Improved beta cell function, with reduction in secretion of intact and 32/33 split proinsulin, after dietary intervention in subjects with type 2 diabetes mellitus

Weight loss improves β-cell function independently of dietary carbohydrate restriction in people with type 2 diabetes: A 6-week randomized controlled trial

Background

Carbohydrate restriction may benefit β-cell function and glucose metabolism in type 2 diabetes (T2D) but also leads to weight loss which in itself is beneficial.

Methods

In order to determine the additional effect of carbohydrate restriction in addition to a fixed body weight loss, we randomly assigned 72 adults with T2D and obesity (mean ± SD HbA_1c 7.4 ± 0.7%, BMI 33 ± 5 kg/m²) to a carbohydrate-reduced high-protein diet (CRHP; energy percent from carbohydrate/protein/fat: 30/30/40) or an isocaloric conventional diabetes diet (CD; 50/17/33) for 6 weeks. All foods were provided free of charge and total energy intake was tailored individually, so both groups lost 6% of baseline body weight.

Results

Despite significantly greater reductions in HbA_1c (mean [95% CI] −1.9 [−3.5, −0.3] mmol/mol) after 6 weeks, the CRHP diet neither improved glucose tolerance, β-cell response to glucose, insulin sensitivity, during a 4-h oral glucose tolerance test, nor basal proinsulin secretion when compared to the CD diet, but increased C-peptide concentration and insulin secretion rate (area under the curve [AUC] and peak) significantly more (~10%, P ≤ 0.03 for all). Furthermore, compared with the CD diet, the CRHP diet borderline increased basal glucagon concentration (16 [−0.1, 34]%, P = 0.05), but decreased glucagon net AUC (−2.0 [−3.4, −0.6] mmol/L ×240 min, P < 0.01), decreased basal triglyceride and total AUC (~20%, P < 0.01 for both), and increased gastric inhibitory polypeptide total AUC (14%, P = 0.01).

Conclusion

A moderately carbohydrate-restricted diet for 6 weeks decreased HbA_1c but did not improve β-cell function or glucose tolerance beyond the effects of weight loss when compared with a conventional diabetes diet in people with T2D.

Clinical trials registration

www.Clinicaltrials.gov, Identifier: NCT02472951.

Effects of carbohydrate restriction on postprandial glucose metabolism, -cell function, gut hormone secretion, and satiety in patients with Type 2 diabetes

Dietary carbohydrate restriction may improve the phenotype of Type 2 diabetes (T2D) patients. We aimed to investigate 6 wk of carbohydrate restriction on postprandial glucose metabolism, pancreatic α- and β-cell function, gut hormone secretion, and satiety in T2D patients. Methods In a crossover design, 28 T2D patients (mean HbA_1c: 60 mmol/mol) were randomized to 6 wk of carbohydrate-reduced high-protein (CRHP) diet and 6 wk of conventional diabetes (CD) diet (energy-percentage carbohydrate/protein/fat: 30/30/40 vs. 50/17/33). Twenty-four-hour continuous glucose monitoring (CGM) and mixed-meal tests were undertaken and fasting intact proinsulin (IP), 32,33 split proinsulin concentrations (SP), and postprandial insulin secretion rates (ISR), insulinogenic index (IGI), β-cell sensitivity to glucose (B_up), glucagon, and gut hormones were measured. Gastric emptying was evaluated by postprandial paracetamol concentrations and satiety by visual analog scale ratings. A CRHP diet reduced postprandial glucose area under curve (net AUC) by 60% (P < 0.001), 24 h glucose by 13% (P < 0.001), fasting IP and SP concentrations (both absolute and relative to C-peptide, P < 0.05), and postprandial ISR (24%, P = 0.015), while IGI and B_up improved by 31% and 45% (both P < 0.001). The CRHP diet increased postprandial glucagon net AUC by 235% (P < 0.001), subjective satiety by 18% (P = 0.03), delayed gastric emptying by 15 min (P < 0.001), decreased gastric inhibitory polypeptide net AUC by 29% (P < 0.001), but had no significant effect on glucagon-like-peptide-1, total peptide YY, and cholecystokinin responses. A CRHP diet reduced glucose excursions and improved β-cell function, including proinsulin processing, and increased subjective satiety in patients with T2D.

