PET-CT reveals increased intestinal glucose uptake after gastric surgery

Intestinal glucose excretion: A potential mechanism for glycemic control

The gut has been increasingly recognized in recent years as a pivotal organ in the maintenance of glucose homeostasis. Specifically, the profound and enduring improvement in glucose metabolism achieved through metabolic surgery to modify the anatomy of the gut has prompted scholars to acknowledge that the most effective strategy for treating type 2 diabetes mellitus (T2DM) involves the gut. The mechanisms underlying the regulation of glucose metabolism by the gut encompass gut hormones, bile acids, intestinal gluconeogenesis, gut microbiota, and signaling interactions between the gut and other organs (liver, brain, adipose, etc.). Recent studies have also revealed a novel phenomenon of glucose lowering through the gut: metabolic surgery and metformin promote the excretion of glucose from the circulation into the intestinal lumen by enterocytes. However, there is still limited understanding regarding the underlying mechanisms of intestinal glucose excretion and its contribution to glycemic control. This article reviews current research on intestinal glucose excretion while focusing on its role in T2DM management as well as potential mechanisms.

Bariatric surgery alters mitochondrial function in gut mucosa

BACKGROUND

The obesity pandemic has worsened global disease burden, including type 2 diabetes, cardiovascular disease, and cancer. Metabolic/bariatric surgery (MBS) is the most effective and durable obesity treatment, but the mechanisms underlying its long-term weight loss efficacy remain unclear. MBS drives substrate oxidation that has been linked to improvements in metabolic function and improved glycemic control that are potentially mediated by mitochondria-a primary site of energy production. As such, augmentation of intestinal mitochondrial function may drive processes underlying the systemic metabolic benefits of MBS. Herein, we applied a highly sensitive technique to evaluate intestinal mitochondrial function ex vivo in a mouse model of MBS.

METHODS

Mice were randomized to surgery, sham, or non-operative control. A simplified model of MBS, ileal interposition, was performed by interposition of a 2-cm segment of terminal ileum into the proximal bowel 5 mm from the ligament of Treitz. After a four-week recovery period, intestinal mucosa of duodenum, jejunum, ileum, and interposed ileum were assayed for determination of mitochondrial respiratory function. Citrate synthase activity was measured as a marker of mitochondrial content.

RESULTS

Ileal interposition was well tolerated and associated with modest body weight loss and transient hypophagia relative to controls. Mitochondrial capacity declined in the native duodenum and jejunum of animals following ileal interposition relative to controls, although respiration remained unchanged in these segments. Similarly, ileal interposition lowered citrate synthase activity in the duodenum and jejunum following relative to controls but ileal function remained constant across all groups.

CONCLUSION

Ileal interposition decreases mitochondrial volume in the proximal intestinal mucosa of mice. This change in concentration with preserved respiration suggests a global mucosal response to segment specific nutrition signals in the distal bowel. Future studies are required to understand the causes underlying these mitochondrial changes.

Glucose metabolism after bariatric surgery: implications for T2DM remission and hypoglycaemia

Although promising therapeutics are in the pipeline, bariatric surgery (also known as metabolic surgery) remains our most effective strategy for the treatment of obesity and type 2 diabetes mellitus (T2DM). Of the many available options, Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG) are currently the most widely used procedures. RYGB and VSG have very different anatomical restructuring but both surgeries are effective, to varying degrees, at inducing weight loss and T2DM remission. Both weight loss-dependent and weight loss-independent alterations in multiple tissues (such as the intestine, liver, pancreas, adipose tissue and skeletal muscle) yield net improvements in insulin resistance, insulin secretion and insulin-independent glucose metabolism. In a subset of patients, post-bariatric hypoglycaemia can develop months to years after surgery, potentially reflecting the extreme effects of potent glucose reduction after surgery. This Review addresses the effects of bariatric surgery on glucose regulation and the potential mechanisms responsible for both the resolution of T2DM and the induction of hypoglycaemia.

Regulation of body weight: Lessons learned from bariatric surgery

BACKGROUND

Bariatric or weight loss surgery is currently the most effective treatment for obesity and metabolic disease. Unlike dieting and pharmacology, its beneficial effects are sustained over decades in most patients, and mortality is among the lowest for major surgery. Because there are not nearly enough surgeons to implement bariatric surgery on a global scale, intensive research efforts have begun to identify its mechanisms of action on a molecular level in order to replace surgery with targeted behavioral or pharmacological treatments. To date, however, there is no consensus as to the critical mechanisms involved.

SCOPE OF REVIEW

The purpose of this non-systematic review is to evaluate the existing evidence for specific molecular and inter-organ signaling pathways that play major roles in bariatric surgery-induced weight loss and metabolic benefits, with a focus on Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG), in both humans and rodents.

