Effects of proximal gut bypass on glucose tolerance and insulin sensitivity in humans

Physiology Reconfigured: How Does Bariatric Surgery Lead to Diabetes Remission?

Bariatric surgery improves glucose homeostasis and glycemic control in patients with type 2 diabetes. Over the past 20 years, a breadth of studies has been conducted in humans and rodents aimed to identify the regulatory nodes responsible for surgical remission of type 2 diabetes. The review herein discusses central mechanisms of type 2 diabetes remission associated with weight loss and surgical modification of the gastrointestinal tract.

Hypothesis: Bolus jejunal feeding via an enteral feeding tube simulates key features of gastric bypass to initiate similar clinical benefits

BACKGROUND

Therapy for obesity and related comorbidities should be clinically effective, widely available and acceptable, and used in conjunction with an optimized lifestyle. Dieting is widely available and acceptable but has poorly sustained clinical efficacy. By contrast, Roux-en-Y gastric bypass (GB) is highly effective but cost and safety concerns limit widespread use. In this article this we discuss the hypothesis that bolus jejunal feeding (BJ) via an enteral feeding tube simulates key features of GB with the potential for similar clinical benefits. We further hypothesize that a practical manner of providing BJ therapeutically is via an externally inapparent orojejunal feeding tube.

RATIONALE

The first hypothesis is underpinned by the outcomes of research in three fields: 1) investigations into the mechanisms underlying the benefit of GB, 2) studies investigating gastrointestinal physiology and pathophysiology using enteral feeding tubes, and3) investigations into the mechanism underlying involuntary anorexia and weight loss in clinical situations that entail rapid nutrient delivery to the jejunum. There is compelling evidence that a supraphysiologic rate of delivery of nutrient to the jejunum suppresses appetite and energy intake and improves glucose homeostasis, and that these effects can be achieved non-surgically using an enteral feeding tube. The second hypothesis is supported by clinical demonstration of the feasibility of administering intermittent cycles of bolus feeds via an intraorally anchored feeding tube in ambulatory obese adults.

CONCLUSION

The hypotheses are testable in clinical studies. If validated, BJ could be used to induce the clinical benefits of GB, but without its costs or safety concerns.

Cardiovascular Risk Reduction Following Metabolic and Bariatric Surgery

Cardiovascular disease (CVD) remains a leading cause of morbidity and mortality in developed countries, with worsening pandemics of type 2 diabetes mellitus and obesity as major cardiovascular (CV) risk factors. Clinical trials of nonsurgical obesity treatments have not shown benefits in CVD, although recent diabetes trials have demonstrated major CV benefits. In many retrospective and prospective cohort studies, however, metabolic (bariatric) surgery is associated with substantial and reproducible CVD benefits. Despite a lack of prospective, randomized clinical trials, data suggest metabolic surgery may be the most effective modality for CVD risk reduction, likely through weight loss and weight loss-independent mechanisms.

Metabolic networks of the human gut microbiota

The human gut microbiota controls factors that relate to human metabolism with a reach far greater than originally expected. Microbial communities and human (or animal) hosts entertain reciprocal exchanges between various inputs that are largely controlled by the host via its genetic make-up, nutrition and lifestyle. The composition of these microbial communities is fundamental to supply metabolic capabilities beyond those encoded in the host genome, and contributes to hormone and cellular signalling that support the dynamic adaptation to changes in food availability, environment and organismal development. Poor functional exchange between the microbial communities and their human host is associated with dysbiosis, metabolic dysfunction and disease. This review examines the biology of the dynamic relationship between the reciprocal metabolic state of the microbiota-host entity in balance with its environment (i.e. in healthy states), the enzymatic and metabolic changes associated with its imbalance in three well-studied diseases states such as obesity, diabetes and atherosclerosis, and the effects of bariatric surgery and exercise.

