Human subjects with severe obesity are increasingly treated with bariatric surgery to promote weight loss via procedures that reduce the capacity of the stomach and/or the absorptive surface area of the small bowel, resulting in reduced food intake and/or energy malabsorption. Some of these surgical procedures may also be associated with changes in plasma levels of one or more gut hormones, due to anatomical alterations in gut motility, incomplete nutrient digestion, and disruption of neural innervation. Changes in the levels of circulating gut hormones have been commonly observed after gut surgery, and in some instances, relative changes in the numbers of specific enteroendocrine cell subsets have also been described. Diversion of nutrients away from the proximal gut and consequent exposure of the distal gut to a greater load of incompletely digested nutrients is often associated with a reductions in levels of circulating peptides derived from the proximal gut, and an increase in levels of peptide hormones derived from the distal gut, such as neurotensin, PYY and enteroglucagon (a surrogate for levels of glicentin, oxyntomodulin, GLP-1 and GLP-2) Gut hormone changes after jejunoileal (JIB) or biliopancreatic (BPB) bypass surgery for morbid obesity. Int J Obes 1981; 5:471-80. and Plasma enteroglucagon after jejunoileal bypass with 3:1 or 1:3 jejunoileal ratio. Scand J Gastroenterol 1979; 14:205-7. and Morphological and functional alterations to a sub-group of regulatory peptides in human pancreas and intestine after jejuno-ileal bypass. Int J Obes Relat Metab Disord 1993 17:109-113.
As many human subjects experience significant weight loss, and even more remarkable improvement or complete resolution of their diabetes within days of the surgical procedure, there is great interest in understanding the potential roles of gut hormones in the amelioration of the diabetic state, and in the factors contributing to weight loss. An increase in the levels of anorectic hormones such as PYY or GLP-1, or a decrease in levels of orexigenic hormones such as ghrelin Plasma ghrelin levels after diet-induced weight loss or gastric bypass surgery. N Engl J Med 2002: 346:1623-1630 may contribute to changes in appetite and body weight in subsets of patients following bypass surgery.
Gastric bypass surgery, GLP-1, and weight loss
Many patients with obesity experience rapid weight loss together with striking amelioration of their diabetes often within days of gastric bypass surgery. As marked amelioration or complete resolution of the associated diabetes frequently precedes weight loss, a role for GI hormones secreted from the distal GI tract, such as GLP-1 has been invoked to explain these impressive clinical improvements. As different surgical procedures result in distinct anatomical rearrangement of the normal proximal-distal gut orientation and integrity, it is important to distinguish between the various surgical procures and associated changes in levels of gut hormones.
Naslund and colleagues assessed plasma levels of GIP and GLP-1 from 9 months to 20 years after jejunoileal bypass surgery. Both fasting and postprandial levels of GIP were markedly elevated in female obese human subjects 20 years after bypass surgery, in association with improvement in glucose tolerance as described in Importance of small bowel peptides for the improved glucose metabolism 20 years after jejunoileal bypass for obesity. Obes Surg 1998 8:253-260.
Valverde and colleagues studied plasma levels of GLP-1, together with serial analysis of glucose tolerance in two groups of patients; after Larrad's pancreaticobiliary diversion (BPD) or following vertical banded gastroplasty (VBG). Basal and glucose-stimulated plasma GLP-1 increased after surgery, with GLP-1 levels comparatively greater in subjects following BPD. See Changes in glucagon-like peptide-1 (GLP-1) secretion after biliopancreatic diversion or vertical banded gastroplasty in obese subjects. Obes Surg. 2005 Mar;15(3):387-97. Similarly, the GLP-1 response to meal ingestion was flat or absent prior to surgery for obesity, but significantly improved in obese subjects 6 weeks following Roux-en-Y Gastric Bypass (RYGBP) GLP-1, PYY, Hunger and Satiety Following Gastric Bypass Surgery In Morbidly Obese Subjects. J Clin Endocrinol Metab. 2006 Feb 14; [Epub ahead of print]
Le Roux and colleagues examined changes in plasma levels of gut hormones and glucose tolerance following different gastric bypass procedures in rats and human subjects. Patients with Roux-en-Y gastric bypass (RYGB) had increased postprandial levels of plasma PYY and GLP-1 together with early and exaggerated insulin responses, and improved glycemic control. In contrast, these hormonal changes were not seen in subjects after gastric banding.Gut hormone profiles following bariatric surgery favor an anorectic state, facilitate weight loss, and improve metabolic parameters. Ann Surg. 2006 Jan;243(1):108-14.
