Accelerated intestinal glucose absorption in morbidly obese humans: relationship to glucose transporters, incretin hormones, and glycemia.

ORIGINAL E n d o c r i n e

ARTICLE R e s e a r c h

Accelerated intestinal glucose absorption in morbidly obese humans – relationship to glucose transporters, incretin hormones and glycaemia Nam Q Nguyen1,2, MBBS, PhD, FRACP, Tamara L Debreceni1, BSc (Hon), Jenna E Bambrick1, BSc, Bridgette Chia3, BSc (Hon), Judith Wishart2, BSc (Hon), Adam M Deane2,4, MBBS, PhD, FCICM, FRACP; Chris K Rayner1,2, MBBS, PhD, FRACP, Michael Horowitz2, MBBS, PhD, FRACP, Richard L Young2,3, BSc (Hon), PhD 1 Department of Gastroenterology and Hepatology, Level 7, Royal Adelaide Hospital, N Terr, Adelaide, S Australia, Australia 5000.; 2 Discipline of Medicine, University of Adelaide, Royal Adelaide Hospital, Level 6 Eleanor Harrald Bldg, N Terr, Adelaide, S Australia, Australia 5000.; 3 Discipline of Medicine, Nerve-Gut Research Laboratory, Level-1 Hanson Institute, Frome Rd, Adelaide, S Australia, 5000, Australia.; 4 Intensive Care Unit, Level 4, Emergency Services Bldg, Royal Adelaide Hospital, N Terr, Adelaide, S Australia, Australia 5000.

Context: Intestinal glucose absorption is mediated by sodium dependent glucose transporter-1 (SGLT-1) and glucose transporter-2 (GLUT2), which are linked to sweet taste receptor (STR) signaling and incretin responses. Objective: This study aimed to examine intestinal glucose absorption in morbidly obese humans, and its relationship to the expression of STR and glucose transporters, glycaemia, and incretin responses. Design/Setting/Participants: 17 non-diabetic, morbidly obese subjects (BMI:48⫾4kg/m2) and 11 lean controls (BMI:25⫾1kg/m2) underwent endoscopic duodenal biopsies before and after a 30min intraduodenal glucose infusion (30g glucose & 3g 3-O-methylglucose (3-OMG)). Main Outcome Measures: Blood glucose and plasma concentrations of 3-OMG, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), insulin, and glucagon were measured over 270min. Expression of duodenal SGLT-1, GLUT2 and STR was quantified by PCR. Results: The rise in plasma 3-OMG (P⬍0.001) and blood glucose (P⬍0.0001) were greater in obese than lean subjects. Plasma 3-OMG correlated directly with blood glucose (r⫽0.78, P⬍0.01). In response to intraduodenal glucose, plasma GIP (P⬍0.001), glucagon (P⬍0.001), and insulin (P⬍0.001) were higher, but GLP-1 (P⬍0.001) was less, in the obese compared to lean. Expression of SGLT-1 (P⫽0.035), but not GLUT2 or T1R2, was higher in the obese, and related to peak plasma 3-OMG (r⫽0.60, P⫽0.01), GIP (r⫽0.67, P⫽0.003) and insulin (r⫽0.58, P⫽0.02). Conclusions: In morbid obesity, proximal intestine glucose absorption is accelerated and related to increased SGLT-1 expression, leading to an incretin-glucagon profile promoting hyperinsulinemia and hyperglycemia. These findings are consistent with the concept that accelerated glucose absorption in the proximal gut underlies the foregut theory of obesity and type 2 diabetes.

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he causes of the obesity crisis over the last 30 years remain unclear but are thought to relate primarily to increased consumption of energy-dense foods and bever-

ages and decreased physical activity. Increased caloric intake is the leading culprit for weight gain, with ‘fast food’ meals (1) and sweetened beverages (2). Although it has

ISSN Print 0021-972X ISSN Online 1945-7197 Printed in U.S.A. Copyright © 2014 by the Endocrine Society Received August 7, 2014. Accepted November 18, 2014.