Use of a Low-carbohydrate Enteral Nutrition Formula with Effective Inhibition of Hypoglycemia and Post-infusion Hyperglycemia in Non-diabetic Patients Fed via a Jejunostomy Tube

Objective As direct jejunal feeding often causes great fluctuation in glucose levels, continuous or slow infusion is recommended for jejunal tube-fed patients. However, continuous feeding results in prolonged immobility and the loss of activities of daily living. We investigated whether or not intermittent feeding of a low-carbohydrate high-monounsaturated fatty acid (LC/HM) nutrient formula reduces glucose fluctuation in patients who have undergone jejunotomy. Methods Ten bed-ridden non-diabetic patients receiving enteral feeding via a jejunostomy tube were enrolled in this study. LC/HM formula and standard control formula were infused in cross-over order for each patient at a speed of 160 kcal/h. Blood glucose levels were monitored by a continuous glucose monitoring system during the investigation period. Results The mean and standard deviation of the glucose concentrations and mean amplitude of glucose excursion (MAGE) were markedly lower while receiving LC/HM formula than while receiving control standard formula (104 vs. 136 mg/dL, 18.1 vs. 58.1 mg/dL, 50.8 vs. 160 mg/dL, respectively). The post-infusion hyperglycemia [area under the curve (AUC) >140 mg/dL] and peak value of the glucose level were also significantly lower in patients fed LC/HM than the control (25.7 vs. 880 mg・h/dL and 153 vs. 272 mg/dL, respectively). Reactive hypoglycemia (AUC <70 mg/dL) was also significantly lower (0.63 vs. 16.7 mg・h/dL) and the minimum value of the glucose level higher (78.4 vs. 61.8 mg/dL) in patients fed LC/HM than the control. Conclusion The LC/HM formula is considered to markedly inhibit glycemic spikes and prevent rebound hypoglycemia in patients who receive enteral feeding after jejunostomy.

GLP-1: Molecular mechanisms and outcomes of a complex signaling system

Meal ingestion provokes the release of hormones and transmitters, which in turn regulate energy homeostasis and feeding behavior. One such hormone, glucagon-like peptide-1 (GLP-1), has received significant attention in the treatment of obesity and diabetes due to its potent incretin effect. In addition to the peripheral actions of GLP-1, this hormone is able to alter behavior through the modulation of multiple neural circuits. Recent work that focused on elucidating the mechanisms and outcomes of GLP-1 neuromodulation led to the discovery of an impressive array of GLP-1 actions. Here, we summarize the many levels at which the GLP-1 signal adapts to different systems, with the goal being to provide a background against which to guide future research.

Role of Bile Acids and GLP-1 in Mediating the Metabolic Improvements of Bariatric Surgery

BACKGROUND & AIMS

Bile diversion to the ileum (GB-IL) has strikingly similar metabolic and satiating effects to Roux-en-Y gastric bypass (RYGB) in rodent obesity models. The metabolic benefits of these procedures are thought to be mediated by increased bile acids, although parallel changes in body weight and other confounding variables limit this interpretation.

METHODS

Global G protein-coupled bile acid receptor-1 null (Tgr5^-/-) and intestinal-specific farnesoid X receptor null (Fxr^Δ/E) mice on high-fat diet as well as wild-type C57BL/6 and glucagon-like polypeptide 1 receptor deficient (Glp-1r^-/-) mice on chow diet were characterized following GB-IL.

RESULTS

GB-IL induced weight loss and improved oral glucose tolerance in Tgr5^-/-, but not Fxr^Δ/E mice fed a high-fat diet, suggesting a role for intestinal Fxr. GB-IL in wild-type, chow-fed mice prompted weight-independent improvements in glycemia and glucose tolerance secondary to augmented insulin responsiveness. Improvements were concomitant with increased levels of lymphatic GLP-1 in the fasted state and increased levels of intestinal Akkermansia muciniphila. Improvements in fasting glycemia after GB-IL were mitigated with exendin-9, a GLP-1 receptor antagonist, or cholestyramine, a bile acid sequestrant. The glucoregulatory effects of GB-IL were lost in whole-body Glp-1r^-/- mice.

CONCLUSIONS

Bile diversion to the ileum improves glucose homeostasis via an intestinal Fxr-Glp-1 axis. Altered intestinal bile acid availability, independent of weight loss, and intestinal Akkermansia muciniphila appear to mediate the metabolic changes observed after bariatric surgery and might be manipulated for treatment of obesity and diabetes.

