‐induced insulin resistance is the lack of relevant animal models. Numerous studies, mostly in rodents, have demonstrated that diets high in fructose induce insulin resistance; however, important metabolic differences exist between rodents and primates. Thus, the results of metabolic studies performed in primates are substantively more translatable to human physiology, underscoring the importance of establishing nonhuman primate models of common metabolic conditions. In this report, we demonstrate that a high‐fructose diet in rhesus monkeys produces insulin resistance and many features of the metabolic syndrome, including central obesity, dyslipidemia, and inflammation within a short period of time; moreover, a subset of monkeys developed type 2 diabetes. Given the rapidity with which the metabolic changes occur, and the ability to control for many factors that cannot be controlled for in humans, fructose feeding in rhesus monkeys represents a practical and efficient model system in which to investigate the pathogenesis, prevention, and treatment of diet‐induced insulin resistance and its related comorbidities. Clin Trans Sci 2011; Volume 4: 243–252

Ileal interposition surgery improves glucose and lipid metabolism and delays diabetes onset in the UCD-T2DM rat.

BACKGROUND & AIMS Bariatric surgery has been shown to reverse type 2 diabetes; however, mechanisms by which this occurs remain undefined. Ileal interposition (IT) is a surgical model that isolates the effects of increasing delivery of unabsorbed nutrients to the lower gastrointestinal tract. In this study we investigated effects of IT surgery on glucose tolerance and diabetes onset in UCD-T2DM (University of California at Davis type 2 diabetes mellitus) rats, a polygenic obese animal model of type 2 diabetes. METHODS IT or sham surgery was performed on 4-month-old male UCD-T2DM rats. All animals underwent oral glucose tolerance testing (OGTT). A subset was killed 2 months after surgery for tissue analyses. The remainder was followed until diabetes onset and underwent oral fat tolerance testing (OFTT). RESULTS IT surgery delayed diabetes onset by 120 +/- 49 days compared with sham surgery (P < .05) without a difference in body weight. During OGTT, IT-operated animals exhibited lower plasma glucose excursions (P < .05), improved early insulin secretion (P < .01), and 3-fold larger plasma glucagon-like peptide-1(7-36) (GLP-1(7-36)) excursions (P < .001), and no difference in glucose-dependent insulinotropic polypeptide responses compared with sham-operated animals. Total plasma peptide YY (PYY) excursions during OFTT were 3-fold larger in IT-operated animals (P < .01). IT-operated animals exhibited lower adiposity (P < .05), smaller adipocyte size (P < .05), 25% less ectopic lipid deposition, lower circulating lipids, and greater pancreatic insulin content compared with sham-operated animals (P < .05). CONCLUSIONS IT surgery delays the onset of diabetes in UCD-T2DM rats which may be related to increased nutrient-stimulated secretion of GLP-1(7-36) and PYY and improvements of insulin sensitivity, beta-cell function, and lipid metabolism.

Development and characterization of a novel rat model of type 2 diabetes mellitus: the UC Davis type 2 diabetes mellitus UCD-T2DM rat

The prevalence of type 2 diabetes (T2DM) is increasing, creating a need for T2DM animal models for the study of disease pathogenesis, prevention, and treatment. The purpose of this project was to develop a rat model of T2DM that more closely models the pathophysiology of T2DM in humans. The model was created by crossing obese Sprague-Dawley rats with insulin resistance resulting from polygenic adult-onset obesity with Zucker diabetic fatty-lean rats that have a defect in pancreatic beta-cell function but normal leptin signaling. We have characterized the model with respect to diabetes incidence; age of onset; longitudinal measurements of glucose, insulin, and lipids; and glucose tolerance. Longitudinal fasting glucose and insulin data demonstrated progressive hyperglycemia (with fasting and fed glucose concentrations >250 and >450 mg/dl, respectively) after onset along with hyperinsulinemia resulting from insulin resistance at onset followed by a progressive decline in circulating insulin concentrations, indicative of beta-cell decompensation. The incidence of diabetes in male and female rats was 92 and 43%, respectively, with an average age of onset of 6 mo in males and 9.5 mo in females. Results from intravenous glucose tolerance tests, pancreas immunohistochemistry, and islet insulin content further support a role for beta-cell dysfunction in the pathophysiology of T2DM in this model. Diabetic animals also exhibit glycosuria, polyuria, and hyperphagia. Thus diabetes in the UC Davis-T2DM rat is more similar to clinical T2DM in humans than in other existing rat models and provides a useful model for future studies of the pathophysiology, treatment, and prevention of T2DM.

