Incretin and islet hormonal responses to fat and protein ingestion in healthy men

Is there any interplay between incretins and bile acids? What is the role of bariatric surgery?

Obesity and type 2 diabetes are associated with the impairment of the incretin effect. Evidence has revealed that bile acids are involved in glucose homeostasis. Bariatric surgery, referred also as metabolic surgery, exerts beyond weight loss an important metabolic effect by inducing amelioration or remission of type 2 diabetes. Surgical procedures that involve rearrangements of the gastrointestinal tract and therefore rerouting of the food such as laparoscopic Roux en Y gastric by pass (LRYGB), induce an increase in glucagon-like peptide 1 (GLP-1) levels and of bile acids, which will both promote an early improvement of glycemic control. Emerging data have revealed that there might be an interplay between GLP-1 and bile acids regarding glycemic control, raising the question about considering bile acids as the new gut hormones.

A randomized cross-over trial of the postprandial effects of three different diets in patients with type 2 diabetes

BACKGROUND

In the clinic setting both fasting levels of glucose and the area under the curve (AUC) of glucose, by determination of HbA1c levels, are used for risk assessments, in type 2 diabetes (NIDDM). However little is known about postprandial levels, and hence AUC, regarding other traditional risk factors such as insulin and blood-lipids and how this is affected by different diets.

OBJECTIVE

To study postprandial effects of three diets, during a single day, in NIDDM.

METHODS

A low-fat diet (45-56 energy-% from carbohydrates), and a low-carbohydrate diet (16-24 energy-% from carbohydrates) was compared with a Mediterranean-style diet (black coffee for breakfast and the same total-caloric intake as the other two diets for lunch with red wine, 32-35 energy-% from carbohydrates) in a randomized cross-over design. Total-caloric intake/test-day at the clinic from food was 1025-1080 kCal in men and 905-984 kCal in women. The test meals were consumed at a diabetes ward under supervision.

RESULTS

Twenty-one participants were recruited and 19 completed the studies. The low-carbohydrate diet induced lower insulin and glucose excursions compared with the low-fat diet (p<0.0005 for both AUC). The insulin-response following the single Mediterranean-style lunch-meal was more pronounced than during the low-fat diet lunch (insulin increase-ratio of the low-fat diet: 4.35 ± 2.2, of Mediterranean-style diet: 8.12 ± 5.2, p = 0.001) while postprandial glucose levels were similar. The increase-ratio of insulin correlated with the elevation of the incretin glucose-dependent insulinotropic-polypeptide following the Mediterranean-style diet lunch (Spearman, r = 0.64, p = 0.003).

CONCLUSIONS

The large Mediterranean-style lunch-meal induced similar postprandial glucose-elevations as the low-fat meal despite almost double amount of calories due to a pronounced insulin-increase. This suggests that accumulation of caloric intake from breakfast and lunch to a single large Mediterranean style lunch-meal in NIDDM might be advantageous from a metabolic perspective.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01522157 NCT01522157.

Impaired glucose tolerance in rats fed low-carbohydrate, high-fat diets

Moderate low-carbohydrate/high-fat (LC-HF) diets are widely used to induce weight loss in overweight subjects, whereas extreme ketogenic LC-HF diets are used to treat neurological disorders like pediatric epilepsy. Usage of LC-HF diets for improvement of glucose metabolism is highly controversial; some studies suggest that LC-HF diets ameliorate glucose tolerance, whereas other investigations could not identify positive effects of these diets or reported impaired insulin sensitivity. Here, we investigate the effects of LC-HF diets on glucose and insulin metabolism in a well-characterized animal model. Male rats were fed isoenergetic or hypocaloric amounts of standard control diet, a high-protein "Atkins-style" LC-HF diet, or a low-protein, ketogenic, LC-HF diet. Both LC-HF diets induced lower fasting glucose and insulin levels associated with lower pancreatic β-cell volumes. However, dynamic challenge tests (oral and intraperitoneal glucose tolerance tests, insulin-tolerance tests, and hyperinsulinemic euglycemic clamps) revealed that LC-HF pair-fed rats exhibited impaired glucose tolerance and impaired hepatic and peripheral tissue insulin sensitivity, the latter potentially being mediated by elevated intramyocellular lipids. Adjusting visceral fat mass in LC-HF groups to that of controls by reducing the intake of LC-HF diets to 80% of the pair-fed groups did not prevent glucose intolerance. Taken together, these data show that lack of dietary carbohydrates leads to glucose intolerance and insulin resistance in rats despite causing a reduction in fasting glucose and insulin concentrations. Our results argue against a beneficial effect of LC-HF diets on glucose and insulin metabolism, at least under physiological conditions. Therefore, use of LC-HF diets for weight loss or other therapeutic purposes should be balanced against potentially harmful metabolic side effects.

Postprandial responses of incretin and pancreatic hormones in non-diabetic patients with end-stage renal disease

BACKGROUND

Patients with end-stage renal disease (ESRD) have glucometabolic disturbances resulting in a high prevalence of prediabetes. The underlying pathophysiology remains unclear, but may prove important for the strategies employed to prevent progression to overt diabetes. Meal-induced release of the insulinotropic gut-derived incretin hormones and pancreatic hormones play a critical role in the maintenance of a normal postprandial glucose tolerance.

METHODS

We studied patients with ESRD and either normal (n = 10) or impaired (n = 10) glucose tolerance, and control subjects (n = 11). Plasma concentrations of glucose, insulin, glucagon, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and paracetamol were measured repeatedly during a standardized 4-h liquid meal including 1.5 g paracetamol (added for evaluation of gastric emptying).

RESULTS

Fasting glucose and postprandial glucose responses were comparable between groups (P > 0.082). Patients with ESRD exhibited higher fasting levels of GIP and glucagon compared with controls (P < 0.001). Baseline-corrected GLP-1 and glucagon responses were enhanced (P < 0.002), baseline-corrected insulin responses and insulin excursions were reduced (P < 0.035), and paracetamol excursions were delayed (P < 0.024) in patients with ESRD compared with controls. None of the variables differed between the two ESRD subgroups.

CONCLUSIONS

Non-diabetic patients with ESRD were characterized by reduced postprandial insulin responses despite increased secretion of the insulinotropic incretin hormone GLP-1. Fasting levels and baseline-corrected responses of glucagon were elevated and gastric emptying was delayed in the ESRD patients. These perturbations seem to be caused by uraemia per se and may contribute to the disturbed glucose metabolism in ESRD patients.

Effects of intraduodenal lipid and protein on gut motility and hormone release, glycemia, appetite, and energy intake in lean men

BACKGROUND

Intraduodenal lipid modulates gastrointestinal motility and hormone release and suppresses energy intake (EI) more than does intraduodenal glucose. Oral protein is the most satiating macronutrient and modulates postprandial glycemia; the comparative effects of intraduodenal protein and lipid and their combined effects are unclear.

