Exenatide challenge in oral glucose tolerance test is insufficient for predictions of glucose metabolism and insulin secretion after sleeve gastrectomy (SG) in obese patients with type 2 diabetes: a pilot study to establish a preoperative model to estimate β-cell function following augmented glucagon-like peptide-1 secretion after SG

Abstract

The postoperative increase in glucagon-like peptide-1 (GLP-1) is the main factor to improve glucose metabolism following sleeve gastrectomy (SG) in obese patients with type 2 diabetes. We investigated whether the β-cell responsiveness to an injection of exogenous GLP-1 in the preoperative period could determine the postoperative glucose tolerance in 18 patients underwent SG. In the preoperative period, a regular oral glucose tolerance test (OGTT) and an exenatide-challenge during OGTT (Ex-OGTT) were performed to evaluate the β-cell function and its responsiveness to GLP-1. The postoperative glucose tolerance was evaluated by another regular OGTT performed at 3 months after SG. The significant decrease in glucose levels with enhanced secretions of insulin and GLP-1 was observed in OGTT at 3 months after SG. The area under the curve of glucose from 0 to 120 minutes (AUC glucose_{0–120 min}) and the insulinogenic index (I.I.) in OGTT at 3 months post-SG were significantly improved compared to those in preoperative period, but comparable with those in Ex-OGTT. AUC glucose_{0–120 min} and I.I. in OGTT at 3 months post-SG were significantly correlated with not only those in Ex-OGTT, but also those in the preoperative regular OGTT. Conversely, the correlations calculated by the Spearman’s ρ were stronger in the latter than the former. This exenatide-challenge protocol might be useful to estimate glucose tolerance and insulin secretion after SG, however, it may be insufficient to improve predictability of a patient who is likely to achieve a significant benefit on glucose metabolism from receiving SG.

SLEEVE GASTRECTOMY (SG) and Roux-en-Y gastric bypass (RYGB) are the principal bariatric/metabolic procedures accepted worldwide as effective interventions for morbid obesity and type 2 diabetes (T2D) [1]. In the early postoperative period after these procedures, many patients achieve a marked amelioration of their glucose tolerance before any substantial weight loss has occurred [2]. This mechanism is not completely understood, but it is thought to be largely related to dramatic postoperative changes in the secretion of gastrointestinal hormones typified by glucagon-like peptide-1 (GLP-1) [2, 3]. The rapid and massive delivery of intact nutrients to the distal ileum enhances the postprandial secretion of GLP-1, which results in a stimulation of insulin secretion [4]. This “hindgut hypothesis” has been proposed as the main mechanism underlying the immediate improvement of glucose tolerance following bariatric surgeries including SG and RYGB [5]. This is supported by several reports that glucose tolerance was worsened by the significant reduction of postprandial insulin secretion following an administration of the GLP-1 antagonist exendin 9–39 in patients with T2D who underwent these surgeries [6].

The exaggerated GLP-1 response is a key mediator of the improvement in glucose metabolism during the early postoperative period, but it is probably insufficient in patients whose β-cells are unable to secrete a sufficient level of insulin to achieve diabetes remission. The postprandial GLP-1 levels after SG or RYGB were similar between patients who achieved T2D remission and those who did not [7, 8]. We thus hypothesized that the sensitivity of β-cells to GLP-1 could determine whether glucose tolerance will be recovered in patients who undergo bariatric surgery.

In this pilot study, we investigated whether the preoperative β-cell responsiveness to exenatide, a short-acting GLP-1 receptor agonist (GLP-1RA), correlates with the postoperative glucose metabolism outcome in morbidly obese patients with T2D.

Methods

Patients

We recruited 18 Japanese adults with T2D and a body mass index ≥35 kg/m² who were scheduled to undergo SG.

Study design

This was a single-center, single-arm, observational pilot study carried out at Nagasaki University Hospital. The study was registered in the University Hospital Medical Information Network (no. UMIN000034407).

In the preoperative period, the participants were admitted to the department of internal medicine at Nagasaki University Hospital for their medical examination and taking preoperative education. During their hospitalization, the patients achieved a good level of glycemic control by using intensified insulin therapy while discontinuing all other antidiabetic medications excluding metformin. As shown in Fig. 1, we investigated the patients’ glucose tolerance by administering a 75-g oral glucose tolerance test (OGTT) after overnight fasting (i.e., Baseline OGTT). To estimate the patient’s β-cell responsiveness to the GLP-1RA exenatide, the glycemic and insulin responses to a subcutaneous injection of 5 μg exenatide 5 minutes before ingestion of glucose during an OGTT (i.e., Exenatide-challenge OGTT) were evaluated on the following day after Baseline OGTT was performed.