TGR5 contributes to glucoregulatory improvements after vertical sleeve gastrectomy in mice

Objective Vertical sleeve gastrectomy (VSG) produces high rates of type 2 diabetes remission; however, the mechanisms responsible remain incompletely defined. VSG increases circulating bile acid concentrations and bile acid signalling through TGR5 improves glucose homeostasis. Therefore, we investigated the role of TGR5 signalling in mediating the glucoregulatory benefits of VSG. Design VSG or sham surgery was performed in high-fat-fed male Tgr5+/+ (wild type) and Tgr5−/− (knockout) littermates. Sham-operated mice were fed ad libitum or food restricted to match their body weight to VSG-operated mice. Body weight, food intake, energy expenditure, insulin signalling and circulating bile acid profiles were measured and oral glucose tolerance testing, islet immunohistochemistry and gut microbial profiling were performed. Results VSG decreased food intake and body weight, increased energy expenditure and circulating bile acid concentrations, improved fasting glycaemia, glucose tolerance and glucose-stimulated insulin secretion, enhanced nutrient-stimulated glucagon-like peptide 1 secretion and produced favourable shifts in gut microbial populations in both genotypes. However, the body weight-independent improvements in fasting glycaemia, glucose tolerance, hepatic insulin signalling, hepatic inflammation and islet morphology after VSG were attenuated in Tgr5−/− relative to Tgr5+/+ mice. Furthermore, VSG produced metabolically favourable alterations in circulating bile acid profiles that were blunted in Tgr5−/− relative to Tgr5+/+ mice. TGR5-dependent regulation of hepatic Cyp8b1 expression may have contributed to TGR5-mediated shifts in the circulating bile acid pool after VSG. Conclusions These results suggest that TGR5 contributes to the glucoregulatory benefits of VSG surgery by promoting metabolically favourable shifts in the circulating bile acid pool.

Vertical Sleeve Gastrectomy Improves Glucose and Lipid Metabolism and Delays Diabetes Onset in UCD- T2DM Rats

Vertical sleeve gastrectomy (VSG) has gained interest as a low morbidity bariatric surgery, which is effective in producing weight loss and causing type 2 diabetes resolution. However, the efficacy of VSG to prevent the onset of type 2 diabetes has not been previously investigated. VSG or sham surgery was performed on 2-month-old prediabetic male University of California Davis-type 2 diabetes mellitus rats. Sham-operated animals were either sham-operated ad libitum fed (S-AL) or were weight-matched to VSG-operated animals (S-WM). Diabetes onset was determined by weekly nonfasting blood glucose measurements. Animals underwent oral glucose tolerance tests at 1 and 4 months after surgery and indirect calorimetry at 1.5 months after surgery. VSG surgery significantly delayed diabetes onset compared with both S-AL and S-WM animals. VSG-operated animals ate 23% less and weighed 20% less than S-AL. Energy expenditure did not differ between VSG-operated animals and controls. Results from the oral glucose tolerance tests demonstrate improved glucose tolerance and islet function in VSG-operated animals compared with S-AL and S-WM. Nutrient-stimulated glucagon-like peptide (GLP)-1, GLP-2, and peptide YY excursions were greater in VSG-operated animals. VSG surgery resulted in decreased fasting plasma insulin, ghrelin and lipid concentrations, and markedly higher fasting plasma adiponectin and bile acid concentrations, independent of body weight. Increases of circulating bile acid concentrations were due to selective increases of taurine-conjugated bile acids. Thus, VSG delays type 2 diabetes onset in the University of California Davis-type 2 diabetes mellitus rat, independent of body weight. This is potentially mediated by increases of circulating bile acids, adiponectin, and nutrient-stimulated GLP-1 secretion and decreased circulating ghrelin concentrations.