Clinical and Laboratory Evaluation of a New Specific Point-of-Care Test for Intact Proinsulin

BACKGROUND

Intact proinsulin is a biomarker for pancreatic ß-cell dysfunction. In large prospective studies in nondiabetic subjects, elevated intact proinsulin predicted development of type 2 diabetes and/or macrovascular events up to 7 years in advance. This study was performed to evaluate a new semiquantitative lateral flow-based point-of-care rapid test (POCT) for elevated intact proinsulin (cutoff: 15 pmol/L). The test requires 10 µL of capillary whole blood, with visual readout after 5 minutes. It is best applied at 2 hours after a glucose challenge or a meal.

METHODS

POCT results were obtained by health care professionals from 60 patients and healthy subject (33 female, 27 male, 28 type 2 diabetes, age: 53.6 ± 12.3 years). An additional venous blood sample was obtained from all participants for measurement of intact proinsulin by means of a quantitative ELISA reference method (TecoMedical, Sissach, Switzerland).

RESULTS

Elevated intact proinsulin levels (>15 pmol/L) were determined by the reference method in 26 participants, of whom 22 were also positive with the POCT (sensitivity: 85%). All 34 subjects with low intact proinsulin levels were tested negative by the POCT (specificity: 100%).

CONCLUSIONS

The test successfully detected elevated postprandial intact proinsulin levels in 85% of the tested subjects and no false positive test result occurred. This POCT can therefore serve as a simple screening tool for identification of patients with prevalent ß-cell dysfunction, who are at high risk for development of type 2 diabetes and/or macrovascular events within the next 5-7 years.

Elevated intact proinsulin levels are indicative of Beta-cell dysfunction, insulin resistance, and cardiovascular risk: impact of the antidiabetic agent pioglitazone

BACKGROUND

Insulin resistance (IR) and deterioration of beta-cell secretion are main features in the development of type 2 diabetes, which is reflected in increasing serum intact proinsulin levels in later disease stage. Introduction of stable assays that are able to distinguish between intact proinsulin and its specific and unspecific cleavage products has resulted in the finding that serum intact proinsulin values can serve as a direct marker for beta-cell dysfunction, are a highly specific indicator of IR, and can predict cardiovascular risk.

METHOD

Determination of fasting intact proinsulin may be used to monitor and optimize antidiabetic therapeutic approaches. Our study group has been involved in a variety of clinical studies investigating drug effects on beta-cell secretory capacity, IR, and intact proinsulin levels. One focus was on the impact of insulin-sensitizing therapy with pioglitazone on the pancreatic beta-cell load.

RESULTS

Treatment with pioglitazone resulted in significant decreases in elevated proinsulin levels in type 2 diabetes patients. This effect was independent from glycemic control.

CONCLUSIONS

Measurement of fasting intact proinsulin values allows a staging of beta-cell dysfunction and evaluation of IR, thus providing an interesting diagnostic tool for both selection of appropriate therapy and monitoring of treatment success.

Proinsulin levels in patients with pancreatic diabetes are associated with functional changes in insulin secretion rather than pancreatic beta-cell area

INTRODUCTION

Hyperproinsulinaemia has been reported in patients with type 2 diabetes. It is unclear whether this is due to an intrinsic defect in β-cell function or secondary to the increased demand on the β-cells. We investigated whether hyperproinsulinaemia is also present in patients with secondary diabetes, and whether proinsulin levels are associated with impaired β-cell area or function.

PATIENTS AND METHODS

Thirty-three patients with and without diabetes secondary to pancreatic diseases were studied prior to pancreatic surgery. Intact and total proinsulin levels were compared with the pancreatic β-cell area and measures of insulin secretion and action.

RESULTS

Fasting concentrations of total and intact proinsulin were similar in patients with normal, impaired (including two cases of impaired fasting glucose) and diabetic glucose tolerance (P=0.58 and P=0.98 respectively). There were no differences in the total proinsulin/insulin or intact proinsulin/insulin ratio between the groups (P=0.23 and P=0.71 respectively). There was a weak inverse association between the total proinsulin/insulin ratio and pancreatic β-cell area (r(2)=0.14, P=0.032), whereas the intact proinsulin/insulin ratio and the intact and total proinsulin levels were unrelated to β-cell area. However, a strong inverse relationship between homeostasis model assessment index of β-cell function and both the total and the intact proinsulin/insulin ratio was found (r(2)=0.55 and r(2)=0.48 respectively). The association of insulin resistance (IR) with intact proinsulin was much weaker than the correlation with fasting insulin.

CONCLUSIONS

Hyperproinsulinaemia is associated with defects in insulin secretion rather than a reduction in β-cell area. The weak association between intact proinsulin and IR argues against the usefulness of this parameter in clinical practice.