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doi: 10.1210/jc.2014-3144

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Glucose Absorption in Obesity


hyperinsulinaemia, insulin resistance, and lipogenesis observed in obesity. The current study examines intestinal glucose absorption in morbidly obese subjects, and its relationship to glycaemia, incretin hormones, and the expression of intestinal STR (T1R2), SGLT-1 and GLUT2 transcripts in comparison to healthy lean subjects.

Figure 1. Outline of study protocol.

been suggested that intestinal absorption of nutrients is more rapid and efficient in obese than lean humans predisposing to both weight gain and type 2 diabetes (3), this issue has attracted little attention. Recently, we reported that the rise in blood glucose after a carbohydrate-containing drink was greater in obese compared to lean subjects (4). While this is likely to partly reflect the effect of insulin resistance related to obesity, it is also potentially attributable to accelerated intestinal glucose absorption. The outcome of animal experiments indicates that intestinal glucose absorption is strongly regulated in the presence of luminal glucose or sweet tastants via activation of intestinal sweet taste receptors (STRs), which are heterodimers of G-protein-coupled receptors T1R2 and T1R3 (5, 6). Upon activation, STRs have the capacity to promote increased availability of intestinal sodium-dependent glucose cotransporter-1 (SGLT-1) (5, 7) and glucose transporter-2 (GLUT2) (8), the critical transporters for glucose absorption. Activation of STRs and/or glucose transporters (GTs) has also been reported to trigger the release of the ‘incretin’ hormones glucagon-like-peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) (5, 7, 9, 10). Genetically modified mice that have 7-fold higher levels of SGLT-1 protein exhibit an increased rate of intestinal glucose absorption and develop visceral obesity (11, 12). While there are no information regarding the expression of either STRs or SGLT-1 in obese humans, increased accumulation of both apical and basolateral membrane GLUT2 during fasting has been reported in morbidly obese subjects with type 2 diabetes (13). Although noncaloric sweeteners fail to trigger GLP-1 release acutely in humans (14), lactisole (inhibitor of STRs) (15) or phloridzin (the competitive inhibitor of SGLT-1) (16) attenuate the GLP-1 response to enteral glucose. Together, these observation support the potential role of the intestinal STR/GTs system in the regulation of glucose absorption, glycaemia, and body weight. We hypothesized that morbid obesity is associated with increased intestinal glucose absorption, resulting from dysregulation of the intestinal STR system, GTs or both. Increased intestinal glucose absorption may, in turn, lead to incretin responses that promote the hyperglycaemia,

Materials and Methods Subjects. Morbidly obese subjects (BMI ⬎ 35 kg/m2) and healthy volunteers (BMI ⬍ 28 kg/m2) were recruited for the study between January 2013 and November 2013 from advertisement placed at the hospital notice boards and the Obesity Clinic of the Royal Adelaide Hospital. Subjects were excluded if they were aged less than 18 years, pregnant, known to have diabetes, had a contraindication to endoscopy, a history of surgery on the stomach, or small intestine, were receiving drugs known to alter platelet aggregation or thrombus formation, had an International Normalized Ratio (INR) ⬎ 1.5, platelet count ⬍ 50 000 or abnormal plasma creatinine. Healthy subjects were selected to match the age of the obese subjects (Table 1). The study was approved by the Human Research Ethics Committee of the Royal Adelaide Hospital. Informed written consent was obtained from all subjects. Protocol. The study protocol is summarized in Figure 1. Each subject was studied after an overnight fast and at 0900h. Female subjects were studied exclusively during the follicular phrase of the menstrual cycle to limit the potential confounding effects on gut hormone concentrations. An intravenous (IV) cannula was inserted in each subject for blood sampling. A small diameter (5.3 mm) video endoscope (GIF-XP160, Olympus, Tokyo, Japan) was passed via an anesthetized nostril into the second part of the duodenum from which mucosal biopsies were collected using standard biopsy forcep and placed into RNAlater (Qiagen, Sydney, NSW, Australia). At T ⫽ 0min, an intraduodenal infusion of 30 g glucose and 3 g of the glucose absorption marker 3 O-methyglucose (3-OMG, Sigma-Aldrich, St Louis, MO, USA) was commenced via the biopsy channel of the endoscope. The infusion was maintained at a constant rate of 4 kcal/min (ie, 1 g