Fig. 1

The study design. 1) In the preoperative period, a regular OGTT was administered to evaluate the patients’ glucose tolerance (Baseline OGTT). 2) On the following day after Baseline OGTT was performed, 5 μg of exenatide was injected subcutaneously 5 minutes before ingestion of glucose as Exenatide-challenge OGTT to estimate the patients’ β-cell responsiveness to augmented GLP-1. 3) At 3 months after receiving SG, another regular OGTT was performed to evaluate the patients’ postoperative glucose tolerance.

All the patients underwent SG after achieving a sufficient weight reduction at least 5% of their baseline body weight by diet therapy. We performed another OGTT 3 months after SG to evaluate postoperative glucose metabolism of the patients. All antidiabetic medications were discontinued at the preceding day in each OGTT if the patients were treated with any antidiabetic medications.

The study was approved by the ethical committee of Nagasaki University Hospital (no. 17032726) and performed in accordance with the Declaration of Helsinki. Written informed consent was obtained from all participants.

Statistical analysis

We performed a repeated-measures analysis of variance to test the differences in the values at each time-point during the OGTTs. The paired t-test and Spearman’s rank correlation coefficient were used to compare the data obtained from the OGTTs. The statistical analyses were carried out using JMP Pro 15 (SAS Institute, Cary, NC, USA). P-values <0.05 were considered significant.

Results

Table 1 summarizes the patients’ baseline characteristics when they admitted to the department of internal medicine in the preoperative period (Fig. 1). The means of body weight and HbA1c levels were significantly decreased at 3 months post-SG (90.2 ± 19.3 kg and 5.8 ± 0.7%) compared to those of baseline (both, p < 0.001). Fifteen (83%) of the 18 patients had discontinued their antidiabetic medications by 3 months after SG. In OGTT at 3 months post-SG, 13 (72%) patients showed normal glucose tolerance, 4 (22%) patients showed impaired glucose tolerance, and only one (6%) patient remained diabetes.

Table 1 Baseline characteristics of the participants (n = 18)

Male/female, n	11/7
Age, yrs	40.0 ± 6.2
ABCD score	4.7 ± 2.4
Weight, kg	115.9 ± 21.8
BMI, kg/m2	43.1 ± 9.2
HbA1c, NGSP, %	7.7 ± 2.1
AST, IU/L	36 ± 16
ALT, IU/L	54 ± 26
Creatinine, mg/dL	0.82 ± 0.20
LDL-C, mg/dL	122 ± 25
HDL-C, mg/dL	40 ± 10
Triglyceride, mg/dL	156 ± 42
UACR, mg/gCr	16.4 (3.2–222.7)
Pharmacological treatment, n (%):
Insulin	6 (33.3)
GLP-1 receptor agonist*	2 (11.1)
DPP-4 inhibitor	6 (33.3)
SGLT2 inhibitor	2 (11.1)
Metformin	10 (55.6)
Others	5 (27.8)
HOMA-IR	3.30 ± 2.08
Matsuda index	3.57 ± 3.04
Insulinogenic index	0.63 ± 0.60

Data are mean ± standard deviation or median (25%–75% confidence interval) for continuous variables. ALT: alanine aminotransferase, AST: aspartate aminotransferase, BMI: body mass index, DPP-4: dipeptidyl peptidase-4, GLP-1: glucagon-like peptide-1, HDL-C: high-density lipoprotein cholesterol, HOMA-IR: homeostatic model assessment of insulin resistance, LDL-C: low-density lipoprotein cholesterol, SGLT2: sodium-glucose cotransporter 2, UACR: urine albumin/creatinine ratio. * GLP-1 receptor agonist used in the 2 patient was liraglutide.

Fig. 2 illustrates the results of OGTTs. The patients’ glycemic profiles were significantly improved at 3 months post-SG (Fig. 2a). The early-phase insulin secretory response was restored post-SG with a marked increase in GLP-1 secretion (Fig. 2b, 2d). The glucose-dependent insulinotropic polypeptide levels were very similar in the pre- and post-operative periods (Fig. 2c). At 3 months post-SG, the area under the curve of glucose from 0 to 120 minutes (AUC glucose_{0–120 min}) and the insulin secretory capacity measured by the insulinogenic index (I.I.) [9] were significantly improved compared to those in baseline (Fig. 2e, 2f), but insulin sensitivity indexes including the homeostatic model assessment of insulin resistance (HOMA-IR) [10] and Matsuda index [11] were not (Fig. 2f).

Fig. 2

The results of the 75-g OGTTs. The data are mean ± standard error. * p < 0.05 between the results of the Baseline and Exenatide-challenge OGTTs. ** p < 0.05 between the results of the Baseline OGTT and the OGTT at 3 months post-SG. Black circles and bars: the results of the Baseline OGTT. Blue triangles and bars: the results of the Exenatide-challenge OGTT. Red squares and bars: the results of the 3-month post-SG OGTT. Open circles with dotted lines in the Exenatide-challenge OGTT and the 3-month post-SG OGTT represent the values of Baseline OGTT to highlight those differences. AUC: area under the curve, GIP: glucose-dependent insulinotropic polypeptide, N.D.: no data.