IRIS II study: intact proinsulin is confirmed as a highly specific indicator for insulin resistance in a large cross-sectional study design

BACKGROUND

The cross-sectional IRIS-II study tried to assess the prevalence of insulin resistance and macrovascular disease in orally treated patients with Type 2 diabetes.

METHODS

In total, 4,270 patients were enrolled into the study (2,146 male, 2,124 female; mean +/- SD age 63.9 +/- 11.1 years; body mass index 30.1 +/- 5.5 kg/m2; duration of disease 5.4 +/- 5.6 years; hemoglobin A1c 6.8 +/- 1.3%). The study consisted of a single morning visit with completion of a standardized questionnaire and collection of a fasting blood sample.

RESULTS

The mean intact proinsulin value was 11.4 +/- 12.4 pmol/L (normal range < 10 pmol/L). Homeostasis model assessment resulted in 1,147 insulin-sensitive patients (26.9%) and 3,123 patients (73.1%) with insulin resistance. Of the latter patients 1,465 (34.3% of all patients) had also elevated intact proinsulin values, while 1,658 (38.8%) had no proinsulin elevation. In contrast, 1,042 (24.4%) of the insulin-sensitive patients had normal intact proinsulin, and only 105 (2.4%) had elevated intact proinsulin concentrations (chi2 test P < 0.0001). A specificity of 93.2% (sensitivity 46.9%) was calculated for elevated intact proinsulin as an indirect marker for insulin resistance. Of the 1,451 patients treated with sulfonylurea 52% had elevated intact proinsulin values and increased prevalence of cardiovascular complications (odds ratio 1.45).

CONCLUSION

Type 2 patients with elevated fasting intact proinsulin values can be regarded as being insulin resistant. The results confirm that fasting intact proinsulin is a suitable measure for beta-cell dysfunction and insulin resistance in type 2 diabetes and may be used to support therapeutic decisions.

Role of intact proinsulin in diagnosis and treatment of type 2 diabetes mellitus

Insulin resistance in patients with type 2 diabetes is associated with an increased risk of cardiovascular events. While this can be partly explained by an impairment of direct insulin action on the endothelial cell, an independent contribution can be assigned also to the secretory dysfunction of the beta-cell. If the demand for insulin triggered by insulin resistance is arriving at a certain threshold, an insufficiency of the cleavage capacity of beta-cell carboxypeptidase H leads to an increased secretion of intact proinsulin in addition to the desired insulin molecule. Proinsulin, however, has been demonstrated to be an independent cardiovascular risk factor by stimulating plasminogen activator inhibitor-1 secretion and blocking fibrinolysis. A recently introduced intact proinsulin assay is able to distinguish between intact proinsulin and its specific and non-specific cleavage products. This assay allows for a pathophysiological staging of type 2 diabetes based on beta-cell secretion. It could be confirmed by a large epidemiological study (IRIS-2, 4,265 patients) that intact proinsulin is a highly specific marker for insulin resistance. It could also be shown in other studies that successful resistance treatment with insulin or glitazones led to a decrease in elevated proinsulin levels and, thus, to a decrease of cardiovascular risk, while the levels remained high during sulfonylurea therapy. Therefore, patients with increased fasting intact proinsulin values should be treated with a therapy focusing on insulin resistance. Assessment of beta-cell function by determination of intact proinsulin may facilitate the selection of the most promising therapy and may also serve to monitor treatment success in the further course of the disease.

Fasting intact proinsulin is a highly specific predictor of insulin resistance in type 2 diabetes

OBJECTIVE

In later stages of type 2 diabetes, proinsulin and proinsulin-like molecules are secreted in increasing amounts with insulin. A recently introduced chemiluminescence assay is able to detect the uncleaved "intact" proinsulin and differentiate it from proinsulin-like molecules. This investigation explored the predictive value of intact proinsulin as an insulin resistance marker.

RESEARCH DESIGN AND METHODS

In total, 48 patients with type 2 diabetes (20 women and 28 men, aged 60 +/- 9 years [means +/- SD], diabetes duration 5.1 +/- 3.8 years, BMI 31.2 +/- 4.8 kg/m2, and HbA1c 6.9 +/- 1.2%) were studied by means of an intravenous glucose tolerance test and determination of fasting values of intact proinsulin, insulin, resistin, adiponectin, and glucose. Insulin resistance was determined by means of minimal model analysis (MMA) (as the gold standard) and homeostatis model assessment (HOMA).