Table 1. Demographics and clinical characteristics of morbidly obese and healthy subjects. Lean healthy Morbidly obese subjects (n ⴝ 11) subjects (n ⴝ 17) Age (years) Male:Female ratio Glycated haemoglobin (HbA1c, %) Body mass index (kg/m2)

44 ⫾ 6 10:1 5.6 ⫾ 0.4

45 ⫾ 3 5:12 * 5.8 ⫾ 0.3

25 ⫾ 1

48 ⫾ 4 *

Data are mean ⫹/- SD, * P ⬍ 0.01, vs. healthy subjects


doi: 10.1210/jc.2014-3144

glucose/min) for 30 minutes. At T ⫽ 30 minutes additional biopsies were collected, and the endoscope was then removed. Blood samples (20 mL) were taken at baseline (fasting), then every 30 minutes over 270 minutes for determination of blood glucose and plasma concentrations of 3-OMG, GLP-1, GIP, insulin and glucagon. For blood glucose and plasma of 3-OMG and insulin, more frequent blood sampling was performed within the first 90 minutes (Figure 1). Blood samples for measurements of glycated hemoglobin (HbA1c), glucagon, GIP and GLP-1 were collected into chilled 5-mL ethylene-di-amine-tetraacetic acid tubes. Both serum and plasma were separated by centrifugation (3200 rpm for 15 mins at 4°C). Samples were then stored at –70°C until assayed. Absolute expression of SGLT-1, GLUT2 and T1R2 transcripts was quantified from duodenal mucosal biopsies by quantitative real time reverse transcriptase PCR (RT-PCR). In each subject, the degree of insulin resistance was estimated at baseline by homeostasis model assessment (HOMA) (17). Insulin resistance score (HOMA-IR) was computed using the formula: fasting plasma glucose (mmol/l) times fasting serum insulin (mU/l) divided by 22.5. Low HOMA-IR values are indicative of high insulin sensitivity, whereas high HOMA-IR values indicate low insulin sensitivity (ie, insulin resistance) (17).

Measurements Intestinal glucose absorption. Plasma concentrations of 3-OMG were measured using high performance exchange chromatography with an assay sensitivity of 0.010 mmol/L (18 –20). Peak and time to peak 3-OMG concentrations, as well as the area under the curve (AUC) at 120 minutes (AUC0 –120min) and 270 minutes (AUC0 –270min), were quantified. Glycated hemoglobin (HbA1c) and blood glucose. HbA1c was measured using cation exchange high performance liquid chromatography. Blood glucose was measured at the bedside using a portable glucometer (Medisense Optimum, USA) (19, 20). Plasma insulin, GIP, GLP-1 and glucagon (19, 20). Plasma insulin was measured by enzyme-linked immunosorbent assay (ELISA) (ELISA, Diagnostic Systems Laboratories, Inc., Webster, TX) with assay sensitivity of 0.26 mU/L and coefficient of variation was 2.6% within, and 6.2% between, assays. Total plasma GIP was measured by radioimmunoassay (RIA) where the sensitivity was 2 pmol/L, and both coefficients of variation were 15%. Total plasma GLP-1 was measured by RIA where the sensitivity was 1.5 pmol/L, and coefficient of variation was 17% within, and 18% between, assays. Plasma glucagon was measured by RIA (GL-32K, Millipore, Billerica, MA) where the sensitivity was 20pg/ml and coefficient of variation was 15% within, and 12% between, assays. Gene expression of intestinal STRs and GTs. RNA was extracted from duodenal mucosal biopsies using RNeasy Mini or Micro kits (Qiagen) following manufacturer instructions with RNA yield and quality determined by NanoDrop (NanoDrop Technologies, USA). Quantitative real time reverse transcriptase PCR (RT-PCR) was used to determine absolute levels of T1R2, SGLT-1 and GLUT2 transcript in 25 ng of total human RNA. As the absolute number of transcripts was to be quantified, comparison to housekeeper genes was not required. Primers for these targets were used as validated primer assays (QuantiTect, Qia-