OBJECTIVE

We investigated the effects of intraduodenal protein and lipid, alone or in combination, on antropyloroduodenal motility, gastrointestinal hormone release, glycemia, and EI.

DESIGN

Twenty lean men were studied on 5 randomized, double-blind occasions. Antropyloroduodenal motility, cholecystokinin, glucagon-like peptide-1 (GLP-1), insulin, glucagon, blood glucose, appetite, and nausea were measured during 90-min isocaloric (3 kcal/min) intraduodenal infusions of lipid [pure lipid condition (L3)], protein [pure protein condition (P3)], a 2:1 combination of lipid and protein [2:1 lipid:protein condition (L2P1)], a 1:2 combination of lipid and protein [1:2 lipid:protein condition (L1P2)], or a control. Immediately after the infusion, EI from a buffet lunch was quantified.

RESULTS

In comparison with the control, all nutrient infusions suppressed antral and duodenal and stimulated pyloric pressures (P < 0.05). Cholecystokinin and GLP-1 release and pyloric stimulation were lipid-load dependent (r ≥ 0.39, P < 0.01), insulin and glucagon releases were protein-load dependent (r = 0.83, P < 0.001), and normoglycemia was maintained. L3 but not P3 increased nausea (P < 0.05). Compared with the control, L3 and P3 but not L2P1 or L1P2 suppressed EI (P < 0.05) without major effects on appetite.

CONCLUSIONS

In lean men, despite differing effects on gut function, intraduodenal lipid and protein produce comparable reductions in energy intake. The effects of lipid may be a result of nausea. Protein also regulates blood glucose by stimulating insulin and glucagon. In contrast, at the loads selected, lipid:protein combinations did not suppress energy intake, suggesting that a threshold load is required to elicit effects. This trial was registered at Australia and New Zealand Clinical Trial Registry (http://www.anzctr.org.au) as 12609000949280.

Ingestion of thermally oxidized sunflower oil decreases postprandial lipemia mainly in younger individuals

Animal studies have shown that diets rich in thermally oxidized fat increase glucose and decrease insulin and triglyceride (TG) concentrations in the blood. We hypothesized that ingestion of a potato meal rich in thermally oxidized sunflower oil (TOSO) would decrease postprandial concentrations of insulin, incretins, and TG and increase plasma glucose concentrations. Twenty healthy subjects aged 22 to 70 years consumed meals rich in TOSO or unheated sunflower oil and containing paracetamol (1.5 g) in a randomized, crossover trial. Blood samples were taken at baseline and 10, 20, 30, 60, 90, and 120 minutes after the meals and glucose, insulin, TG, nonesterified fatty acids, glucagon-like polypeptide-1, glucose-independent polypeptide, and paracetamol (as a marker of gastric emptying) were measured in plasma or serum. The incremental areas under the curve of glucose, insulin, nonesterified fatty acid, incretins, and paracetamol levels were not significantly different between the meals. Plasma TG incremental area under the curve was 44% lower after the TOSO meal at a marginal level of significance (P = .06) in the total study population and was significantly (P = .04) and 61% lower in those of median age and younger (n = 11). These data suggest that ingestion of TOSO may acutely decrease plasma TG mainly in younger individuals and does not acutely affect glucose and insulin metabolism or gastric emptying in healthy subjects.

High protein intake stimulates postprandial GLP1 and PYY release

OBJECTIVE

Meals high in protein induce greater intermeal satiety than meals high in fat and carbohydrates. We studied the gut hormone response and subsequent food intake after breakfasts high in protein, carbohydrate or high in fat controlled for volume, calories and appearance.

DESIGN AND METHODS

Eight healthy volunteers participated in this randomized three-way crossover study. Study breakfasts were calculated to provide 20% of daily energy requirements and provided either 60% of energy from protein, fat or carbohydrate. Blood was drawn half-hourly for 4 h; energy intake at a subsequent ad libitum meal was measured.

RESULTS

Total ghrelin decreased after food intake equally with the three breakfasts. PYY levels were highest after the high protein breakfast (P = 0.005). Indeed, PYY at 240 min was highest after the high protein breakfast compared to the high fat breakfast and to the high carbohydrate breakfast (P = 0.011 and P = 0.012, respectively). GLP-1 levels were highest after the high protein breakfast (P = 0.041) at 120 min and remained higher throughout the study. These differences in gut hormones did not translate into differences in food intake (1023 ± 390 kcal after high protein, 1016 ± 388 kcal after high fat and 1158 ± 433 kcal after high carbohydrate).

CONCLUSION

We conclude that a high protein meal increases circulating concentrations of the gut hormones PYY and GLP-1, but when meals are matched for volume, appearance and caloric value, these gut hormone changes do not translate into a reduction in ad libitum food intake.

Glucose-lowering effect of the DPP-4 inhibitor sitagliptin after glucose and non-glucose macronutrient ingestion in non-diabetic subjects

AIM

Recent studies suggest that the incretin concept is not restricted to glucose ingestion but relevant also after non-glucose macronutrient administration. We therefore hypothesized that raising incretin hormones reduces circulating glucose after both glucose and non-glucose macronutrient ingestion in healthy subjects.

METHODS

Twelve healthy subjects received the dipeptidyl peptidase-4 inhibitor sitagliptin (100 mg) or placebo before ingestion of glucose, fat (olive oil) or protein mix in equicaloric amounts (8 kcal/kg) plus paracetamol (1.5 g). The 120-min areas under curve (AUC) of intact glucagon-like peptide-1 (GLP-1), glucose, insulin, C-peptide, glucagon and paracetamol, and model-derived insulin secretion rate (ISR), insulin sensitivity, insulin clearance and glucose absorption were measured.

RESULTS

The increased plasma intact GLP-1 levels after each macronutrient was augmented by sitagliptin. This was associated with a robust lowering of glucose: glucose excursion after oral glucose was diminished, and glucose fell below baseline after oral fat and protein. In spite of lower glucose, AUCC -peptide and ISR did not differ significantly between sitagliptin and placebo after any macronutrient. AUCglucagon , insulin sensitivity and insulin clearance were also not different between sitagliptin and placebo. Glucose absorption after oral glucose was reduced by sitagliptin, whereas AUCparacetamol was not statistically different between sitagliptin and placebo.

CONCLUSIONS

Physiological elevation of intact GLP-1 levels after ingestion of glucose and non-glucose macronutrients is robustly glucose-lowering in healthy subjects. Hence, the incretin concept is not restricted to glucose ingestion in normal physiology. The glucose-lowering action of sitagliptin at these low glucose levels in healthy subjects may have complex mechanisms, involving both islet-dependent and islet-independent mechanisms.