RESULTS

There was a significant correlation between intact proinsulin values and insulin resistance (MMA P<0.05 and HOMA P<0.01). Elevation of intact proinsulin values above the reference range (>10 pmol/l) showed a very high specificity (MMA 100% and HOMA 92.9%) and a moderate sensitivity (MMA 48.6% and HOMA 47.1%) as marker for insulin resistance. Adiponectin values were slightly lower in the insulin resistant group, but no correlation to insulin resistance could be detected for resistin in the cross-sectional design.

CONCLUSIONS

Elevated intact proinsulin seems to indicate an advanced stage of beta-cell exhaustion and is a highly specific marker for insulin resistance. It might be used as arbitrary marker for the therapeutic decision between secretagogue, sensitizer, or insulin therapy in type 2 diabetes.

Improvements in insulin sensitivity and beta-cell function (HOMA) with weight loss in the severely obese. Homeostatic model assessment

AIMS

To examine the effect of weight loss on insulin sensitivity and beta-cell function in severely obese subjects of varying glycaemic control.

PATIENTS AND METHODS

Subjects were 254 (F:M 209:45) patients having adjustable gastric banding for severe obesity, with paired biochemical data from before operation and at 1-year follow up. The homeostatic model assessment method was used to calculate insulin sensitivity (HOMA%S) and beta-cell function (HOMA%B). Subjects were grouped by diabetic status and by pre-weight loss HbA1c.

RESULTS

Initial mean (sd) weight and body mass index were 128 (26) kg and 46.2 (7.7) kg/m2, respectively, and at 1-year were 101 (22) kg and 36.4 (6.7) kg/m2. The percentage of excess weight lost (%EWL) was 44.3 (14)%. HOMA%S improved from 37.5 (16)% presurgery to 62 (25)% (P < 0.001). %EWL was the only predictor of HOMA%S improvement (r = 0.28, P < 0.001). Subjects with normal fasting glucose, impaired fasting glucose and Type 2 diabetes had a fall, no change and increase in HOMA%B, respectively. The improvement in HOMA%B in subjects with diabetes (n = 39) was inversely related to the time with diabetes (r = -0.36, P = 0.02). In non-diabetic subjects the HOMA%S-HOMA%B relationship was favourably altered with weight loss, so that for any given HOMA%S there was an increase in HOMA%B (f = 11.8, P = 0.001). This improvement in HOMA%B was positively related to %EWL (r = 0.25, P = 0.019).

DISCUSSION

There are beneficial changes in both insulin sensitivity and beta-cell function with weight loss. Modern laparoscopic obesity surgery may have an important early role in the management of Type 2 diabetes in obese subjects.

Effect of pioglitazone on blood proinsulin levels in patients with type 2 diabetes mellitus

The objective of this study was to clarify the influence of pioglitazone (Pio) on proinsulin (PI) in patients with type 2 diabetes mellitus. The subjects were 55 patients with type 2 diabetes. Among them, 18, 18, and 19 patients were respectively treated with Pio alone (group P), gliclazide (Gli) alone (group G), or Pio plus Gli (group PG) for 12 weeks. Fasting blood samples were obtained before and after treatment and were used to measure fasting plasma glucose (FPG), HbA1C, immunoreactive insulin (IRI), and PI. The levels of FPG, HbA1C, and IRI showed a significant decrease after treatment with Pio in groups P and PG. Treatment with Pio also caused PI to decrease significantly (group P: from 24.7 +/- 12.9 (mean +/- SD) to 14.0 +/- 6.2 pmol/L, p < 0.01, group PG: from 24.3 +/- 11.3 to 14.4 +/- 6.5 pmol/L, p < 0.01). In group G, treatment with Gli caused FPG and HbA1C to decrease significantly, but PI showed no change (21.5 +/- 12.3 to 21.6 +/- 10.4 pmol/L, p = n.s.). In patients with type 2 diabetes, treatment with Pio achieved an improvement of glycemic control and reduced the load on the pancreatic beta cells.

Fasting proinsulin and 2-h post-load glucose levels predict the conversion to NIDDM in subjects with impaired glucose tolerance: the Hoorn Study