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gen; Supplemental Table 1) or for absolute PCR standards, designed from target sequences obtained from the NCBI nucleotide database (Supplemental Table 2) (6, 20). RT-PCR was performed on a Chromo4 (MJ Research) real time instrument attached to a PTC-200 Peltierthermal cycler (MJ Research) using a QuantiTect SYBR Green one-step RT-PCR kit (Qiagen) according to the manufacturer’s specifications. Cycling conditions were reverse transcription at 50°C for 30 minutes, 95°C for 15min, then 45 cycles PCR at 94°C for 15s, followed by 55°C for 30s and 72°C for 30s. A melt curve was generated (60 –95°C) to verify the specificity and identity of the RT-PCR products; product size was confirmed on an electrophoresis gel (Bio-Rad Laboratories). Each assay was performed in triplicate and included internal no-template and no-RT controls. Minimum detectable levels for SGLT-1 and T1R2 transcripts were 2 ⫻ 103 copies; when levels were not detected, this threshold value was substituted (20).

Data analysis The sample size of 17 obese and 11 lean subjects was based on our previous work (6, 21, 22) comparing duodenal RNA levels in type 2 diabetes, critically ill and healthy subjects, giving 80% power to detect a difference in RNA expression between groups of 20%, accepting an ␣ error of 0.05. The primary endpoints were differences in plasma 3-OMG concentrations and the number of STR and TGs transcripts between the groups. Secondary outcomes included the differences in incretin responses, glycaemia, HOMA-IR scores and relationships between these variables between the groups. Differences in blood glucose, plasma 3-OMG, insulin, GIP, GLP-1 and glucagon concentrations between morbidly obese and healthy subjects were assessed by two-way repeated measures analysis of variance (ANOVA) with treatment and time as factors. In addition, the differences in the copy numbers of intestinal SGLT-1, T1R2 and GLUT2 RNA transcript between the groups were compared using unpaired t-tests. Changes in the expression of intestinal SGLT-1, T1R2 and GLUT2 among the groups were assessed by paired t-tests. Linear relationships were evaluated using Spearman’s Rank Test. Difference in gender between the groups was analyzed by Fisher’s exact test. Analyses were performed using GraphPad Prism statistical software, version 6 (GraphPad Software Inc., La Jolla, CA, USA). Significance was accepted at P ⬍ .05; data are presented as mean ⫾ SD.

Results Seventeen morbidly obese subjects and 11 lean healthy subjects were studied - demographics in the two groups, including differences between them, are summarized in Table 1. All subjects were Caucasian, tolerated the procedure well and had a normal macroscopic appearance of the small intestinal mucosa. There were no complications related to the endoscopy, duodenal mucosal biopsy, intraduodenal glucose infusion or blood sampling. Intestinal glucose absorption Both the rate of plasma 3-OMG increase and peak concentrations were higher in morbidly obese than healthy