Novel therapeutic targets for nonalcoholic fatty liver disease

INTRODUCTION

Nonalcoholic fatty liver disease (NAFLD) is a serious public health problem. It is now estimated to affect 30% of adults and about 10% of children in the U.S. Hispanics are disproportionably affected with not only higher rates of NAFLD but also more severe disease. Treatment options are currently limited.

AREAS COVERED

In this review, we will focus on a series of novel findings related to the pathobiology of liver damage in nonalcoholic steatohepatitis (NASH) that are attractive targets for development of novel therapeutic strategies for human NASH. In particular, we will discuss four different areas due to their novelty and growing importance including microparticles, the inflammasomes, gut-liver axis and dietary lipids.

EXPERT OPINION

There is an urgent need to develop novel safe and effective therapies for the growing NAFLD epidemic. The data discussed in this article provide strong rational to think out of the box when considering novel therapeutic targets for patients with NAFLD.

Postprandial gut hormone responses and glucose metabolism in cholecystectomized patients

Preclinical studies suggest that gallbladder emptying, via bile acid-induced activation of the G protein-coupled receptor TGR5 in intestinal L cells, may play a significant role in the secretion of the incretin hormone glucagon-like peptide-1 (GLP-1) and, hence, postprandial glucose homeostasis. We examined the secretion of gut hormones in cholecystectomized subjects to test the hypothesis that gallbladder emptying potentiates postprandial release of GLP-1. Ten cholecystectomized subjects and 10 healthy, age-, gender-, and body mass index-matched control subjects received a standardized fat-rich liquid meal (2,200 kJ). Basal and postprandial plasma concentrations of glucose, insulin, C-peptide, glucagon, GLP-1, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-2 (GLP-2), cholecystokinin (CCK), and gastrin were measured. Furthermore, gastric emptying and duodenal and serum bile acids were measured. We found similar basal glucose concentrations in the two groups, whereas cholecystectomized subjects had elevated postprandial glucose excursions. Cholecystectomized subjects had reduced postprandial concentrations of duodenal bile acids, but preserved postprandial plasma GLP-1 responses, compared with control subjects. Also, cholecystectomized patients exhibited augmented fasting glucagon. Basal plasma CCK concentrations were lower and peak concentrations were higher in cholecystectomized patients. The concentrations of GIP, GLP-2, and gastrin were similar in the two groups. In conclusion, cholecystectomized subjects had preserved postprandial GLP-1 responses in spite of decreased duodenal bile delivery, suggesting that gallbladder emptying is not a prerequisite for GLP-1 release. Cholecystectomized patients demonstrated a slight deterioration of postprandial glycemic control, probably because of metabolic changes unrelated to incretin secretion.

Evaluation of second phase insulin secretion with simple surrogates derived from the mixed meal tolerance test in patients with type 2 diabetes

The major contributors to the pathogenesis of type 2 diabetes are impaired insulin action and insulin secretion, including second phase insulin secretion (2nd ISEC). This study aimed to compare surrogates derived from the mixed meal tolerance test (MTT) with 2nd ISEC derived from modified low-dose graded glucose infusion (M-LDGGI) in patients with type 2 diabetes. We were subsequently able to decide which surrogate would be performed easily and accurately. Twenty type 2 diabetes patients were enrolled. They received both MTT and M-LDGGI. The standardized MTT meals were provided at 8:00 A.M. and 12:00 P.M. The M-LDGGI was a simplified version of the Polonsky method; only two 80-min stages of glucose infusion (2 and 6 mg/kg/min) were given. The slopes of the insulin to glucose curve during the test were regarded as the 2nd ISEC. First, we used the area under the insulin curve (AUC(IN)) during MTT to quantify the 2nd ISEC. The best correlated AUC(IN) was from 60-240 min. Second, the slopes between any two time points of the plasma insulin to glucose level (SLOPE(I/G)) were also assessed. The time period best correlated with 2nd ISEC was from 0-120 min (SLOPE₀₋₁₂₀). Finally, the insulin-to-glucose ratio (IGr) of each time point was used to estimate the 2nd ISEC, and the best correlation was observed at 180 min. In conclusion, estimating 2nd ISEC surrogates derived from MTT proved to be possible. The most accurate surrogate is the SLOPE₀₋₁₂₀, while IG(r180) is another less precise but more convenient method.

Review article: the emerging interplay among the gastrointestinal tract, bile acids and incretins in the pathogenesis of diabetes and non-alcoholic fatty liver disease

BACKGROUND

Recent research has led to an interest in the role of the gut and liver in type 2 diabetes mellitus (T2DM).

AIM

To review the role of the gastrointestinal system in glucose homoeostasis, with particular focus on the effects of incretin hormones, hepatic steatosis and bile acids.

METHODS

PubMed and Google Scholar were searched using terms such as incretin, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), dipeptidyl peptidase-4 (DPP-4), hepatic steatosis, bile acid and gastric bypass. Additional relevant references were identified by reviewing the reference lists of articles.

RESULTS

Perturbations of incretin hormones and bile acid secretion contribute to the pathogenesis of T2DM, leading to their potential as therapeutic targets. The incretin hormones (GIP and GLP-1) are deactivated by DPP-4. GLP-1 agonists and DPP-4 inhibitors improve glycaemic control in patients with T2DM. Hepatic steatosis, along with insulin resistance, may precede the development of T2DM, and may benefit from anti-diabetes medications. Bile acids play an important role in glucose homoeostasis, with effects mediated via the farnesoid X receptor (FXR) and the cell surface receptor TGR5. The bile acid sequestrant colesevelam has been shown to be effective in improving glycaemic control in patients with T2DM. Altered gastrointestinal anatomy after gastric bypass surgery may also affect enterohepatic recirculation of bile acids and contribute to improved glycaemic control.

CONCLUSIONS

Research in recent years has led to new pathways and processes with a role in glucose homoeostasis, and new therapeutic targets and options for type 2 diabetes mellitus.

GLP-1 agonists for type 2 diabetes: pharmacokinetic and toxicological considerations

INTRODUCTION

Within recent years, glucagon-like peptide 1 receptor agonists (GLP-1-RA) have emerged as a new treatment option for type 2 diabetes. The GLP-1-RA are administered subcutaneously and differ substantially in pharmacokinetic profiles.

AREAS COVERED

This review describes the pharmacokinetics and safety aspects of the currently available GLP-1 receptor agonists, liraglutide (based on the structure of native GLP-1), exenatide twice daily and exenatide once weekly (based on exendin-4) in relation to the kinetics and toxicology of native GLP-1. The review is based on electronic literature searches and legal documents in the form of assessment reports from the European Medicines Agency and the United States Food and Drug Administration.