A negative correlation between AUC glucose_{0–120 min} in 3-month postoperative OGTT and I.I. in Exenatide-challenge OGTT was observed (Fig. 3a), but there were no significant correlations between AUC glucose_{0–120 min} in 3-month postoperative OGTT and insulin resistance indexes (HOMA-IR and Matsuda index) in Exenatide-challenge OGTT (Fig. 3b, 3c).

Fig. 3

The correlations between the AUC glucose_{0–120 min} in 3-month post-SG OGTT and insulin secretion/sensitivity indexes in Exenatide-challenge OGTT. Each Spearman’s ρ and its p-value were shown in each figure.

The values of AUC glucose_{0–120 min} and I.I. in Exenatide-challenge OGTT conducted in the preoperative period were comparable (Fig. 2e, 2f), and significantly correlated to those in the OGTT at 3 months post-SG, respectively (Fig. 4a, 4b). The values of AUC glucose_{0–120 min} and I.I. in Baseline OGTT were also significantly correlated with those in 3-month postoperative OGTT (Fig. 4c, 4d). Conversely, the correlations calculated by the Spearman’s ρ between Baseline OGTT and 3-month postoperative OGTT were stronger than those between Exenatide-challenge OGTT and 3-month postoperative OGTT. The values of AUC glucose_{0–120 min} and I.I. in Exenatide-challenge OGTT were also significantly correlated with those in Baseline OGTT (Fig. 4e, 4f).

Fig. 4

The correlations between the values of each OGTTs. Each correlation was determined based on Spearman’s rank correlation coefficient. Each Spearman’s ρ and its p-value were shown in each figure.

We performed an additional analysis in the patients dividing into 2 groups depending on the ABCD score, which was reported as useful in predicting the success of T2D treatment using metabolic surgery [12] (Supplementary Fig. 1). The values of AUC glucose_{0–120 min} and I.I. in Exenatide-challenge OGTT were significantly correlated with those in 3-month postoperative OGTT only in patients with higher ABCD score (≥5, n = 9). However, the correlations in the indexes between Exenatide-challenge OGTT and 3-month postoperative OGTT were not stronger than those between Baseline OGTT and 3-month postoperative OGTT in either patients with higher ABCD score or patients with lower ABCD score.

We could follow up 17 of the 18 patients until 12 months after SG, and analyzed the 17 patients dividing into 2 groups depending on whether a patient had kept diabetes remission (defined by HbA1c levels <6.0% without any use of anti-diabetic agents) for postoperative 12 months or not (Supplementary Table 1). There were no significant differences in the indexes in preoperative OGTTs regardless of exenatide administration between the remission group (n = 11) and the non-remission group (n = 6).

Discussion

Our results demonstrated significant improvements in glucose tolerance and insulin secretion with a marked increase in GLP-1 secretion in obese patients with T2D who underwent SG, as observed in other studies [6, 13, 14]. Insulin secretory capacity measured by I.I. significantly increased 3 months after SG, whereas insulin sensitivity measured by HOMA-IR or Matsuda index did not. It seems likely that elevated GLP-1 secretion contributes to improve early-postoperative glucose tolerance via increase in insulin secretion rather than reduction in appetite and weight loss after SG. These results may support the hindgut hypothesis of SG, i.e., the main role of increasing GLP-1 secretion.

Although a previous study showed that the sensitivity of insulin secretion to continuous infusion of GLP-1 during hyperglycemic cramp was significantly reduced more than 2 years after gastric bypass [15], another group demonstrated that the insulinotropic action of a single dose injection of GLP-1 was preserved 3 months after gastric bypass [16]. Thus, β-cell responsiveness to enhanced GLP-1 secretion might be a key factor of deciding glucose tolerance especially in early postoperative period.

In this study, we examined whether preoperative β-cell response to a single dose of the short-acting GLP-1RA exenatide could predict glucose metabolism in the early postoperative period by imitating the augmented postprandial GLP-1 secretion. The glucose excursion and insulin secretion in Exenatide-challenge OGTT were comparable to those in the 3-month postoperative OGTT. However, the values of AUC glucose_{0–120 min} and I.I. in 3-month postoperative OGTT were significantly correlated with those in preoperative OGTT regardless of the exenatide injection. Those correlations between post- and preoperative OGTTs were not strengthened by the addition of exenatide-challenge. The responsiveness of β-cells to postoperative augmented GLP-1 secretion could be to some extent predicted by a preoperative regular OGTT.

Our findings indicate the exenatide-challenge protocol might be useful to estimate postoperative glucose tolerance but is insufficient to distinguish between individuals who will receive a beneficial effect of SG on glucose metabolism and those who will not.