MAJOR CONCLUSIONS

Gut-brain communication and its brain targets of food intake control and energy balance regulation are complex and redundant. Although the relatively young science of bariatric surgery has generated a number of hypotheses, no clear and unique mechanism has yet emerged. It seems increasingly likely that the broad physiological and behavioral effects produced by bariatric surgery do not involve a single mechanism, but rather multiple signaling pathways. Besides a need to improve and better validate surgeries in animals, advanced techniques, including inducible, tissue-specific knockout models, and the use of humanized physiological traits will be necessary. State-of-the-art genetically-guided neural identification techniques should be used to more selectively manipulate function-specific pathways.

Gut adaptation after gastric bypass in humans reveals metabolically significant shift in fuel metabolism

OBJECTIVE

Roux-en-Y gastric bypass surgery (RYGB) is among the most effective therapies for obesity and type 2 diabetes, and intestinal adaptation is a proposed mechanism for these effects. It was hypothesized that intestinal adaptation precedes and relates to metabolic improvement in humans after RYGB.

METHODS

This was a prospective, longitudinal, first-in-human study of gene expression (GE) in the "Roux limb" (RL) collected surgically/endoscopically from 19 patients with and without diabetes. GE was determined by microarray across six postoperative months, including at an early postoperative (1 month ± 15 days) time point.

RESULTS

RL GE demonstrated tissue remodeling and metabolic reprogramming, including increased glucose and amino acid use. RL GE signatures were established early, before maximal clinical response, and persisted. Distinct GE fingerprints predicted concurrent and future improvements in HbA1c and in weight. Human RL exhibited GE changes characterized by anabolic growth and shift in metabolic substrate use. Paradoxically, anabolic growth in RL appeared to contribute to the catabolic state elicited by RYGB.

CONCLUSIONS

These data support a role for a direct effect of intestinal energy metabolism to contribute to the beneficial clinical effects of RYGB, suggesting that related pathways might be potential targets of therapeutic interest for patients with obesity with or without type 2 diabetes.

Foregut Exclusion Enhances Incretin and Insulin Secretion After Roux-en-Y Gastric Bypass in Adults With Type 2 Diabetes

INTRODUCTION

Patients with type 2 diabetes experience resolution of hyperglycemia within days after Roux-en-Y gastric bypass (RYGB) surgery. This is attributed, in part, to enhanced secretion of hindgut factors following exclusion of the gastric remnant and proximal intestine during surgery. However, evidence of the mechanisms of remission remain limited due to the challenges of metabolic evaluation during the early postoperative period. The purpose of this investigation was to determine the role of foregut exclusion in the resolution of type 2 diabetes after RYGB.

METHODS

Patients with type 2 diabetes (n = 15) undergoing RYGB had a gastrostomy tube (G-tube) placed in their gastric remnant at time of surgery. Patients were randomized to receive a mixed meal tolerance test via oral or G-tube feeding immediately prior to and 2 weeks after surgery in a repeated measures crossover design. Plasma glucose, insulin, C-peptide, incretin responses, and indices of meal-stimulated insulin secretion and sensitivity were determined.

RESULTS

Body weight, fat mass, fasting glucose and insulin, and circulating lipids were significantly decreased 2 weeks after surgery. The glycemic response to feeding was reduced as a function of total area under the curve but not after adjustment for the reduction in fasting glucose. Oral feeding significantly enhanced insulin and incretin secretion after RYGB, which was entirely ablated by G-tube feeding.

CONCLUSION

Foregut exclusion accounts for the rise in incretin and insulin secretion but may not fully explain the early improvements in glucose metabolism after RYGB surgery.

The Role of Positron Emission Tomography in Bariatric Surgery Research: a Review

Bariatric surgery, initially understood as restricting or bypassing the amount of food that reaches the stomach to reduce food intake and/or increase malabsorption of food to promote weight loss, is now recognized to also affect incretin signaling in the gut and promote improvements in system-wide metabolism. Positron emission tomography (PET) is an imaging technique whereby patients are injected with picomolar concentrations of radioactive molecules, below the threshold of having physiological effects, to measure spatial distributions of blood flow, metabolism, receptor, and enzyme pharmacology. Recent advances in both whole-body PET imaging and radioligand development will allow for novel research that may help clarify the roles of peripheral and central receptor/enzyme systems in treating obesity with bariatric surgery.

Glycemic Patterns Are Distinct in Post-Bariatric Hypoglycemia After Gastric Bypass (PBH-RYGB)

CONTEXT

Severe hypoglycemia with neuroglycopenia, termed post-bariatric hypoglycemia (PBH). typically occurs postprandially, but it is also reported after activity or mid-nocturnally.

OBJECTIVE

To quantify glycemia, glycemic variability, and magnitude/duration of low sensor glucose (SG) values in patients with PBH after Roux-en-Y gastric bypass (PBH-RYGB).