EXPERT OPINION

GLP-1-based therapy combines several unique mechanisms of action and have the potential to gain widespread use in the fight against diabetes and obesity. The difference in chemical structure have strong implications for key pharmacokinetic parameters such as absorption and clearance, and eventually the safety and efficacy of the individual GLP-1-RA. The main safety concerns are pancreatitis and neoplasms, for which there are no identifiable differences in risk between the available agents. Antibody formation and injection site reactions are more frequent with the exendin-4-based compounds. The efficacy with regard to Hb(A1c) reduction is superior with the longer-acting agonists, whereas the shorter-acting GLP-1-RA seems to provide greater postprandial glucose control and lower tolerability as a possible consequence of less induction of tachyphylaxis. The future place of these agents will depend on the added safety and efficacy data in the several ongoing cardiovascular outcome trials.

Synergism by individual macronutrients explains the marked early GLP-1 and islet hormone responses to mixed meal challenge in mice

Apart from glucose, proteins and lipids also stimulate incretin and islet hormone secretion. However, the glucoregulatory effect of macronutrients in combination is poorly understood. We therefore developed an oral mixed meal model in mice to 1) explore the glucagon-like peptide-1 (GLP-1) and islet hormone responses to mixed meal versus isocaloric glucose, and 2) characterize the relative contribution of individual macronutrients to these responses. Anesthetized C57BL/6J female mice were orally gavaged with 1) a mixed meal (0.285 kcal; glucose, whey protein and peanut oil; 60/20/20% kcal) versus an isocaloric glucose load (0.285 kcal), and 2) a mixed meal (0.285 kcal) versus glucose, whey protein or peanut oil administered individually in their mixed meal caloric quantity, i.e., 0.171, 0.055 and 0.055 kcal, respectively. Plasma was analyzed for glucose, insulin and intact GLP-1 before and during oral challenges. Plasma glucose was lower after mixed meal versus after isocaloric glucose ingestion. In spite of this, the peak insulin response (P=0.02), the peak intact GLP-1 levels (P=0.006) and the estimated β-cell function (P=0.005) were higher. Furthermore, the peak insulin (P=0.004) and intact GLP-1 (P=0.006) levels were higher after mixed meal ingestion than the sum of responses to individual macronutrients. Compared to glucose alone, we conclude that there is a marked early insulin response to mixed meal ingestion, which emanates from a synergistic, rather than an additive, effect of the individual macronutrients in the mixed meal and is in part likely caused by increased levels of GLP-1.

Current evidence for a role of GLP-1 in Roux-en-Y gastric bypass-induced remission of type 2 diabetes

Weight-reducing surgical procedures such as Roux-en-Y gastric bypass (RYGB) have proven efficient as means of decreasing excess body weight. Furthermore, some studies report that up to 80% of patients with type 2 diabetes mellitus (T2DM) undergoing RYGB experience complete remission of their T2DM. Interestingly, the majority of remissions occur almost immediately following the operation and long before significant weight loss has taken place. Following RYGB, dramatic increases in postprandial plasma concentrations of the incretin hormone glucagon-like peptide-1 (GLP-1) have been recorded, and the known antidiabetic effects of GLP-1 are thought to be key mediators in RYGB-induced remission of T2DM. However, the published studies on the impact of RYGB on GLP-1 secretion are few, small and often not controlled properly. Furthermore, mechanistic studies delineating the role of endogenous GLP-1 secretion in RYGB-induced remission of T2DM are lacking. This article critically evaluates the current evidence for a role of GLP-1 in RYGB-induced remission of T2DM.

Diet: friend or foe of enteroendocrine cells--how it interacts with enteroendocrine cells

Gut hormones play a key role in the regulation of food intake, energy expenditure, glucose homeostasis, lipid metabolism, and a wide range of metabolic functions in response to food ingestion. These hormones are altered in metabolic diseases, such as obesity and type 2 diabetes, and are thus proposed to be possible targets for the prevention or treatment of these diseases. It is clear that food composition, macronutrients, and other non-nutrient components as well as the physical properties of food not only modulate the secretion of gut peptides but also modulate transcription and enteroendocrine cell differentiation, which ultimately modifies gut hormone response. The specific mechanisms or sensing machinery that respond to the different components of the diet have been studied for many years; however, over the last few years, new molecular genetic techniques have led to important advances, thereby allowing a deeper understanding of these mechanisms. This review addresses the current knowledge regarding enteroendocrine cells and how diet interacts with this machinery to stimulate and regulate the secretion of gut peptides. The potential for diet interventions as a promising strategy for modulating gut hormone responses to food ingestion and, ultimately, preventing or treating metabolic diseases is being emphasized considering that these diseases are currently a public health burden.

Glucose-dependent insulinotropic polypeptide: a bifunctional glucose-dependent regulator of glucagon and insulin secretion in humans

OBJECTIVE

To evaluate the glucose dependency of glucose-dependent insulinotropic polypeptide (GIP) effects on insulin and glucagon release in 10 healthy male subjects ([means ± SEM] aged 23 ± 1 years, BMI 23 ± 1 kg/m(2), and HbA(1c) 5.5 ± 0.1%).

RESEARCH DESIGN AND METHODS

Saline or physiological doses of GIP were administered intravenously (randomized and double blinded) during 90 min of insulin-induced hypoglycemia, euglycemia, or hyperglycemia.

RESULTS

During hypoglycemia, GIP infusion caused greater glucagon responses during the first 30 min compared with saline (76 ± 17 vs. 28 ± 16 pmol/L per 30 min, P < 0.008), with similar peak levels of glucagon reached after 60 min. During euglycemia, GIP infusion elicited larger glucagon responses (62 ± 18 vs. -11 ± 8 pmol/L per 90 min, P < 0.005). During hyperglycemia, comparable suppression of plasma glucagon (-461 ± 81 vs. -371 ± 50 pmol/L per 90 min, P = 0.26) was observed with GIP and saline infusions. In addition, during hyperglycemia, GIP more than doubled the insulin secretion rate (P < 0.0001).

CONCLUSIONS

In healthy subjects, GIP has no effect on glucagon responses during hyperglycemia while strongly potentiating insulin secretion. In contrast, GIP increases glucagon levels during fasting and hypoglycemic conditions, where it has little or no effect on insulin secretion. Thus, GIP seems to be a physiological bifunctional blood glucose stabilizer with diverging glucose-dependent effects on the two main pancreatic glucoregulatory hormones.