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subjects (Figure 2A). The time taken to reach to peak plasma 3-OMG after the intra-duodenal glucose infusion was also earlier in morbidly obese subjects than healthy subjects (54 ⫾ 3 vs. 72 ⫾ 8 minutes, P ⫽ .01). However, neither integrated plasma 3-OMG concentrations over 120 minutes (obese vs. healthy AUC0 –120min: 30.9 ⫾ 1.5 vs. 27.5 ⫾ 2.6 mmol/L.min; P ⫽ .24) nor 270 minutes (AUC0 –270min: 65.4 ⫾ 2.8 vs. 66.5 ⫾ 4.6 mmol/L.min, respectively; P ⫽ .68) differed between the groups, though plasma 3-OMG had not returned to baseline (or zero) at 270min. Glycaemia and plasma insulin, glucagon, GIP and GLP-1 Fasting blood glucose concentrations tended to be higher in the morbidly obese than the healthy subjects (6.1 ⫾ 0.1 vs. 5.8 ⫾ 0.2 mmol/L, P ⫽ .14, Figure 2B). Fasting plasma insulin (P ⬍ .001) and GIP (P ⬍ .001) were higher in obese subjects (Figure 3C,E), while fasting plasma GLP-1 and glucagon concentrations were comparable (Figure 3D,F). Blood glucose and plasma insulin, GIP and GLP-1 concentrations were increased markedly in morbidly obese


and healthy subjects following intra-duodenal glucose infusion (Figure 2). Plasma glucagon concentrations increased in morbidly obese subjects, but not healthy subjects. The increases, reflected by peak concentrations, in blood glucose and plasma concentrations of insulin, glucagon and GIP after glucose infusion were greater (Figure 2B,C,E,F), while the rise in GLP-1 was less, in the morbidly obese than healthy subjects (Figure 2D). Duodenal transcript levels of sweet taste receptor and glucose transporters SGLT-1 was the most abundant transcript in the duodenal mucosa of fasted obese and healthy subjects, with ⬃30% lower levels of GLUT2 (P ⬍ .001), and much lower levels of T1R2 (P ⬍ .001, Figure 3). Fasting or baseline transcript levels of SGLT-1, but not GLUT2 or T1R2, were higher in morbidly obese subjects when compared to healthy subjects (P ⫽ .04, Figure 3B). In obese subjects, SGLT-1 transcript levels decreased after duodenal glucose infusion, an effect that was not evident in healthy subjects (P ⬍ .001, Figure 3B). Expressions of GLUT-2 and T1R2 were not affected by duodenal glucose in either group.

Figure 2. Intestinal glucose absorption, reflected by changes in plasma 3-OMG concentrations, blood glucose and plasma concentration of insulin, GLP-1, GIP and glucagon during fasting and in response to intra-duodenal glucose infusion in morbidly obese and healthy subjects. *P ⬍ .001, vs. healthy subjects.


doi: 10.1210/jc.2014-3144


Figure 3. Changes in the mRNA expression of intestinal sweet taste receptor (T1R2) and glucose transporters (SGLT-1 and GLUT-2) in morbidly obese and healthy subjects.

Relationships between intestinal glucose absorption, glycaemia, incretin responses and HOMA-IR Peak plasma 3-OMG concentrations correlated closely with blood glucose concentrations in obese (r ⫽ 0.70, P ⫽ .0006; Figure 4A) and healthy (r ⫽ 0.58, P ⫽ .05) subjects. In obese subjects, peak plasma 3-OMG concentrations were related directly to peak plasma concentrations of GIP (r ⫽ 0.70, P ⫽ .001) and insulin (r ⫽ 0.61, P ⫽ .01) but inversely to peak plasma GLP-1 (r⫽-0.51, P ⫽ .04) concentrations (Figure 4B-D). No relationships between peak plasma 3-OMG and incretin hormone concentrations were observed in healthy subjects. HOMA-IR scores were higher in obese than healthy subjects (3.9 ⫾ 0.5 vs. 1.4 ⫾ 0.3, P ⬍ .001). In the obese, but not healthy, subjects, HOMA-IR scores correlated modestly with peak (r ⫽ 0.59, P ⫽ .02) and integrate (AUC0 –270min: r ⫽ 0.61, P ⫽ .01) plasma concentrations of 3-OMG. Relationships between intestinal glucose absorption, transporters and incretin responses Expression of duodenal SGLT-1 transcripts during fasting correlated positively with GLUT2 transcript levels