Subcutaneous administration of leptin normalizes fasting plasma glucose in obese type 2 diabetic UCD-T2DM rats

Leptin has been shown to reduce hyperglycemia in rodent models of type 1 diabetes. We investigated the effects of leptin administration in University of California, Davis, type 2 diabetes mellitus (UCD-T2DM) rats, which develop adult-onset polygenic obesity and type 2 diabetes. Animals that had been diabetic for 2 mo were treated with s.c. injections of saline (control) or murine leptin (0.5 mg/kg) twice daily for 1 mo. Control rats were pair-fed to leptin-treated animals. Treatment with leptin normalized fasting plasma glucose and was accompanied by lowered HbA1c, plasma glucagon, and triglyceride concentrations and expression of hepatic gluconeogenic enzymes compared with vehicle (P < 0.05), independent of any effects on body weight and food intake. In addition, leptin-treated animals exhibited marked improvement of insulin sensitivity and glucose homeostasis compared with controls, whereas pancreatic insulin content was 50% higher in leptin-treated animals (P < 0.05). These effects coincided with activation of leptin and insulin signaling pathways and down-regulation of the PKR-like endoplasmic reticulum (ER) kinase/eukaryotic translation inhibition factor 2α (PERK-eIF2α) arm of ER stress in liver, skeletal muscle, and adipose tissue as well as increased pro-opiomelanocortin and decreased agouti-related peptide in the hypothalamus. In contrast, several markers of inflammation/immune function were elevated with leptin treatment in the same tissues (P < 0.05), suggesting that the leptin-mediated increase of insulin sensitivity was not attributable to decreased inflammation. Thus, leptin administration improves insulin sensitivity and normalizes fasting plasma glucose in diabetic UCD-T2DM rats, independent of energy intake, via peripheral and possibly centrally mediated actions, in part by decreasing circulating glucagon and ER stress.

Chronic Administration of the Glucagon-Like Peptide-1 Analog, Liraglutide, Delays the Onset of Diabetes and Lowers Triglycerides in UCD-T2DM Rats

OBJECTIVE The efficacy of liraglutide, a human glucagon-like peptide-1 (GLP-1) analog, to prevent or delay diabetes in UCD-T2DM rats, a model of polygenic obese type 2 diabetes, was investigated. RESEARCH DESIGN AND METHODS At 2 months of age, male rats were divided into three groups: control, food-restricted, and liraglutide. Animals received liraglutide (0.2 mg/kg s.c.) or vehicle injections twice daily. Restricted rats were food restricted to equalize body weights to liraglutide-treated rats. Half of the animals were followed until diabetes onset, whereas the other half of the animals were killed at 6.5 months of age for tissue collection. RESULTS Before diabetes onset energy intake, body weight, adiposity, and liver triglyceride content were higher in control animals compared with restricted and liraglutide-treated rats. Energy-restricted animals had lower food intake than liraglutide-treated animals to maintain the same body weights, suggesting that liraglutide increases energy expenditure. Liraglutide treatment delayed diabetes onset by 4.1 ± 0.8 months compared with control (P < 0.0001) and by 1.3 ± 0.8 months compared with restricted animals (P < 0.05). Up to 6 months of age, energy restriction and liraglutide treatment lowered fasting plasma glucose and A1C concentrations compared with control animals. In contrast, liraglutide-treated animals exhibited lower fasting plasma insulin, glucagon, and triglycerides compared with both control and restricted animals. Furthermore, energy-restricted and liraglutide-treated animals exhibited more normal islet morphology. CONCLUSIONS Liraglutide treatment delays the development of diabetes in UCD-T2DM rats by reducing energy intake and body weight, and by improving insulin sensitivity, improving lipid profiles, and maintaining islet morphology.