Physiology of incretins in health and disease

The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), are gut peptides which are secreted by endocrine cells in the intestinal mucosa. Their plasma concentrations increase quickly following food ingestion, and carbohydrate, fat, and protein have all been shown to stimulate GLP-1 and GIP secretion. Although neural and hormonal mechanisms have also been proposed to regulate incretin hormone secretion, direct stimulation of the enteroendocrine cells by the presence of nutrients in the intestinal lumen is probably the most important factor in humans. The actions of the incretin hormones are crucial for maintaining normal islet function and glucose homeostasis. Furthermore, it is also now being recognized that incretin hormones may have other actions in addition to their glucoregulatory effects. Studies have shown that GLP-1 and GIP levels and actions may be perturbed in disease states, but interpretation of the precise relationship between disease and incretins is difficult. The balance of evidence seems to suggest that alterations in secretion and/or action of incretin hormones arise secondarily to the development of insulin resistance, glucose intolerance, and/or increases in body weight rather than being causative factors. However, these impairments may contribute to the deterioration of glycemic control in diabetic patients.

Contributions of fat and protein to the incretin effect of a mixed meal

BACKGROUND

The relative contributions of fat and protein to the incretin effect are still largely unknown.

OBJECTIVE

This study assessed the incretin effects elicited by a mixed meal, and by its fat and protein components alone, with the use of a hyperglycemic clamp combined with oral nutrients.

DESIGN

Eight healthy volunteers were studied over 6 h after ingestion of a sandwich containing 1) dried meat, butter, and white bread; 2) dried meat alone; 3) butter alone; or 4) no meal (fasting control). Meals were ingested during a hyperglycemic clamp, and the incretin effect was calculated as the increment in plasma insulin after food intake relative to the concentrations observed during the control study.

RESULTS

A significant augmentation of postprandial insulin secretion, independent of plasma glycemia, occurred after ingestion of the mixed nutrients and the lipid component of the mixed meal (203 ± 20.7% and 167.4 ± 22.9% of control, respectively; both P < 0.05), whereas the protein component did not induce a significant incretin effect (129.0 ± 7.9% of control; P = 0.6)

CONCLUSIONS

Fat ingestion, in an amount typical of a standard meal, increases insulin secretion during physiologic hyperglycemia and thus contributes to the incretin effect. In contrast, ingestion of protein typical of normal meals does not contribute to the augmentation of postprandial insulin secretion. This trial was registered at clinicaltrials.gov as NCT00869453.

Dissociated incretin hormone response to protein versus fat ingestion in obese subjects

Protein elicits a stronger early (30 min) glucose-dependent insulinotropic polypeptide (GIP) response than fat ingestion in lean individuals, with no difference in glucagon-like peptide-1 (GLP-1). We assessed the incretin hormone response to protein versus fat ingestion in obesity. Equicaloric (8 kcal/kg) fat (olive oil) or protein (whey protein) was ingested by non-diabetic obese male volunteers [body mass index (BMI) >30 kg/m(2) ; n = 12] and plasma GIP and GLP-1 were determined. We found no difference in the early GIP or GLP-1 responses to fat versus protein. However, the total 300-min GIP response was greater after fat than after protein ingestion (20.3 ± 3.9 vs. 10.0 ± 2.8 nmol/l × min; p = 0.026), whereas the 300-min GLP-1 responses were the same. Thus, in obesity, protein and fat ingestion elicit similar early (30 min) incretin hormone responses, whereas 300-min GIP secretion is more pronounced after fat than protein ingestion.

Clinical Efficacy of Colesevelam in Type 2 Diabetes Mellitus

Purpose: The clinical experience and role in therapy of colesevelam in type 2 diabetes mellitus (T2DM) is discussed. Summary: Colesevelam HCl is a bile acid sequestrant (BAS) with proven efficacy in reducing elevated low-density lipoprotein cholesterol (LDL-C) in patients with primary hyperlipidemia. Colesevelam HCl gained food and drug administration (FDA) approval in 2008 as an adjunct to diet and exercise to improve glycemic control in adults with T2DM. In randomized controlled studies, colesevelam (add-on therapy with metformin, sulfonylureas, and insulin) has shown significant percentage reductions in glycosylated hemoglobin A1c (HbA1c) ranging from 0.5% to 0.54%. Reductions in LDL-C and non-high-density lipoprotein cholesterol (non-HDL-C) ranging from –12.8% to –16.7% and –4.0% to –10.3%, respectively, were also observed. Although no direct comparisons have been made, the safety and tolerability profile of this agent appears to be better than other BAS, with the most common side effects being gastrointestinal related. Conclusion: Colesevelam is effective as an adjunct to diet and exercise to improve glycemic control in adults with T2DM. Due to its effects upon LDL-C and glycemic parameters and favorable safety profile, colesevelam can play a role in an array of T2DM patients.

Two incretin hormones GLP-1 and GIP: comparison of their actions in insulin secretion and β cell preservation

Gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are the two primary incretin hormones secreted from the intestine upon ingestion of glucose or nutrients to stimulate insulin secretion from pancreatic β cells. GIP and GLP-1 exert their effects by binding to their specific receptors, the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R), which belong to the G-protein coupled receptor family. Receptor binding activates and increases the level of intracellular cAMP in pancreatic β cells, thereby stimulating insulin secretion glucose-dependently. In addition to their insulinotropic effects, GIP and GLP-1 have been shown to preserve pancreatic β cell mass by inhibiting apoptosis of β cells and enhancing their proliferation. Due to such characteristics, incretin hormones have been gaining mush attention as attractive targets for treatment of type 2 diabetes, and indeed incretin-based therapeutics have been rapidly disseminated worldwide. However, despites of plethora of rigorous studies, molecular mechanisms underlying how GIPR and GLP-1R activation leads to enhancement of glucose-dependent insulin secretion are still largely unknown. Here, we summarize the similarities and differences of these two incretin hormones in secretion and metabolism, their insulinotropic actions and their effects on pancreatic β cell preservation. We then try to discuss potential of GLP-1 and GIP in treatment of type 2 diabetes.

Evolutionary expression of glucose-dependent-insulinotropic polypeptide (GIP)

Glucose-dependent insulinotropic polypeptide (GIP) is a mammalian incretin hormone released into the circulation following nutrient ingestion. We examined the functional evolution of GIP and its relationship with insulin to delineate their respective roles in promoting nutrient efficiency. Expression patterns were examined in the sea lamprey (Petromyzon marinus), a basal vertebrate lacking a distinct pancreas, and in the zebrafish, Xenopus laevis, chicken, and mouse, organisms possessing extraintestinal pancreata. Although sea lamprey genomic analysis predicted a potential GIP-like gene, transcripts were not detected, and insulin expression was confined to the caudal pancreatic bud. GIP was detected in both the intestine and pancreas of the zebrafish and X. laevis. In contrast, GIP and insulin expression were limited to the intestine and pancreas, respectively, in chicken and mouse. Phylogenetic analysis of the glucagon-like ligands suggested proglucagon as the common ancestor, supporting the theory that GIP arose as a gene duplication of proglucagon. Insulin-secreting cells in the sea lamprey intestine may have obviated the need for an enteroinsular axis, and zebrafish may represent an evolutionary transition where GIP does not yet function as an incretin hormone. These observations are consistent with the hypothesis that GIP and insulin influence survival advantage by enhancing the efficiency of nutrient absorption and energy storage.