METHODS

This retrospective analysis of data from an academic medical center included individuals with PBH-RYGB (n = 40), reactive hypoglycemia without gastrointestinal surgery (Non-Surg Hypo, n = 20), prediabetes (Pre-DM, n = 14), newly diagnosed T2D (n = 5), and healthy controls (HC, n = 38). Masked continuous glucose monitoring (Dexcom G4) was used to assess patterns over 24 hours, daytime (6 am-midnight) and nighttime (midnight-6 am). Prespecified measures included mean and median SG, variability, and percent time at thresholds of sensor glucose.

RESULTS

Mean and median SG were similar for PBH-RYGB and HC (mean: 99.8 ± 18.6 vs 96.9 ± 10.2 mg/dL; median: 93.0 ± 14.8 vs 94.5 ± 7.4 mg/dL). PBH-RYGB had a higher coefficient of variation (27.3 ± 6.8 vs 17.9 ± 2.4%, P < 0.0001) and range (154.5 ± 50.4 vs 112.0 ± 26.7 mg/dL, P < 0.0001). Nadir was lowest in PBH-RYGB (42.5 ± 3.7 vs HC 49.0 ± 11.9 mg/dL, P = 0.0046), with >2-fold greater time with SG < 70 mg/dL vs HC (7.7 ± 8.4 vs 3.2 ± 4.1%, P = 0.0013); these differences were greater at night (12.6 ± 16.9 vs 1.0 ± 1.5%, P < 0.0001). Non-Surg Hypo also had 4-fold greater time with SG < 70 at night vs HC (SG < 70: 4.0 ± 5.9% vs 1.0 ± 1.5%), but glycemic variability was not increased.

CONCLUSION

Patients with PBH-RYGB experience higher glycemic variability and frequency of SG < 70 compared to HC, especially at night. These data suggest that additional pathophysiologic mechanisms beyond prandial changes contribute to PBH.

Bariatric Surgery Affects Plasma Levels of Alanine Aminotransferase Independent of Weight Loss: A Registry-Based Study

Patients that undergo bariatric surgery experience weight loss and a reduction in the plasma levels of the hepatic enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST). We used the Israeli national bariatric registry, which includes demographic, clinical, and biochemical data on 19,403 patients, of which 1335 patients had two-year follow-up data on ALT, AST, A1C, and BMI, to test the dependence of the reduction in the levels of ALT and AST on weight loss. The data were analyzed using regression models, retrospective matching, and time course analyses. Changes in liver enzymes did not correlate with change in BMI, and linear regression models did not demonstrate that the change in ALT and AST values were dependent on pre-operative levels of BMI or the extent of weight loss. ALT and AST levels were reduced two years after surgery compared with a cohort of retrospectively matched patients for ethnicity, sex, age, BMI, and A1C. Finally, patients who regained weight displayed a reduction in levels of liver enzymes. Our results suggest that bariatric surgery affects AST and ALT levels via weight loss dependent and independent mechanisms. Mechanistic studies that will identify the nature of this effect and the clinical relevance of ALT and AST levels to the post-bariatric liver function are warranted.

Characterization of one anastomosis gastric bypass and impact of biliary and common limbs on bile acid and postprandial glucose metabolism in a minipig model

The alimentary limb has been proposed to be a key driver of the weight-loss-independent metabolic improvements that occur upon bariatric surgery. However, the one anastomosis gastric bypass (OAGB) procedure, consisting of one long biliary limb and a short common limb, induces similar beneficial metabolic effects compared to Roux-en-Y Gastric Bypass (RYGB) in humans, despite the lack of an alimentary limb. The aim of this study was to assess the role of the length of biliary and common limbs in the weight loss and metabolic effects that occur upon OAGB. OAGB and sham surgery, with or without modifications of the length of either the biliary limb or the common limb, were performed in Gottingen minipigs. Weight loss, metabolic changes, and the effects on plasma and intestinal bile acids (BAs) were assessed 15 days after surgery. OAGB significantly decreased body weight, improved glucose homeostasis, increased postprandial GLP-1 and fasting plasma BAs, and qualitatively changed the intestinal BA species composition. Resection of the biliary limb prevented the body weight loss effects of OAGB and attenuated the postprandial GLP-1 increase. Improvements in glucose homeostasis along with changes in plasma and intestinal BAs occurred after OAGB regardless of the biliary limb length. Resection of only the common limb reproduced the glucose homeostasis effects and the changes in intestinal BAs. Our results suggest that the changes in glucose metabolism and BAs after OAGB are mainly mediated by the length of the common limb, whereas the length of the biliary limb contributes to body weight loss.NEW & NOTEWORTHY Common limb mediates postprandial glucose metabolism change after gastric bypass whereas biliary limb contributes to weight loss.