Glucose sensing by gut endocrine cells and activation of the vagal afferent pathway is impaired in a rodent model of type 2 diabetes mellitus.

Glucose in the gut lumen activates gut endocrine cells to release 5-HT, glucagon-like peptide 1/2 (GLP-1/2), and glucose-dependent insulinotropic polypeptide (GIP), which act to change gastrointestinal function and regulate postprandial plasma glucose. There is evidence that both release and action of incretin hormones is reduced in type 2 diabetes (T2D). We measured cellular activation of enteroendocrine and enterochromaffin cells, enteric neurons, and vagal afferent neurons in response to intestinal glucose in a model of type 2 diabetes mellitus, the UCD-T2DM rat. Prediabetic (PD), recent-diabetic (RD, 2 wk postonset), and 3-mo diabetic (3MD) fasted UCD-T2DM rats were given an orogastric gavage of vehicle (water, 0.5 ml /100 g body wt) or glucose (330 μmol/100 g body wt); after 6 min tissue was removed and cellular activation was determined by immunohistochemistry for phosphorylated calcium calmodulin-dependent kinase II (pCaMKII). In PD rats, pCaMKII immunoreactivity was increased in duodenal 5-HT (P < 0.001), K (P < 0.01) and L (P < 0.01) cells in response to glucose; glucose-induced activation of all three cell types was significantly reduced in RD and 3MD compared with PD rats. Immunoreactivity for GLP-1, but not GIP, was significantly reduced in RD and 3MD compared with PD rats (P < 0.01). Administration of glucose significantly increased pCaMKII in enteric and vagal afferent neurons in PD rats; glucose-induced pCaMKII immunoreactivity was attenuated in enteric and vagal afferent neurons (P < 0.01, P < 0.001, respectively) in RD and 3MD. These data suggest that glucose sensing in enteroendocrine and enterochromaffin cells and activation of neural pathways is markedly impaired in UCD-T2DM rats.

Fish Oil Supplementation Ameliorates Fructose-Induced Hypertriglyceridemia and Insulin Resistance in Adult Male Rhesus Macaques

Fish oil (FO) is a commonly used supplemental source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), 2 n-3 (ω-3) polyunsaturated fatty acids (PUFAs) that have been shown to have a variety of health benefits considered to be protective against cardiometabolic diseases. Although the effects of EPA and DHA on lipid metabolism have been extensively studied, not all of the metabolic effects of FO-derived n-3 PUFAs have been characterized. Our laboratory recently showed that a high-fructose diet in rhesus monkeys induces the features of metabolic syndrome (MetS) similar to those observed in humans. Thus, we specifically wanted to evaluate the effects of FO in rhesus monkeys fed a high-fructose diet and hypothesized that FO supplementation would mitigate the development of fructose-induced insulin resistance, dyslipidemia, and other cardiometabolic risk factors. In this study, adult monkeys (aged 12-20 y) received either a standard unpurified diet plus 75 g fructose/d (control group; n = 9) or a standard unpurified diet, 75 g fructose/d, and 4 g FO (16% EPA + 11% DHA)/d (treatment group; n = 10) for 6 mo. Importantly, our results showed that daily FO supplementation in the monkeys prevented fructose-induced hypertriglyceridemia and insulin resistance as assessed by intravenous-glucose-tolerance testing (P ≤ 0.05). Moreover, FO administration in the monkeys prevented fructose-induced increases in plasma apolipoprotein (Apo)C3, ApoE, and leptin concentrations and attenuated decreases in circulating adropin concentrations (P ≤ 0.05). No differences between the control and FO-treated monkeys were observed in body weight, lean mass, fat mass, or fasting glucose, insulin, and adiponectin concentrations. In conclusion, FO administration in a nonhuman primate model of diet-induced MetS ameliorates many of the adverse changes in lipid and glucose metabolism induced by chronic fructose consumption.