Incretin hormone secretion over the day

The two incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are key factors in the regulation of islet function and glucose metabolism, and incretin-based therapy for type 2 diabetes has gained considerable interest during recent years. Regulation of incretin hormone secretion is less well characterized. The main stimulus for incretin hormone secretion is presence of nutrients in the intestinal lumen, and carbohydrate, fat as well as protein all have the capacity to stimulate GIP and GLP-1 secretion. More recently, it has been established that a diurnal regulation exists with incretin hormone secretion to an identical meal being greater when the meal is served in the morning compared to in the afternoon. Finally, whether incretin hormone secretion is altered in disease states is an area with, so far, controversial results in different studies, although some studies have demonstrated reduced incretin hormone secretion in type 2 diabetes. This review summarizes our knowledge on regulation of incretin hormone secretion and its potential changes in disease states.

Dietary effects on incretin hormone secretion

The delivery of nutrients from the stomach into the duodenum and their subsequent interaction with the small intestine to stimulate incretin hormone release are central determinants of the glycemic response. The incretin effect has hitherto been attributed to the secretion of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) from enteroendocrine cells in the intestinal epithelium. A number of recent studies have yielded fundamental insights into the influence of individual nutrients on incretin release and the mechanisms involved in the detection of carbohydrates, fats, and proteins by enteroendocrine cells, including the K(ATP) channel, sodium-glucose cotransporter 1 (SGLT1), sweet taste receptors, G-protein-coupled receptors (GPRs), and oligopeptide transporter 1 (PepT1). Dietary modification, including modifying macronutrient composition or the consumption of "preloads" in advance of a meal, represents a novel approach to manipulate the incretin response and thereby regulate glucose homeostasis in patients with type 2 diabetes. This review focuses on the effects of individual nutrients on incretin hormone secretion, our current understanding of the signaling mechanisms that trigger secretion by enteroendocrine cells, and the therapeutic implications of these observations.

An oral lipid challenge and acute intake of caffeinated coffee additively decrease glucose tolerance in healthy men

Lipid-induced insulin resistance has been investigated primarily with i.v. infusions, and caffeine-induced insulin resistance, with alkaloid caffeine. The effects of orally consumed lipids and coffee have not been established and to our knowledge have never been simultaneously investigated. The goals of this study were to determine whether an oral lipid challenge and caffeinated coffee would disrupt glucose homeostasis and to characterize their respective incretin responses. It was hypothesized that oral ingestion of saturated lipids would impair glucose tolerance and that caffeinated coffee would further hinder glucose management. Ten young, healthy males participated in 5 trials in a randomized, cross-over design. At time 0 h, they underwent an oral fat tolerance test (OFTT: 1 g lipid/kg body weight) or consumed water, followed 5 h later by caffeinated (5 mg/kg) coffee, decaffeinated coffee, or water. At 6 h, volunteers underwent an oral glucose tolerance test (OGTT). Consumption of the OFTT increased glucose concentrations (P < 0.05) after a subsequent OGTT. At 7 h, caffeinated coffee produced the highest glucose concentrations (P < 0.05). Glucagon-like peptide-1 active (GLP-1a) and glucose-dependent insulinotropic polypeptide (GIP) were both increased for up to 6 h in all OFTT trials (P < 0.05). Compared to all other treatments, caffeinated and decaffeinated coffee produced higher GLP-1a response at 6.25 h (P < 0.05), whereas only caffeinated coffee increased GIP secretion (P < 0.05). These results show that oral consumption of lipids and caffeinated coffee can independently and additively decrease glucose tolerance. Incretin hormones could explain at least in part this impaired glucose homeostasis.

Milk signalling in the pathogenesis of type 2 diabetes

The presented hypothesis identifies milk consumption as an environmental risk factor of Western diet promoting type 2 diabetes (T2D). Milk, commonly regarded as a valuable nutrient, exerts important endocrine functions as an insulinotropic, anabolic and mitogenic signalling system supporting neonatal growth and development. The presented hypothesis substantiates milk's physiological role as a signalling system for pancreatic β-cell proliferation by milk's ability to increase prolactin-, growth hormone and incretin-signalling. The proposed mechanism of milk-induced postnatal β-cell mass expansion mimics the adaptive prolactin-dependent proliferative changes observed in pregnancy. Milk signalling down-regulates the key transcription factor FoxO1 leading to up-regulation of insulin promoter factor-1 which stimulates β-cell proliferation, insulin secretion as well as coexpression of islet amyloid polypeptide (IAPP). The recent finding that adult rodent β-cells only proliferate by self-duplication is of crucial importance, because permanent milk consumption beyond the weaning period may continuously over-stimulate β-cell replication thereby accelerating the onset of replicative β-cell senescence. The long-term use of milk may thus increase endoplasmic reticulum (ER) stress and toxic IAPP oligomer formation by overloading the ER with cytotoxic IAPPs thereby promoting β-cell apoptosis. Both increased β-cell proliferation and β-cell apoptosis are hallmarks of T2D. This hypothesis gets support from clinical states of hyperprolactinaemia and progeria syndromes with early onset of cell senescence which are both associated with an increased incidence of T2D and share common features of milk signalling. Furthermore, the presented milk hypothesis of T2D is compatible with the concept of high ER stress in T2D and the toxic oligomer hypothesis of T2D and may explain the high association of T2D and Alzheimer disease.

The effect of colesevelam hydrochloride on insulin sensitivity and secretion in patients with type 2 diabetes: a pilot study

BACKGROUND

This study evaluated the effect of colesevelam hydrochloride on insulin sensitivity, potential binding to glucose, and chronic effect(s) on fasting and postprandial glucose and insulin in patients with type 2 diabetes mellitus.

METHODS

Patients meeting inclusion criteria were withdrawn from all antidiabetes agents for 2 weeks and randomized to colesevelam 3.75 grams/day (n = 17) or placebo (n = 18) for 8 weeks. Hyperinsulinemic-euglycemic clamp studies were performed at baseline (week -1) and weeks 2 and 8. A meal tolerance test was conducted at weeks -1, 0, 2, and 8. The meal tolerance test and study drug were coadministered at week 0 to assess the direct effect of colesevelam on glucose absorption.

