Predictors of Incretin Concentrations in Subjects With Normal, Impaired, and Diabetic Glucose Tolerance

Abstract

OBJECTIVE-Defects in glucagon-like peptide 1 (GLP-1) secretion have been reported in some patients with type 2 diabetes after meal ingestion. We addressed the following questions: 1) Is the quantitative impairment in GLP-1 levels different after mixed meal or isolated glucose ingestion? 2) Which endogenous factors are associated with the concentrations of GLP-1? In particular, do elevated fasting glucose or glucagon levels diminish GLP-1 responses?RESEARCH DESIGN AND METHODS-Seventeen patients with mild type 2 diabetes, 17 subjects with impaired glucose tolerance, and 14 matched control subjects participated in an oral glucose tolerance test (75 g) and a mixed meal challenge (820 kcal), both carried out over 240 min on separate occasions. Plasma levels of glucose, insulin, C-peptide, glucagon, triglycerides, free fatty acids (FFAs), gastric inhibitory polypeptide (GIP), and GLP-1 were determined.RESULTS-GIP and GLP-1 levels increased significantly in both experiments (P Ͻ 0.0001). In patients with type 2 diabetes, the initial GIP response was exaggerated compared with control subjects after mixed meal (P Ͻ 0.001) but not after oral glucose ingestion (P ϭ 0.98). GLP-1 levels were similar in all three groups in both experiments. GIP responses were 186 Ϯ 17% higher after mixed meal ingestion than after the oral glucose load (P Ͻ 0.0001), whereas GLP-1 levels were similar in both experiments. There was a strong negative association between fasting glucagon and integrated FFA levels and subsequent GLP-1 concentrations. In contrast, fasting FFA and integrated glucagon levels after glucose or meal ingestion and female sex were positively related to GLP-1 concentrations. Incretin levels were unrelated to measures of glucose control or insulin secretion.CONCLUSIONS-Deteriorations in glucose homeostasis can develop in the absence of any impairment in GIP or GLP-1 levels. This suggests that the defects in GLP-1 concentrations previously described in patients with long-standing type 2 diabetes are likely secondary to other hormonal and metabolic alterations, such as hyperglucagonemia. GIP and GLP-1 concentrations appear to be regulated by different factors and are independent of each other. Diabetes 57:678-687, 2008 P ostprandial glucose homeostasis is controlled not only by the direct stimulation of insulin release by the absorbed nutrients but also through the secretion of incretin hormones, namely gastric inhibitory polypeptide (GIP) and glucagonlike peptide 1 (GLP-1) (1-4). In healthy, nondiabetic subjects, the quantitative contribution of this incretin effect to the overall postprandial insulin secretion has been estimated to be 50 -70% (5,6), depending on meal size and composition. In contrast, a marked reduction of the incretin effect is characteristic of patients with type 2 diabetes (7), thereby contributing to the excess postprandial glucose excursions in such patients. Although the exact mechanisms underlying the loss of incretin activity in patients with type 2 diabetes are still not completely understood, two d...

Dietary Resistant Starch Reduces Levels of Glucose-Dependent Insulinotropic Polypeptide mRNA along the Jejunum-Ileum in Both Normal and Type 2 Diabetic Rats

“…Some recent studies have reported that the circulating GIP concentrations were associated with the development of diabetes in humans. 6,7) Although it is assumed that there are many reasons for the improvement of diabetes by the RS intake, and it needs to be investigated whether the repression of excessive GIP secretion can lead to the prevention/improvement of diabetes and the associated complications, the results of our current study support this notion.…”

“…Indeed, some studies have observed that the circulating GIP levels were higher in obese and diabetic subjects than in healthy subjects. [4][5][6][7] It is thus believed that repressing the excessive GIP and insulin secretion caused by delaying the digestion/absorption of carbohydrates is important for preventing/improving such diseases. Several human studies have already shown that the postprandial induction of circulating GIP was repressed by a simultaneous intake of acarbose, [8][9][10] an inhibitor of disaccharidases in the small intestine, a soluble fiber guar gum 11) and resistant starch (RS), 12) which consists of an autoclaved high-amylose starch, is known to undergo digestion slowly and has the characteristics of a dietary fiber.…”

Incretin levels in polycystic ovary syndrome

“…In particular, leptin stimulates GLP-1 secretion, and this effect was attenuated in subjects with leptin resistance (29), while glucagon correlated negatively with GLP-1 during OGTT in patients with T2DM. On the other hand, no relationships between the parameters of glucose control, insulin sensitivity, and secretion with GLP-1 levels have been found (30).…”

Preserved GLP-1 and exaggerated GIP secretion in type 2 diabetes and relationships with triglycerides and ALT

“…Nevertheless, heterogeneity was apparent between studies that used mixed meal (lower GLP-1 among diabetes) and oral glucose load (higher GLP-1). Studies that directly compared GLP-1 responses following oral glucose vs mixed meal are scarce and the available studies did not find an altered GLP-1 response after both interventions among diabetic patients (7). GIP responses were found to be reduced (8), increased (7,9), but mostly unchanged in patients with diabetes (10,11,12,13).…”