Jejunal administration of glucose enhances acyl ghrelin suppression in obese humans

Ghrelin is a gastric hormone that stimulates hunger and worsens glucose metabolism. Circulating ghrelin is decreased after Roux-en-Y gastric bypass (RYGB) surgery; however, the mechanism(s) underlying this change is unknown. We tested the hypothesis that jejunal nutrient exposure plays a significant role in ghrelin suppression after RYGB. Feeding tubes were placed in the stomach or jejunum in 13 obese subjects to simulate pre-RYGB or post-RYGB glucose exposure to the gastrointestinal (GI) tract, respectively, without the confounding effects of caloric restriction, weight loss, and surgical stress. On separate study days, the plasma glucose curves obtained with either gastric or jejunal administration of glucose were replicated with intravenous (iv) infusions of glucose. These "isoglycemic clamps" enabled us to determine the contribution of the GI tract and postabsorptive plasma glucose to acyl ghrelin suppression. Plasma acyl ghrelin levels were suppressed to a greater degree with jejunal glucose administration compared with gastric glucose administration (P < 0.05). Jejunal administration of glucose also resulted in a greater suppression of acyl ghrelin than the corresponding isoglycemic glucose infusion (P ≤ 0.01). However, gastric and isoglycemic iv glucose infusions resulted in similar degrees of acyl ghrelin suppression (P > 0.05). Direct exposure of the proximal jejunum to glucose increases acyl ghrelin suppression independent of circulating glucose levels. The enhanced suppression of acyl ghrelin after RYGB may be due to a nutrient-initiated signal in the jejunum that regulates ghrelin secretion.

Hypoglycaemia after gastric bypass: mechanisms and treatment

Hypoglycaemia after gastric bypass can be severe, but is uncommon, and is sometimes only revealed through monitoring glucose concentrations. The published literature is limited by the heterogeneity of the criteria used for diagnosis, arguing in favour of the Whipple triad with a glycaemia threshold of 55 mg/dl as the diagnostic reference. Women who lost most of their excess weight after gastric bypass, long after the surgery was performed, and who did not have diabetes before surgery are at the greatest risk. In this context, hypoglycaemia results from hyperinsulinism, which is either generated by pancreas anomalies (nesidioblastosis) and/or caused by an overstimulation of β cells by incretins, mainly glucagon-like peptide-1 (GLP-1). Glucose absorption is both accelerated and increased because of the direct communication between the gastric pouch and the jejunum. This is a post-surgical exaggeration of a natural adaptation that is seen in patients who have not undergone surgery in whom glucose is infused directly into the jejunum. There is not always a correspondence between symptoms and biological traits; however, hyperinsulinism is constant if hypoglycaemia is severe and there are neuroglucopenic symptoms. The treatment relies firstly on changes in eating habits, splitting food intake into five to six daily meals, slowing gastric emptying, reducing the glycaemic load and glycaemic index of foods, using fructose and avoiding stress at meals. Pharmacological treatment with acarbose is efficient, but other drugs still need to be validated in a greater number of subjects (insulin, glucagon, calcium channel blockers, somatostatin analogues and GLP-1 analogues). Lastly, if the surgical option has to be used, the benefits (efficient symptom relief) and the risks (weight regain, diabetes) should be weighed carefully.

Post-prandial hypoglycemia results from a non-glucose-dependent inappropriate insulin secretion in Roux-en-Y gastric bypassed patients

BACKGROUND

After Roux-en-Y gastric bypass (RYGB), hypoglycemia can occur and be associated with adverse events such as intense malaise and impaired quality of life.

OBJECTIVE

To compare insulin secretion, sensitivity, and clearance between two groups of patients, with or without hypoglycemia, after an oral glucose tolerance test (OGTT 75-g), and also to compare real-life glucose profiles within these two groups.

SETTING

Bariatric surgery referral center.

METHODS

This study involves a prospective cohort of 46 consecutive patients who complained of malaise compatible with hypoglycemia after RYGB, in whom an OGTT 75-g was performed. A plasma glucose value of lower than 2.8 mmol/L (50 mg/dl) between 90 and 120 min after the load was considered to be a significant hypoglycemia. The main outcome measures were insulin sensitivity, beta-cell function, and glycemic profiles during the test. Glucose parameters were also evaluated by continuous glucose monitoring (CGM) in a real-life setting in 43 patients.

