Introduction
Obesity is a leading cause of morbidity and mortality, placing growing demands on healthcare systems. Bariatric surgery induces significant long-lasting weight loss, ameliorates obesity-associated co-morbidities and reduces mortality [
1]. Laparoscopic Roux-en-Y gastric bypass (LRYGBP) is the ‘gold standard’ procedure, resulting in 65–80 % excess bodyweight loss, decreased appetite, and rapid weight-independent amelioration of type-2 diabetes mellitus (T2DM) [
2‐
5]. LRYGBP is cost-effective, but technically challenging with associated mortality—albeit low ∼0.09 % [
6]—and micronutrient deficiencies risks, necessitating lifelong follow-up. Laparoscopic sleeve gastrectomy (LSG) (originally undertaken as a first-step in super-obese patients with subsequent conversion to a hybrid
restrictive–malabsorptive procedure) is technically less complex, with lower complications and nutritional deficiencies rates than LRYGBP [
2]. In light of reports of comparable weight loss and metabolic outcomes to LRYGBP, LSG is increasingly undertaken as a stand-alone procedure [
7‐
10]. However, its long-term efficacy for weight loss and metabolic benefit remains unclear [
8,
10].
Understanding the mechanisms mediating the weight loss and metabolic effects of bariatric surgery is key for developing less invasive procedures and medical obesity treatments. Post-operative changes in circulating gut hormones, including ghrelin, peptide YY (PYY) and glucagon-like peptide-1 (GLP-1), are thought to play a key role to the beneficial outcomes of bariatric surgery [
2].
Ghrelin is produced by
X/A-like cells predominantly located in the stomach fundus and proximal small intestine. Its circulating levels increase with fasting and decrease post-prandially. Acyl-ghrelin, the bioactive form, exerts orexigenic properties and is produced by ghrelin octanoylation in serine-3 mediated by
ghrelin-O-acyl transferase (GOAT). Acyl-ghrelin is rapidly converted by endogenous esterases to the main circulating form, des-acyl-ghrelin [
11]. Moreover, ghrelin is labile, with highest stability in acidic states [
12]. Thus, plasma acyl-ghrelin assessment requires specific sample processing by addition of esterase inhibitor and plasma acidification [
13].
GLP-1 and PYY are released post-prandially by distal gut entero-endocrine L-cells [
2]. Bioactive forms of GLP-1 augment glucose-dependent insulin release and decrease appetite [
2]. Active GLP-1 is rapidly inactivated by the protease di-peptidyl-peptidase-4 (DPP-4). PYY3-36 reduces appetite, bodyweight and adiposity and is produced by truncation of the full-length PYY1-36 by DPP-4 [
14]. The effects of PYY1-36 on appetite and feeding are less clear. In rodents, central PYY1-36 administration stimulates feeding [
15], whereas equipotent or weaker anorexigenic effects to PYY3-36 have been attributed to peripheral PYY1-36 administration [
16,
17]. However, to date, there is no evidence that systemically administered PYY1-36 alters human appetite [
18]. Accurate assessment of plasma active GLP-1 and PYY3-36 requires rapid addition of DPP4-inhibitor to samples [
13]. Diet-induced weight loss increases ghrelin and reduces PYY, but has little effect on GLP-1 [
4,
19,
20], whilst in post-LRYGBP despite the weight loss, ghrelin decreases and marked, weight-independent, increments in nutrient-stimulated plasma PYY and GLP-1 occur [
2‐
5,
21‐
24].
LRYGBP and LSG differentially alter gastrointestinal anatomy. LRYGBP reduces stomach volume and bypasses the majority of the stomach, duodenum and proximal jejunum, with direct nutrient delivery to the distal gut [
2]. In LSG, the gastric fundus (the major source of ghrelin source) is excised, and this accelerates gastric emptying and intestinal transit post-operatively resulting in rapid nutrient delivery to the duodenum and hindgut [
25‐
27]. Thus, LSG and LRYGBP produce differential nutrient exposure of entero-endocrine cells, and as such would be anticipated to differentially alter circulating gut hormones. Most studies comparing the two procedures report larger decreases in circulating fasting and/or meal-stimulated ghrelin after LSG versus LRYGBP [
21,
28‐
30]; however, findings of their effects on hindgut hormones have been inconsistent, reporting either comparable increases in GLP-1 and/or total-PYY [
8,
21,
28,
29,
31]; or superior total-PYY [
32] and GLP-1 [
30] increases post-LRYGBP versus LSG. Technical procedural variations, differences in hormonal isoforms assessed, time-lapse from surgery, subjects’ HOMA-IR and glycaemic status, and differences in subject standardization and sample processing may account for these discrepancies.