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in healthy subjects, (r ⫽ 0.71; P ⫽ .009) but not in the obese. There were no relationships apparent between transcript levels of T1R2 and SGLT-1 or GLUT2 in either group. In contrast, in fasted obese subjects, SGLT-1 transcript levels correlated positively with peak plasma 3-OMG (r ⫽ 0.60; P ⫽ .01), GIP (r ⫽ 0.67; P ⫽ .003) and insulin (r ⫽ 0.58; P ⫽ .015) (Figure 5A-C). Both at baseline and after intra-duodenal glucose, transcript copy number for T1R2 or GLUT2 did not correlate with peak plasma concentrations, or AUC0 – 270min, of 3-OMG, GIP, GLP-1 or

insulin.

Discussion This study has demonstrated for the first time that the rate of glucose absorption is increased in the proximal intestine of morbidly obese subjects, and that this is associated with increased duodenal expression of the apical glucose transporter SGLT-1. Moreover, we showed that these changes in glucose absorption in the proximal intestine are associated with augmented release of both GIP and glucagon, but a reduction in GLP-1, which are likely to account for the observed hyperglycaemia and hyperinsulinaemia. We also revealed relationships of peak plasma 3-OMG with blood glucose, plasma GIP and insulin concentrations, and of duodenal SGLT-1 transcript levels with peak plasma concentrations of 3-OMG, GIP and insulin. Overall, these observations suggest that dysregulated glucose absorption in the proximal gut of the morbidly obese is an important factor in the development of obesity and the related type 2 diabetes, and are also consistent with the ‘foregut theory’ which proposes that, exposure of nutrient

Figure 4. Relationships of blood glucose, plasma GIP, insulin, GLP-1 and intestinal glucose absorption in morbidly obese subjects.


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It should be recognized that the higher postprandial blood glucose in obese subjects will facilitate incretinmediated insulin release given that these were above the ‘threshold’ of ⬃8 mmol/L (29, 34). This imbalanced release of GIP and GLP-1 potentially leads to a vicious circle of hyperglycemia, hyperinsulinaemia, hyperglucagonaemia resulting in insulin resistance, visceral obesity and weight gain. The observed relationship between the rate of intestinal glucose absorption and the rise in plasma GIP in morbidly obese subjects during acute hyperglycaemia is consistent with our recent data in healthy volunteers (35), and is consistent with the concept that accelerated glucose absorption and augmented GIP release are the “unknown factors” responsible for the “foregut theory” of obesity and type 2 diabeFigure 5. Relationships of intestinal glucose absorption, GIP, insulin and intestinal glucose tes. Evidence of higher basal and nutransporter SGLT-1 expression in morbidly obese subjects. trient stimulated GIP levels in obese and glucose-intolerant, than in lean, to the foregut leads to the release of a ‘factor’ that prosubjects are in keeping with this (36, 37). The reduced moted hyperglycemia, hyperinsulinaemia, insulin resisplasma GLP-1 response in conjunction with accelerated tance, and lipogenesis (23, 24). Avoidance of such expoglucose absorption in the proximal intestine in morbidly sure, as is the case with RYGB or, more recently, an endoobese subjects adds support to the “foregut theory”. It is luminal sleeve, has been proposed as a ‘bypass’ mechanism likely that diminished glucose-stimulated GLP-1 release in that leads to improvement in glycaemia and facilitates morbidly obese subjects reflects a reduction in more distal weight loss (25, 26). intestinal exposure of glucose, as a result of increased Physiologically, enhanced glucose absorption in the proximal absorption. Given the documented antidiabetic proximal gut would increase blood glucose concentrations, and trigger the release of gut hormones involved in and appetite suppressing effects of GLP-1 (29), this attenthe incretin response (28, 29). Direct infusion of glucose uated release may contribute to hyperglycemia and weight into the duodenum at a constant rate (4 kcal/min) allows gain in obese subjects. Our findings indicate that intestinal glucose transportits absorption to be reliably assessed, independent of the ers, rather than STRs, play a more important role in the substantial interindividual variations in gastric emptying, regulation of glucose absorption and incretin responses in including the potential effects of obesity (27), and is a major strength of the current study. The secretion of GIP the morbidly obese humans. First, there were no relationis likely to be a consequent of the absorption of glucose via ships evident between STR (T1R2) transcript levels and intestinal SGLT-1 (7, 30, 31), as elevation in blood glucose the expression of either glucose transporters, the rate of does not, per se, increase plasma GIP (29). However, the glucose absorption or incretin responses. This contrasts combination of higher GIP, but lower GLP-1, concentra- with our recent data in healthy subjects and nonobese tions is likely to be responsible for the augmented releases patients with type 2 diabetes, where changes in duodenal of insulin and glucagon, leading to the demonstrated in- T1R2 transcripts in response to duodenal glucose infusion sulin resistance and, potentially, increased lipogenesis (26, were positively associated with glucose absorption and 28, 31–33). Although glucose-dependent insulin release is GIP levels (38). Second, in morbidly obese subjects, we stimulated by both GIP and GLP-1, GIP has a stimulatory showed, for the first time, that duodenal expression of and GLP-1 a suppressive, effect on glucagon release (29). SGLT-1 was up-regulated at baseline, and rapidly modu-