RESULTS

Insulin sensitivity as measured by the insulin clamp method did not change, but the Matsuda Index, a measure of whole-body insulin sensitivity calculated from postmeal tolerance test glucose and insulin levels, increased significantly within the colesevelam group from baseline to week 8 with last observation carried forward (P = 0.020). The postprandial area under the curve for glucose decreased with colesevelam versus placebo at weeks 2 and 8 with last observation carried forward (P = 0.012 and P = 0.061, respectively); the area under the curve for insulin did not decrease in concert with the decrease in area under the curve for glucose at week 2 (P = 0.585). Colesevelam had no effect on postmeal tolerance test glucose levels at week 0.

CONCLUSIONS

These results suggest that colesevelam has no effect on peripheral insulin sensitivity or glucose absorption, but may improve glucose control by improving whole-body insulin sensitivity, although not by an acute effect on glucose absorption. CLINICAL TRIAL IDENTIFIER: NCT00361153.

Differential responses of the incretin hormones GIP and GLP-1 to increasing doses of dietary carbohydrate but not dietary protein in lean rats

Previous studies have shown that oral ingestion of nutrients stimulates secretion of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1); however, it is unclear whether there is a dose-dependent response between the amount of nutrient ingested and the secretion of the hormones in vivo. Using our lymph fistula rat model, we previously demonstrated that both GIP and GLP-1 responded dose dependently to increasing amounts of infused dietary lipid and that the GLP-1-secreting cells were more sensitive to changes in intestinal lipid content. In the present study, we investigated the dose-dependent relationships between incretin secretion and the two remaining macronutrients, carbohydrate and protein. To accomplish this objective, the major mesenteric lymphatic duct of male Sprague-Dawley rats was cannulated. Each animal received a single bolus (3 ml) of saline, dextrin, whey protein, or casein hydrolysate (0.275, 0.55, 1.1, 2.2, 4.4 kcal) via a surgically inserted duodenal or ileal feeding tube. Lymph was continuously collected for 3 h and analyzed for GIP and GLP-1 content. Both GIP and GLP-1 outputs responded dose dependently to increasing amounts of dietary carbohydrate but not protein. Additionally, we found that the GIP-secreting cells were more sensitive than the GLP-1-secreting cells to changes in intestinal carbohydrate content.

Bile-induced secretion of glucagon-like peptide-1: pathophysiological implications in type 2 diabetes?

During the last decades it has become clear that bile acids not only act as simple fat solubilizers, but additionally represent complex hormonal metabolic integrators. Bile acids activate both nuclear receptors (controlling transcription of genes involved in for example bile acid, cholesterol, and glucose metabolism) and the cell surface G protein-coupled receptor TGR5 (modulating energy expenditure in brown fat and muscle cells). It has been shown that TGR5 is expressed in enteroendocrine L cells, which secrete the potent glucose-lowering incretin hormone glucagon-like peptide-1 (GLP-1). Recently it was shown that bile acid-induced activation of TGR5 results in intestinal secretion of GLP-1 and that enhanced TGR5 signaling improves postprandial glucose tolerance in diet-induced obese mice. This Perspectives article presents these novel findings in the context of prior studies on nutrient-induced GLP-1 secretion and outlines the potential implications of bile acid-induced GLP-1 secretion in physiological, pathophysiological, and pharmacological perspectives.

The degree of saturation of fatty acids in dietary fats does not affect the metabolic response to ingested carbohydrate

BACKGROUND

We are interested in the metabolic response to ingested macronutrients, and the interaction between macronutrients in meals. Previously, we and others reported that the postprandial rise in serum glucose following ingestion of 50 g carbohydrate, consumed as potato, was markedly attenuated when butter was ingested with the carbohydrate, whereas the serum insulin response was little affected by the combination.

OBJECTIVE

To determine whether a similar response would be observed with three other dietary fats considerably different in fatty acid composition.

DESIGN

Nine healthy subjects received lard, twelve received olive oil and eleven received safflower oil as a test meal. The subjects ingested meals of 25 g fat (lard, olive oil or safflower oil), 50 g CHO (potato), 25 g fat with 50 g CHO or water only. Glucose, C peptide, insulin, triacylglycerols and nonesterified fatty acids were determined.

RESULTS

Ingestion of lard, olive oil or safflower oil with potato did not affect the quantitative glucose and insulin responses to potato alone. However, the responses were delayed, diminished and prolonged. All three fats when ingested alone modestly increased the insulin concentration when compared to ingestion of water alone. When either lard, olive oil or safflower oil was ingested with the potato, there was an accelerated rise in triacylglycerols. This was most dramatic with safflower oil.

CONCLUSIONS

Our data indicate that the glucose and insulin response to butter is unique when compared with the three other fat sources varying in their fatty acid composition.

Use of DPP-4 inhibitors in type 2 diabetes: focus on sitagliptin

Inhibition of dipeptidyl peptidase-4 (DPP-4) prevents the inactivation of glucagonlike peptide-1 (GLP-1). This increases circulating levels of active GLP-1, stimulates insulin secretion and inhibits glucagon secretion, which results in lowering of glucose levels and improvement of the glycemic control in patients with type 2 diabetes. This review summarizes experiences with DPP-4 inhibition in the treatment of type 2 diabetes, with a focus on sitagliptin. Sitagliptin has in several clinical studies been shown to improve metabolic control in type 2 diabetes, both when used as monotherapy and when used in combination with metformin, sulfonylurea, thiazolidinediones or insulin. The reduction in HbA(1c) is ≈ 0.6% to 1.0% from baseline levels of 7.5% to 8.7% over 6 to 12 months therapy. Sitagliptin has a favorable safety profile, is highly tolerable, and there is a minimal risk of hypoglycemia. Furthermore, sitagliptin is body weight neutral or induces a slight body weight reduction. Sitagliptin may be used in the early stages of type 2 diabetes in combination with metformin or other treatments in subjects with inadequate glycemic control on these treatments alone. Sitagliptin may also be used in monotherapy and, finally, sitagliptin may be used in combination with insulin in more advanced stages of the disease.

Secretion and dipeptidyl peptidase-4-mediated metabolism of incretin hormones after a mixed meal or glucose ingestion in obese compared to lean, nondiabetic men

CONTEXT

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are cleaved by dipeptidyl peptidase-4 (DPP-4); plasma activity of DPP-4 may be increased in obesity. The impact of this increase on incretin hormone secretion and metabolism is not known.

OBJECTIVE

The aim of the study was to assess incretin hormone secretion and degradation in lean and obese nondiabetic subjects.