Several studies have measured total-ghrelin, total-GLP-1 and/or total-PYY post-surgery, which depict hormone production, but may not necessarily reflect circulating levels of their respective bioactive forms. In support of this, DPP-4 activity declines after LRYGBP [
33], whereas
GOAT expression is altered by caloric restriction and plasma GOAT is BMI-dependent and thus may change post-bariatric surgery [
34‐
36]. Moreover, the effects of LSG on the anorectic PYY-isoform, PYY3-36, are unknown. Therefore, we prospectively compared the effects of LSG and LRYGBP on anthropometric indices, leptin, acyl-ghrelin, active GLP-1, PYY3-36, and appetite in non-diabetic patients using our established subject standardisation and stringent sample-processing protocols.
Discussion
LRYGBP and LSG reduced BMI, excess weight, adiposity and plasma leptin at 6w and 12w post-operatively to a similar extent. Hence, the observed differences in appetite and gut hormones were not attributable to differences in weight loss and more likely reflect differences in the surgical procedures per se.
Our study reports the first comparison of plasma acyl-ghrelin in non-diabetic patients’ post-LRYGBP and LSG. These procedures produce differential nutrient contact with ghrelin-producing
X/A-like cells. Post-LRYGBP,
X/A-like cells in the gastric fundus and duodenum remain in situ but are excluded from nutrient contact. Diet-induced weight loss increases ghrelin [
19]. Yet, despite marked weight loss, post-LRYGBP fasting acyl-ghrelin non-significantly decreased at 6w but rose towards baseline values by 12w, whereas at t30 post-meal declined from pre-surgery at 6w and 12w. Our results suggest that stomach fundus and duodenal ghrelin-producing cells contribute to circulating ghrelin post-LRYGBP despite their exclusion from nutrient contact and that mechanisms independent from
X/A-like cell nutrient-sensing, for example the vagus nerve [
39], may signal meal-induced ghrelin suppression. Chronaiou et al. provided direct evidence that fundus ghrelin-producing cells remain active post-LRYGBP by demonstrating superior decreases in ghrelin post-LRYGBP + fundus resection versus LRYGBP with fundus preservation [
23]. Interestingly Barazzoni et al. reported no change in total ghrelin, but significantly increased acyl-ghrelin at 1, 3, 6 and 12 months post-LRYGBP [
24]. Substantial differences in sample handling by addition of esterase inhibitor and plasma acidification in our study may underlie these discrepant findings.
LSG removes gastric fundus ghrelin-producing cells and accelerates nutrient delivery to duodenal ghrelin-producing cells by increasing gastric emptying [
25‐
27]. After LSG, 40–50 % decreases in fasting total-ghrelin have been reported, sustained for up to 5 years post-surgery, with reductions in post-meal circulating ghrelin levels [
21,
28,
40]. We also observed reductions in fasting acyl-ghrelin and acyl-ghrelinAUC
0-180 post-LSG. Moreover, as anticipated in view of the stomach fundus excision, LSG induced superior acyl-ghrelin reductions than LRYGBP. Interestingly, despite removing the majority of the gastric fundus, fasting acyl-ghrelin and acyl-ghrelinAUC
0-180 declined by only ∼20–30 % post-LSG. A possible explanation is that although the majority of ghrelin-producing cells are in the stomach fundus, circulating acyl-ghrelin may primarily originate from the duodenum. Alternatively, plasma acyl-ghrelin is highly regulated, and compensatory up-regulation of duodenal ghrelin-production may occur.