There are study limitations to address. The sample size was small, and the follow-up period was not long. The subcutaneous injection of exenatide 5 μg might not perfectly imitate the postoperative endogenous GLP-1 secretion peaking 30 min after glucose loading. The administration of exenatide might suppress mainly glucose excursion due to its effect on delaying gastric emptying rather than increasing insulin levels [17]. The exogenous GLP-1 used for Exenatide-challenge OGTT and endogenous GLP-1 which is physiologically elevated postoperatively might have different effects on glucose metabolism due to the differences in their hemodynamics and molecular themselves. It remains unclear whether the improvement of glycemic outcomes after bariatric surgery is influenced by other mechanisms including marked changes in other gut hormones including glucagon, peptide YY and ghrelin [6, 18-20], bile acids [21] and bacterial flora [22].

Our results reinforce the possibility that increased GLP-1 secretion might play a significant role in the improvement of glucose metabolism in the early post-SG period. However, evaluating patients’ β-cell responsiveness to GLP-1 in the preoperative period is insufficient to improve predictability of the effectiveness of SG on amelioration of glucose tolerance after SG. Further investigations are needed to clarify the role of gut hormones in the metabolic success of bariatric surgeries.

Abbreviations

AUC, area under the curve; GLP-1, glucagon-like peptide-1; GLP-1 RA, glucagon-like peptide-1 receptor agonist; HOMA-IR, the homeostatic model assessment of insulin resistance; I.I., insulinogenic index; SG, sleeve gastrectomy; OGTT, oral glucose tolerance test; RYGB, Roux-en-Y gastric bypass; T2D, type 2 diabetes

Acknowledgments

We thank all the staffs at the research institution for their support.

Disclosure

The authors declare no conflict of interest.

Supplementary Table 1 Differences of the parameters between the diabetes remission group (Remission group, n = 11) and the diabetes non-remission group (Non-remission group, n = 6)

	Remission	Non-remission	p-value
Baseline characteristics
Male/female, n	7/4	3/3	0.59
Age, yrs	37.6 ± 4.7	46.0 ± 3.5	0.005
ABCD score	5.5 ± 1.8	2.5 ± 1.4	0.005
Weight, kg	118.5 ± 20.5	105.5 ± 19.9	0.12
BMI, kg/m2	43.5 ± 10.3	40.4 ± 5.8	0.73
HbA1c, NGSP, %	7.3 ± 1.8	8.9 ± 2.4	0.044
Baseline OGTT
AUC glucose0–120 min	375 ± 84	462 ± 80	0.063
HOMA-IR	3.51 ± 2.01	2.25 ± 1.39	0.17
Matsuda index	3.01 ± 1.73	5.02 ± 4.57	0.29
Insulinogenic index	0.80 ± 0.71	0.37 ± 0.28	0.15
Exenatide-challenge OGTT
AUC glucose0–120 min	262 ± 55	356 ± 110	0.050
HOMA-IR	3.38 ± 1.74	3.56 ± 1.96	0.65
Matsuda index	3.96 ± 2.53	5.69 ± 7.86	0.51
Insulinogenic index	1.42 ± 1.09	0.63 ± 0.44	0.17
12-month post-SG
Weight, kg	81.1 ± 17.5	82.4 ± 18.1	0.80
BMI, kg/m2	29.6 ± 6.7	31.5 ± 5.2	0.45
HbA1c, NGSP, %	5.5 ± 0.3	6.4 ± 0.4	0.002

Diabetes remission group was defined by HbA1c <6.0% without any glucose-lowering medications at 12 months post-SG. Non-remission group was defined by HbA1c ≥6.0% or/and being treated with glucose-lowering medications at 12 months post-SG. Of 18 patients enrolled in the study, 17 patients were analyzed excluding 1 patient who was lost to follow-up. Data are mean ± standard deviation for continuous variables. P-values for differences between the diabetes remission group and the diabetes non-remission group were calculated using the chi-square test or Wilcoxon’s rank sum test. AUC: area under the curve, BMI: body mass index.

Supplementary Fig. 1

The results of the 75-g OGTTs in patients with higher ABCD score (≥5, n = 9) and those with lower ABCD score (<5, n = 9). * p < 0.05 between Baseline OGTT and Exenatide-challenge OGTT. ** p < 0.05 between Baseline OGTT and 3-month post-SG OGTT. Black circles and bars: Baseline OGTT. Blue triangles and bars: Exenatide-challenge OGTT. Red squares and bars: 3-month post-SG OGTT. Open circles with dotted lines in the Exenatide-challenge OGTT and the 3-month post-SG OGTT represent the values of Baseline OGTT to highlight those differences. Each Spearman’s ρ and its p-value were shown in each figure.

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