DESIGN, SETTINGS, AND PARTICIPANTS

We studied the ingestion of a mixed meal (560 kcal) or oral glucose (2 g/kg) in healthy lean (n = 12; body mass index, 20-25 kg/m(2)) or obese (n = 13; body mass index, 30-35 kg/m(2)) males at a University Clinical Research Unit.

MAIN OUTCOME MEASURES

We measured the area under the curve of plasma intact (i) and total (t) GIP and GLP-1 after meal ingestion and oral glucose.

RESULTS

Plasma DPP-4 activity was higher in the obese subjects (38.5 +/- 3.0 vs. 26.7 +/- 1.6 mmol/min . microl; P = 0.002). Although GIP secretion (AUC(tGIP)) was not reduced in obese subjects after meal ingestion or oral glucose, AUC(iGIP) was lower in obese subjects (8.5 +/- 0.6 vs. 12.7 +/- 0.9 nmol/liter x 300 min; P < 0.001) after meal ingestion. GLP-1 secretion (AUC(tGLP-1)) was reduced in obese subjects after both meal ingestion (7.3 +/- 0.9 vs. 10.0 +/- 0.6 nmol/liter x 300 min; P = 0.022) and oral glucose (6.6 +/- 0.8 vs. 9.6 +/- 1.1 nmol/liter x 180 min; P = 0.035). iGLP-1 was reduced in parallel to tGLP-1.

CONCLUSIONS

1) Release and degradation of the two incretin hormones show dissociated changes in obesity: GLP-1 but not GIP secretion is lower after meal ingestion and oral glucose, whereas GIP but not GLP-1 metabolism is increased after meal ingestion. 2) Increased plasma DPP-4 activity in obesity is not associated with a generalized augmented incretin hormone metabolism.

The hypoglycemic effect of fat and protein is not attenuated by insulin resistance

BACKGROUND

The glucose-lowering effect of fat and protein is attenuated or absent in diabetic patients, which suggests that the same may occur in insulin-resistant subjects without diabetes.

OBJECTIVE

The objective was to determine whether the postprandial metabolic responses elicited by fat and protein were influenced by the insulin sensitivity of the subjects and whether fat and protein modulate glucose responses through different mechanisms.

DESIGN

Healthy nondiabetic subjects aged 18-45 y took 50 g oral glucose with 0-30-g doses of canola oil and whey protein on 11 separate mornings after fasting overnight. The subjects were classified into 3 fasting serum insulin (FSI) groups: FSI < 40 pmol/L (n = 9), 40 < or = FSI < 70 pmol/L (n = 8), and FSI > or = 70 pmol/L (n = 8). The relative glycemic response was expressed as the incremental area under the curve (AUC) after each test meal divided by the mean AUC of the glucose control in each subject.

RESULTS

Protein significantly decreased glucose (P < 0.0001) and hepatic insulin extraction (P <0.0001) and increased insulin (P < 0.0001) and glucagon-like peptide 1 (P = 0.004); however, protein had no significant effect on C-peptide (P = 0.69) or on the insulin secretion rate (P = 0.13). No significant FSI x fat (P = 0.19) or FSI x protein (P = 0.08) interaction effects on glucose AUC were observed. In addition, the changes in relative glycemic response per gram of fat (r = -0.05, P = 0.82) or protein (r = -0.08, P = 0.70) were not related to FSI.

CONCLUSIONS

The hypoglycemic effect of fat and protein was not blunted by insulin resistance. Protein increased insulin but had no effect on C-peptide or the insulin secretion rate, which suggests decreased hepatic insulin extraction or increased C-peptide clearance.

Expression of glucose-dependent insulinotropic polypeptide in the zebrafish

In mammals, glucose-dependent insulinotropic polypeptide (GIP) is synthesized predominately in the small intestine and functions in conjunction with insulin to promote nutrient deposition. However, little is known regarding GIP expression and function in early vertebrates like the zebrafish, a model organism representing an early stage in the evolutionary development of the compound vertebrate pancreas. Analysis of GIP and insulin (insa) expression in zebrafish larvae by RT-PCR demonstrated that although insa was detected as early as 24 h postfertilization (hpf), GIP expression was not demonstrated until 72 hpf, shortly after the completion of endocrine pancreatic development but prior to the commencement of independent feeding. Furthermore, whole mount in situ hybridization of zebrafish larvae showed expression of GIP and insa in the same tissues, and in adult zebrafish, RT-PCR and immunohistochemistry demonstrated GIP expression in both the intestine and the pancreas. Receptor activation studies showed that zebrafish GIP was capable of activating the rat GIP receptor. Although previous studies have identified four receptors with glucagon receptor-like sequences in the zebrafish, one of which possesses the capacity to bind GIP, a functional analysis of these receptors has not been performed. This study demonstrates interactions between the latter receptor and zebrafish GIP, identifying it as a potential in vivo target for the ligand. Finally, food deprivation studies in larvae demonstrated an increase in GIP and proglucagon II mRNA levels in response to fasting. In conclusion, the results of these studies suggest that although the zebrafish appears to be a model of an early stage of evolutionary development of GIP expression, the peptide may not possess incretin properties in this species.

alpha-Lactalbumin hydrolysate stimulates glucagon-like peptide-2 secretion and small intestinal growth in suckling rats

We investigated whether bovine milk constituents influenced glucagon-like peptide (GLP)-2 secretion and intestinal growth in suckling rats. Male Sprague-Dawley rats (14 d old) received i.g. infusions of a milk protein fraction, a lactose solution, or the cream fraction of milk. The serum concentration of GLP-2, but not GLP-1, markedly increased in rats administered milk protein compared with those given the lactose solution or the cream fraction from 60 to 120 min after administration. In another experiment, both casein (CN) and whey protein isolate stimulated GLP-2 secretion at 120 min after administration, but soy protein and ovalbumin did not. Stimulation of GLP-2 secretion by several milk proteins was similar, including alpha-CN, alpha-lactalbumin (alpha-La), and beta-lactoglobulin, in a separate experiment. A hydrolysate of alpha-La obtained by incubation with protease A extracted from Aspergillus oryzae (LaHPA) caused almost twice the GLP-2 release due to intact alpha-La and other alpha-La hydrolysates. Free amino acid concentrations and molecular size distributions did not differ among alpha-La hydrolysates, including LaHPA. In rat pups reared with milk formulae containing alpha-La or LaHPA, LaHPA significantly promoted small intestinal elongation and increased the number of crypt epithelial cells compared with a formula containing intact alpha-La. LaHPA administration also increased the maltase:lactase activity ratio, a marker of maturation of the intestinal mucosa. In conclusion, milk proteins stimulate GLP-2 secretion and contribute to growth and maturation of the small intestine in suckling rats.