RESULTS

Twenty-five patients had plasma glucose that was lower than 2.8 mmol/L between 90 and 120 from the load (HYPO group). Twenty-one had plasma glucose that was higher than 2.8 mmol/L (NONHYPO group). The HYPO patients were younger, had lost more weight after RYGB, were less frequently diabetic before surgery, and displayed higher early insulin secretion rates compared with the NONHYPO patients after the 75-g OGTT, and they had lower late insulin secretion rates. The HYPO patients had lower interstitial glucose values in real life, which suggests that a continuum exists between observations with an oral glucose load and real-life interstitial glucose concentrations.

CONCLUSIONS

This study suggests that HYPO patients after RYGB display an early increased insulin secretion rate when tested with an OGTT. CGM shows that HYPO patients spend more time below 3.3 mmol/L when compared with NONHYPO patients. This phenotype of patients should be monitored carefully after RYGB.

ß-cell pancreatic dysfunction plays a role in hyperglycemic peaks observed after gastric bypass surgery of obese patients

BACKGROUND

Early and intense hyperglycemic peaks are observed after Roux-en-Y gastric bypass (RYGB).

OBJECTIVES

The aim of this observational study was to compare the ß-cell pancreatic function of patients with (PEAK) and without hyperglycemic peaks (NOPEAK).

SETTING

Referral bariatric surgery center.

METHODS

Insulin secretion rate, clearance, and sensitivity and ß-cell and rate sensitivities were computed after a 75-g oral glucose tolerance test in 42 patients who underwent RYGB.

RESULTS

PEAK patients (n = 18; 30-min glycemia>10.4 mmol/L) did not differ from NOPEAK patients (n = 24) in their presurgery or weight loss characteristics. PEAK patients had significantly higher plasma concentrations of glucose and C-peptide than did NOPEAK patients, whereas insulin and glucagon-like peptide-1 concentrations did not differ. The insulin secretion rate and whole-body insulin clearance (208%) were significantly greater, but insulin sensitivity was significantly less (48%) in PEAK patients. Insulin secretion normalized to plasma glucose was significantly lower in PEAK patients, and the disposition index was reduced (35% to 41% of the values in NOPEAK patients).

CONCLUSION

We conclude that RYGB reveals a series of dysfunctions leading to hyperglycemia in a subset of patients. In PEAK patients, an insufficient adaptation of ß-cell function to glycemia, an increased insulin clearance, and a decreased insulin sensitivity cumulated to contribute to hyperglycemic peaks.

GLP-1: a mediator of the beneficial metabolic effects of bariatric surgery?

There has been increasing interest in the role that gut hormones may play in contributing to the physiological changes produced by certain bariatric procedures, such as Roux-en-Y gastric bypass and sleeve gastrectomy. Here, we review the evidence implicating one such gut hormone, glucagon-like peptide-1, as a mediator of the metabolic benefits of these two procedures.

Striatal dopamine homeostasis is altered in mice following Roux-en-Y gastric bypass surgery

Roux-en-Y gastric bypass (RYGB) is an effective treatment for obesity. Importantly, weight loss following RYGB is thought to result in part from changes in brain-mediated regulation of appetite and food intake. Dopamine (DA) within the dorsal striatum plays an important role in feeding behavior; we therefore hypothesized that RYGB alters DA homeostasis in this subcortical region. In the current study, obese RYGB-operated mice consumed significantly less of a high-fat diet, weighed less by the end of the study, and exhibited lower adiposity than obese sham-operated mice. Interestingly, both RYGB and caloric restriction (pair feeding) resulted in elevated DA and reduced norepinephrine (NE) tissue levels compared with ad libitum fed sham animals. Consequently, the ratio of NE to DA, a measure of DA turnover, was significantly reduced in both of these groups. The RYGB mice additionally exhibited a significant increase in phosphorylation of tyrosine hydroxylase at position Ser31, a key regulatory site of DA synthesis. This increase was associated with augmented expression of extracellular-signal-regulated kinases ERK1/2, the kinase targeting Ser31. Additionally, RYGB has been shown in animal models and humans to improve insulin sensitivity and glycemic control. Curiously, we noted a significant increase in the expression of insulin receptor-β in RYGB animals in striatum (a glucosensing brain region) compared to sham ad libitum fed mice. These data demonstrate that RYGB surgery is associated with altered monoamine homeostasis at the level of the dorsal striatum, thus providing a critical foundation for future studies exploring central mechanisms of weight loss in RYGB.