doi: 10.1210/jc.2014-3144

lated in response to luminal glucose. In contrast, luminal glucose-induced changes in SGLT-1 and GLUT2 were not evident in healthy subjects in the current study, consistent with our previous observations (21). Our results contrast with those of Ait-Omar and colleagues, who reported increased GLUT2 protein expression in obese type 2 diabetic subjects (13). The lack of change in duodenal GLUT2 transcripts in the current study may reflect to differences in ‘diabetic’ status between the cohorts and/or a complex relationship between expression of GLUT2 transcript and protein. Third, and possibly most importantly, increased expression of SGLT-1 has been shown to be associated with accelerated glucose absorption and dysregulated incretin responses. The role of intestinal STRs and GTs in the pathogenesis of human obesity should be explored further by the use of specific inhibitors such as lactisole (STR inhibitor) (15) and/or phloridzin (competitive SGLT-1 inhibitor) (30). The outcomes of the current study highlight the potential for therapeutic intervention in the treatment of obesity and type 2 diabetes with (i) pharmaceutical agent(s) that inhibit glucose transporters, especially intestinal SGLT-1, and/or (ii) endoscopic devices that prevent exposure of the proximal gut to carbohydrate, such as the endo-luminal sleeve. These approaches may confer advantages similar to that of RYGB surgery but without the associated morbidity. There are several limitations in the current study which should be recognized. First, transcript levels of SGLT-1, GLUT2 and T1R2 were quantified, rather than protein. While it is unknown whether changes in transcript accurately reflect the activity of mucosal STRs and GLUT2 proteins, the positive relationship between glucose absorption and SGLT-1 transcript levels in obese subjects is indicative of strong links between transcript and protein changes for SGLT-1. Second, the lack of change in expression of T1R2 and GLUT2 may potentially relate to the relatively short duration of glucose exposure, or to changes that occur subsequent to acute elevations in glycaemia, as we have described for T1R2 (38). The infusion duration of 30 minutes was chosen in part because of concerns that the prolonged presence of an endoscope may prove uncomfortable for subjects and the rate of infusion was at the upper end of the normal range of gastric emptying of glucose. Given that all subjects tolerated the study well, it would now be of interest to determine the effects of more prolonged periods of glucose infusion on the expression of intestinal STRs and GTs. In addition, as changes in SGLT1 protein levels after an intra-luminal glucose challenge may take up to 3 hours to manifest (39), extending assessments of transcriptional activity and protein levels of intestinal STRs and GTs over a longer period