The aims of the present study were to observe the natural history of impaired glucose tolerance and to identify predictors for development of non-insulin-dependent diabetes mellitus (NIDDM). A survey of glucose tolerance was conducted in subjects aged 50-74 years, randomly selected from the registry of the middle-sized town of Hoorn in the Netherlands. Based on the mean values of two oral glucose tolerance tests subjects were classified in categories of glucose tolerance according to the World Health Organization criteria. All subjects with impaired glucose tolerance (n = 224) were invited to participate in the present study, in which 70% (n = 158) were subsequently enrolled. During follow-up subjects underwent a repeated paired oral glucose tolerance test. The mean follow-up time was 24 months (range 12-36 months). The cumulative incidence of NIDDM was 28.5% (95% confidence interval 15-42%). Age, sex, and anthropometric and metabolic characteristics at baseline were analysed simultaneously as potential predictors of conversion to NIDDM using multiple logistic regression. The initial 2-h post-load plasma glucose levels and the fasting proinsulin levels were significantly (p < 0.05) related to the incidence of NIDDM. Anthropometric characteristics, the 2-h post-load specific insulin levels and the fasting proinsulin/fasting insulin ratio were not related to the incidence of NIDDM. These results suggest that beta-cell dysfunction rather than insulin resistance plays the most important role in the future development of diabetes in a high-risk Caucasian population.

Increased secretion of 32,33 split proinsulin after intravenous glucose in glucose-tolerant first-degree relatives of patients with non-insulin dependent diabetes of European, but not Asian, origin

OBJECTIVE

The aetiology of non-insulin dependent diabetes is unknown, but defective insulin secretion is a feature. The disease also has a strong genetic basis and first-degree relatives of patients have an increased risk of future diabetes. To investigate whether beta-cell dysfunction is an early feature of the disease, we studied insulin secretion in healthy first-degree relatives of patients with non-insulin dependent diabetes.

DESIGN

Each subject underwent a 1-hour intravenous glucose tolerance test (0.3 g/kg).

PATIENTS

Seventeen first-degree relatives of patients (10 of European and 7 of Asian (Indian subcontinent) origin) with normal glucose tolerance were compared with 17 matched control subjects with no family history of diabetes.

MEASUREMENTS

Plasma immunoreactive insulin (IRI) was measured by radioimmunoassay, and specific insulin, intact and 32,33 split proinsulin were measured by specific immunoradiometric assays (IRMA) for the 1st phase (0-10 minutes) and 2nd phase (10-60 minutes) responses. Glucose and intermediary metabolites were measured enzymatically.

RESULTS

Fasting concentrations of IRI, IRMA insulin, intact and 32,33 split proinsulin were similar in relatives and controls in each group. Fasting glucose levels were similar in European relatives and controls but lower in Asian relatives compared to their controls (mean +/- SE 4.9 +/- 0.2 vs 5.5 +/- 0.2 mmol/l, P < 0.05). Following intravenous glucose, European relatives had similar IRI and glucose levels to their controls. Secretion of 32,33 split proinsulin was increased in European relatives compared to their controls, significantly so for 2nd phase secretion (1st phase median (range): 71 (7-352) vs 55 (17-118) pmol/l min, NS; 2nd phase: 433 (115-1459) vs 234 (55-745) pmol/l min, P < 0.05). Secretion of IRMA insulin and intact proinsulin were similar in European relatives and controls (IRMA insulin: 1st phase 2757 (700-10,969) vs 2830 (632-4682) pmol/l min; 2nd phase 6387 (3006-15,865) vs 5284 (2060-18,605) pmol/l min; intact proinsulin: 1st phase 31 (13-113) vs 32 (16-72) pmol/l min; 2nd phase: 174 (87-737) vs 159 (97-298) pmol/l min). European relatives had a greater percentage of proinsulin-like molecules (intact + 32,33 split proinsulin) to total insulin (sum of IRMA insulin + intact + 32,33 split proinsulin) during the 2nd phase of secretion (9.1 (5.0-11.8) vs 5.9 (4.3-12.6)%, P < 0.05). In contrast, Asian relatives had similar secretion of IRI, IRMA insulin, intact and 32,33 split proinsulin to their controls. Glucose disappearance (KG) was similar in relatives and controls within each ethnic group (Europeans: relatives 725 +/- 101 vs controls 668 +/- 47/min; Asian: relatives 610 +/- 97 vs controls 783 +/- 936/min). Asian relatives had higher fasting circulating glycerol (65 +/- 7 vs 44 +/- 4 mumol/l, P < 0.05), non-esterified fatty acid (569 +/- 59 vs 375 +/- 64 mumol/l, P < 0.05) and 3-hydroxybutyrate levels (147 (44-187) vs 35 (21-57) mumol/l, P < 0.01) than their controls and this persisted following intravenous glucose. This difference was not observed in the European group.

CONCLUSION

First-degree relatives of European patients with NIDDM possess early signs of beta-cell dysfunction, with increased and disproportionate secretion of 32,33 split proinsulin after intravenous glucose, whilst glucose tolerance is still normal.