PYY and GLP-1 are released from distal gut L-cells. Post-meal their levels rapidly rise, implicating a yet unknown neural and/or humoral mechanism to this initial release. Subsequent PYY and GLP-1 release results from L-cell nutrient-contact. LRYGBP and LSG increase gastric emptying [
25‐
27,
41] but have different effects on gut nutrient-passage. LRYGBP excludes nutrients from foregut contact and expedites nutrient delivery to distal gut L-cells, which is suggested to augment hindgut hormone release;
the ‘hindgut theory’. LSG accelerates gastric emptying, reduces acid production and rapidly transits nutrients into the duodenum and proximal intestine, enhancing foregut stimulation [
2]. Few studies have examined the effects of LRYGBP on circulating PYY3-36, the anorectic PYY-isoform. This is the first report of plasma PYY3-36 post-LSG. LRYGBP and LSG markedly enhanced nutrient-stimulated PYY3-36, with, however, greater, more sustained post-prandial PYY3-36 release post-LRYGBP. These findings are in keeping with the procedural anatomical differences and suggest that factors originating from foregut and hindgut regulate PYY3-36 levels. Our findings of superior PYY3-36 enhancement post-LRYGBP versus LSG are in accord with those of Valderas et al. for total-PYY [
32]. However, they are at odds with reports of comparable post-prandial total-PYY following LRYGBP and LSG [
21,
29,
31]. Differences in PYY-isoforms assessed, sampling time-points, subject standardization, sample handling and in subjects’ HOMA-IR pre-surgery may account for these discrepancies.
Similarly to PYY3-36, both procedures augmented nutrient-stimulated active GLP-1 levels, with again greater, more sustained release observed post-LRYGBP. These findings are at odds with reports by Chambers et al. of similar increases in nutrient-stimulated active GLP-1 post-sleeve gastrectomy and gastric bypass in rats [
8]. These discrepancies may be accounted for by structural inter-species rodent-human stomach differences, which potentially affect gastric emptying post-surgery and hence gut hormone responses. Moreover, Chambers and colleagues measured active GLP-1 5 months post-surgery in weight-stable rats, whilst our studies were undertaken during the acute weight-loss phase. Peterli et al. have reported greater meal-stimulated active GLP-1 responses at 1 week and non-significant increases 12 weeks post-LRYGBP versus LSG [
21]. In another study, they showed non-significantly greater active GLP-1 peak and active GLP-1AUC following LRYGBP [
29]. Again, methodological and subject-related differences may underlie these discrepancies.
Despite greater reductions in the ‘hunger hormone’ acyl-ghrelin post-LSG versus post-LRYGBP, paradoxically the LRYGBP group exhibited lower fasting hunger; post-prandial hunger was reduced comparably by both procedures. We also observed similar post-operative increments in nutrient-stimulated fullness perception in both groups. These findings again are slightly at odds with the accepted notion that PYY3-36 and active GLP-1 mediate satiety, as LRYGBP induced superior increases of these anorectic peptides and thus would be expected to result in greater satiety perception. These findings highlight that additional factors to active GLP-1 and PYY3-36 regulate satiety.
The novel finding of our study is the characterisation for the first time of the effects of LSG on circulating levels of the anorectic PYY-isoform, PYY3-36. Moreover, our study is the first to simultaneously measure bioactive forms of ghrelin, PYY and GLP-1 in the same patient cohort, while concurrently undertaking parallel appetite assessment. The main strength of our study is the use of validated subject-standardisation protocols, stringent sample processing [
13], and tight group-matching pre-operatively for the gut hormone confounders age [
42], sex [
43] and adiposity [
44]. Furthermore, we studied patients without T2DM, dissecting out confounding effects of T2DM on the incretin effect [
45], circulating PYY3-36 [
5] and ghrelin [
46]. The limitations of our study are our small sample sizes, non-randomization and limited follow-up. We studied females only as these represent the majority of patients undergoing bariatric surgical procedures in the UK, and future studies are needed to assess whether gender differences exist in post-operative gut hormone changes. Moreover, studies in patients with T2DM are required to examine whether their inferior weight-loss outcome post-bariatric surgery [
47] results from altered/aberrant gut hormone responses. Future larger, randomised studies with longitudinal assessment of gut hormones, intestinal transit, glycaemic and anthropometric indices are required to further elucidate the mechanisms underlying the beneficial effects of LRYGBP and LSG in an attempt to develop novel, less invasive surgical and non-surgical T2DM and obesity treatments.