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(ie, 4 hours) may reveal differences. Third, since dietary intakes were not systematically recorded, there may be potential for differences in habitual dietary intake to alter expression of STRs and GTs. However, given there were no significant differences in the fasting expression of T1R2 and GLUT2 between the groups, the overall impact of dietary intake is likely to be minimal. Fourth, there was a gender difference between the obese subjects (female predominant) and healthy controls (male predominant). While the impact of gender on the rate of intestinal glucose absorption has not been formally assessed, data derived from studies using an oral glucose tolerance test (OGTT) suggest that gender has no impact on glucose absorption (40, 41), and our earlier studies also indicate that gender does not influence the expression of intestinal STRs or GTs (23, 38). Finally, although diabetes was not formally excluded by a screening OGTT in all subjects, HbA1c was less than 6.5%, and fasting blood glucose less than 7 mmol/L, without differences between the groups. In conclusion, glucose absorption in the proximal intestine is accelerated in morbid obesity, in association with increased expression of SGLT-1 transcripts and an incretin profile that has the potential to promote both hyperglycemia and hyperinsulinemia. While these findings support the concept that accelerated glucose absorption in the proximal gut may underlay the foregut theory of obesity and type 2 diabetes, further investigation, including the use of specific inhibitors of STR or GTs, is required.

Acknowledgments Address all correspondence and requests for reprints to: Associate Professor Nam Nguyen, Department of Gastroenterology and Hepatology, Royal Adelaide Hospital, North Terrace, Adelaide, South Australia, 5000, Australia., Email: [email protected], Phone: ⫹61 8 8222 4000 Fax: ⫹61 8 8222 5885. Disclosure Summary: The authors have nothing to disclose This work was supported by .

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9

Regulation of Intestinal Glucose Absorption by Ion Channels and Transporters.

Effects of intraduodenal hydroxycitrate on glucose absorption, incretin release, and glycemia in response to intraduodenal glucose infusion in health and type 2 diabetes: A randomised controlled trial.

Letter: Intestinal absorption of glucose.

β-cell function, incretin effect, and incretin hormones in obese youth along the span of glucose tolerance from normal to prediabetes to type 2 diabetes.

Effects of sitagliptin on glycemia, incretin hormones, and antropyloroduodenal motility in response to intraduodenal glucose infusion in healthy lean and obese humans and patients with type 2 diabetes treated with or without metformin.

Intestinal Lipoprotein Secretion: Incretin-Based Physiology and Pharmacology Beyond Glucose.

Muscle insulin resistance in uremic humans: glucose transport, glucose transporters, and insulin receptors.

Estriol blunts postprandial blood glucose rise in male rats through regulating intestinal glucose transporters.

Bariatric surgery in morbidly obese insulin resistant humans normalises insulin signalling but not insulin-stimulated glucose disposal.

Is there a role for lymph in intestinal glucose absorption?

Measurement of the incretin hormones: glucagon-like peptide-1 and glucose-dependent insulinotropic peptide.

Relationship between gut hormones and glucose homeostasis after bariatric surgery.

Continuous glucose monitoring and its relationship to hemoglobin A1c and oral glucose tolerance testing in obese and prediabetic youth.

Use of [3-3H]glucose and [6-14C]glucose to measure glucose turnover and glucose metabolism in humans.

Glucose Transporters in Cardiac Metabolism and Hypertrophy.

Intestinal gluconeogenesis is crucial to maintain a physiological fasting glycemia in the absence of hepatic glucose production in mice.

Letter: Glucose absorption and diabetes.

Laparoscopic greater curvature plication in morbidly obese women with type 2 diabetes: effects on glucose homeostasis, postprandial triglyceridemia and selected gut hormones.

Glucose transporters: structure, function, and regulation.

Avian and Mammalian Facilitative Glucose Transporters.

Effect of high sugar intake on glucose transporter and weight regulating hormones in mice and humans.

Regulation of glucose transporters in diabetes.

Effect of diabetes on glucoregulation. From glucose transporters to glucose metabolism in vivo.

Contribution of postprandial insulin and glucose to glucose disposal in normal and insulin-resistant obese subjects.