Effect of macronutrients and mixed meals on incretin hormone secretion and islet cell function

Transcription

1 Effect of macronutrients and mixed meals on incretin hormone secretion and islet cell function Background. Following meal ingestion, several hormones are released from the gastrointestinal tract. Some of these hormones are of importance for the regulation of islet cell function and glucose homeostasis. Incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are among the most important of these hormones which are involved in stimulation of insulin secretion from beta-cells. GLP-1 is also involved in the reduction of glucagon secretion, delaying gastric emptying and weight reduction in obese individuals. The incretin effect is defined as the difference in insulin response between oral and intravenous glucose administration at matched plasma glucose levels. However, incretin hormones are secreted not only after glucose or mixed meal intake, but other macronutrients (fat and protein) are also of importance for stimulating incretin hormones. The incretin effect is retained in type 2 diabetes (T2D) subjects. In this context, both GLP-1 and GIP undergo inactivation within a few minutes after their release through the action of the enzyme dipeptidyl peptidase-4 (DPP-4). Supraphysiologic levels of GLP-1 are able to restore the lost incretin effect and enhance insulin secretion in T2D individuals. One approach to achieve this is via inactivation of DPP-4 enzyme, this led to the development sitagliptin, vildagliptin, saxagliptin and others DPP-4 inhibitors as a tablet form. The other approach is through administration of GLP-1 in a stable dose as GLP-1 agonist. It is now well established that the long term use of both DPP-4 inhibitors and GLP-1 analogues improve glycemia by decreasing HbA1c and fasting plasma glucose. However, the response of islet cells to physiological increases in incretin hormones after ingestion of macronutrients or mixed meals in healthy subjects is still unknown. Most of the clinical studies have been done after an overnight fast, this supposes not to reflect normal physiological daily life. Therefore, the islet cell and incretin hormone responses to increased caloric loads of meals ingested at lunch-time arestill unexplored. It s also known that all macronutrients are of importance for stimulating insulin secretion. Adding protein to glucose more greatly stimulates insulin secretion compared to the glucose alone in healthy and T2D individual. However, it s still unknown how each role of macronutrients (glucose, protein and fat) in mixed meal stimulate islet cell and incretin hormone compared to the meal itself. Aims. In this thesis we aimed to explore how the physiological increase in incretin hormones after intake of sitagliptin affects glucose homeostasis after both glucose and non-glucose macronutrients. We

2 have also for the first time studied the adaptation of incretin and islet hormones after mixed meals of increasing size at lunch time during daily life in healthy subjects. Furthermore, we have studied the integrative impact of macronutrients on postprandial glycemia and islet cell function both in healthy and type 2 diabetes subjects. We also demonstrated the acute effect of sitagliptin after mixed meal ingestion on glucose and islet cell function both in healthy and type 2 diabetic subjects. Study design. The projects were a single-center studies and the Caucasian subjects were included. The participants were challenged with different macronutrients or mixed meal test in randomized cross-over design separated by at least four to eight weeks. Blood samples were taken throughout a 300 minutes period after each challenge test. Paracetamol g administrated together with each challenge test to determine gastric emptying. Water was ingested at the same time of each load to standardized the ingested volume. The work of the thesis was divided in five different projects, which all used the approach of interventional studies in humans. The studies were undertaken according to Good Clinical Practice, approved by the Ethic Committee in Lund, Sweden, and, when, required, also approved by the Swedish Medical Products Agency. The studies were registered at the clinicaltrial.gov and, when required, in the European Clinical Trials and they were externally monitored. Project 1; Glucose-lowering effect of the DPP-4 inhibitor sitagliptin after glucose and non-glucose macronutrient in non-diabetic subjects (NCT , EudraCT , presented at EASD 2012 and published in Diabetes, Obesity and Metabolism 2013) We hypothesized in this study that raising incretin hormone levels reduced circulating glucose after both glucose and non-glucose macronutrients. Twelve healthy subjects were included in this study and they received sitagliptin (100 mg) or placebo before ingestion of pure glucose, protein or fat. Project 2; Incretin and islet hormone responses to meals of increasing size in healthy subjects (NCT , presented at EASD 2013, published in J Clin Endocrinol Metab 2014) In this study we explore how the incretin hormones secretion and action dynamically adapt to demands created by caloric challenges achieved during daily life. Twelve healthy men and twelve healthy women