Chambers and colleagues assessed glucose tolerance, insulin sensitivity, and incretin responses in several groups of rats subjected to either sleeve gastrectomy (VSG; 80% of the stomach is removed) or Roux en Y gastric bypass (RYGB) in which a small remnant gastric puch was connected to the distal jejunum. Glucose clamps were done 2 weeks after surgery and glucose tolerance was assessed 5 weeks and 5 months after surgery in separate groups of animals. RYGP and VSG animals exhibited comparable reductions in food intake, weight loss, and improvements in glucose and insulin tolerance. Hepatic insulin sensitivity improved in both groups of bypassed rats independent of weight loss. Nutrient-stimulated levels of GLP-1 were higher in bypassed rats, glucose tolerance was improved and insulin levels post meal were higher; the majority of these changes were substantially attenuated by co-administration of the GLP-1R antagonist exendin(9-39). Weight-Independent Changes in Blood Glucose Homeostasis after Gastric Bypass or Vertical Sleeve Gastrectomy in Rats Gastroenterology. 2011 Jun 20.
Sequential changes in glucose tolerance, gastric emptying and levels of gut hormones in response to a liquid test meal were also assessed by Falken and colleagues in 12 obese subjects at 3 days, 2 months, and one year after gastric bypass. Change in body mass was highly significant (from 45 to 30 over one year). Plasma levels of several gut hormones, including GLP-1 increased progressively over time, glucose tolerance and b-cell function improved, coincident with satiety and weight loss. However, significant improvements in plasma levels of GLP-1 were also noted as early as day 3. Transit through the GI tract was rapidly improved as early as day 3. Changes in Glucose Homeostasis after Roux-en-Y Gastric Bypass Surgery for Obesity at Day Three, Two Months, and One Year after Surgery: Role of Gut Peptides. J Clin Endocrinol Metab. 2011 May 4. [Epub ahead of print]
Guidone and colleagues studied glucose tolerance, b-cell function, insulin sensitivity, and plasma levels of b-cell function was observed even 1 week after surgery, prior to the development of significant weight loss. Plasma levels of GIP fell, whereas levels of GLP-1 increased, in association with complete resolution of the diabetes. Whether these changes in gut hormones were simply associated with or contributed to improvement in the diabetic state remains unclear. See Mechanisms of recovery from type 2 diabetes after malabsorptive bariatric surgery. Diabetes. 2006 Jul;55(7):2025-31
Gastric bypass surgery and postprandial hypoglycemia
Six patients with hyperinsulinemic hypoglycemia following meal ingestion were detected 0.5-8 years following Roux-en-Y gastric bypass surgery were found to have histological evidence for nesidioblastosis following resection of pancreatic tissue-one patient was found to have multiple insulinomas as described in the July 21 2005 New England Journal of Medicine Hyperinsulinemic hypoglycemia with nesidioblastosis after gastric-bypass surgery. N Engl J Med. 2005 Jul 21;353(3):249-54. The authors speculated, as further discussed in an accompanying editorial, that excessive secretion of gut hormones such as GLP-1 may have contributed to the development of islet proliferation in these human subjects. A similar clinical picture was reported in 3 subjects following gastric bypass, and plasma levels of GLP-1 were markedly elevated in these studies Severe hypoglycaemia post-gastric bypass requiring partial pancreatectomy: evidence for inappropriate insulin secretion and pancreatic islet hyperplasia. Diabetologia. 2005 Sep 30; [Epub ahead of print]. However, a subsequent re-analysis of the pancreas histology, together with control slides obtained from 31 obese subjects and 16 lean control subjects, yielded somewhat modified conclusions. Meier and colleagues reported that b-cell area was not increased in the subjects with gastric bypass-associated hypoglycemia and no evidence of increased islet neogenesis or b-cell proliferation was detected in this group. These findings further emphasis the importance of functional defects, namely changes in gut motility and the acute b-cell response to nutrients/gut hormones in the pathogenesis of the hyperinsulinemic hypoglycemia syndrome seen in some subjects. See Hyperinsulinemic hypoglycemia after gastric bypass surgery is not accompanied by islet hyperplasia or increased beta-cell turnover. Diabetes Care. 2006 Jul;29(7):1554-9
Salehi and colleagues examined the importance of GLP-1/the GLP-1R in glucose-stimulated insulin secretion in asymptomatic subjects after gastric bypass (GB) and in subjects with recurrent hypoglycemia using acute administration of the GLP-1R antagonist, exendin(9-39). Fasting blood glucose and insulin levels were comparable in the two groups. The insulin response to IV glucose and insulin sensitivity was also similar between groups. Administration of Ex9 produced a comparatively greater suppression of postprandial insulin levels in the GB groups however the rate of gastric emptying was not modified by Ex9. The postmeal rise in plasma GLP-1 was significantly higher in the patients experiencing hypoglycemia. Intriguingly, Ex9 infusion produced a further rise in plasma GLP-1 levels, consistent with a negative feedback loop. The control of glucagon secretion was also abnormal and increased in GB patients fater meal ingestion. Whether increased levels of GLP-1 are the predominant factor producing hypoglycemia in these subjects remains uncertain. Gastric Bypass Surgery Enhances Glucagon-Like Peptide 1-Stimulated Postprandial Insulin Secretion in Humans Diabetes. 2011 Sep;60(9):2308-2314