3 with matched age, weight and BMI. On three separate occasions, and after overnight fast, the subjects were served a standardized breakfast at 8:00 AM, and after breakfast,the subjects fast four hours. At 12:00 PM the subjects ingested a lunch meal containing 511 kcal, 743 kcal or 1034 kcal. Meals consumed within minutes and all three meals had identical nutrient compositions [protein energy (E) 18%, fat E 32%, and carbohydrates E 50%]. Project 3; Mixed meal diminishes glucose excursion compared to glucose by several adaptive mechanisms in man (NCT , presented at EASD 2014, revised manuscript submitted to Diabetes Obesity and Metabolism in August, 2015) We examine the integrative impact of macronutrients on postprandial glycemia, incretin hormones and islet cell function. Eighteen healthy and eighteen drug-naïve well controlled T2D subjects ingested macronutrients alone (glucose 330kcal, protein 110kcal or fat 110kcal) or all macronutrients together (550kcal) on four different occasions. Project 4; Islet-independent mechanisms contribute to the glucose-lowering effect of acute DPP-4 inhibition in healthy non diabetic and type 2 diabetes subjects after mixed meal intake ( NCT , EudraCt ; presented at EASD 2015) We explored the effect of acute single dose of sitagliptin (100mg) on incretin hormones and islet cell function after mixed meal ingestion in healthy and T2D. Twelve healthy non-diabetic and twelve well controlled drug naïve T2D individuals included in this study. Project 5>:Comparison of three DPP-4 inhibitors on 24 hour blood glucose, incretin hormones and islet cell function profiles in patients with type 2 diabetes (CODI24, NCT , EudraCt ; on-going project) During recent years, several DPP-4 inhibitors have been introduced on the market. They have all been shown to be efficient, safe and highly tolerable. The various DPP-4 inhibitors differ in regard to chemical structure, pharmacokinetics and metabolism. However, whether this has any consequence for islet function is at present unknown, since no head-to-head study examining DPP-4 inhibition and islet

4 function have been undertaken. Such knowledge would be of great scientific value for the understanding of the mode of action of DPP-4 inhibitors. Such knowledge would also be of huge clinical interest due to the large use of these different DPP-4 inhibitors. In this project we therefore aim at examining the influence of an acute administration of the three DPP-4 inhibitors sitagliptin, vildagliptin and saxagliptin throughout a 24h period in T2D men and women. In this study we are planned to include twelve men and twelve women with well controlled T2D and on stable dose of metformin at last three months. The subjects challenged with standardized meals and DPP-4 inhibitors according to the table below; Day 1/ 7:30 8:00 13:00 18:00 22:00 Time Intake Sitagliptin 100mg Breakfast Lunch Dinner Snacks Vildagliptin 50mg Vildagliptin 50mg Saxagliptin 5mg Ingested 30 minutes before the meal Day 2 Blod sampels after overnight fast Discussion and conclusion. The most important findings of this thesis are that the physiological elevation of incretin hormones after ingestion of sitagliptin followed by ingestion of glucose, non-gluocse macronutrients and mixed meal is robustly glucose lowering in healthy subjects without increasing in beta cell function. However, in type 2 diabetes subjects, we established that sitagliptin also is glucose lowering and have the same mechanism as in healthy subjects after mixed meal ingestion. The results raised the suggestion that the glucose is lowering action of acute and single dose ingestion of DPP-4 inhibitors at the normal and high glucose levels involving both islet-dependent and islet-independent mechanisms. We also found that the mixed lunch meal of increasing size elicit a caloric-dependent insulin response due to increased beta-cell secretion achieved by increased in incretin hormones. Furthermore, we also demonstrated that adding protein and fat macronutrients to glucose in a mixed meal diminish glucose excursion with increased beta-cell function and augmented glucagon and GIPsecretion in healthy and type 2 diabetes subjects.

5 Our findings in this thesis raise the importance of including all macronutrients in a meal intake to achieve glycemia in type 2 diabetes patients. We also suggest that the glucose-lowering effect of incretin hormones not only caused as direct effects of GLP-1 and GIP on insulin secretion but it s also as result of indirect effect on decrease lever glucose production and increase in insulin sensitivity. In conclusion we suggest that the macronutrients and mixed meal test regulates glycemia through both islet and extraislet mechanisms in both healthy subjects and in type 2 diabetes.