While Roux-en-Y gastric bypass (RYGB) leads to substantial and sustained weight loss (WL) in the majority of individuals, along with significant improvements or even resolution of obesity-related conditions, especially type 2 diabetes (T2D) (1), approximately 20–30% experience suboptimal clinical response within 10 years (1–3). These suboptimal outcomes include insufficient total weight loss (TWL <20% at 12 months), weight regain (WR) after initial weight nadir, and/or recurrence of T2D following an initial remission (4).

Although the mechanistic aspects of metabolic bariatric surgery (MBS), consisting in gastric volume restriction and intestinal malabsorption, were initially thought to be the only culprits for the observed response to MBS, it is long known that they are just part of the equation. Gut hormones also play a crucial role, as evidenced by the rapid normalization of blood glucose in individuals with T2D shortly after MBS, even before significant WL occurs (5, 6). Indeed, the mechanical modifications of the gastrointestinal tract in RYGB result in the earlier delivery of nutrients to the distal intestine that stimulate local neuro-endocrine cell populations, responsible for the secretion of several gut-derived hormones, such as Glucagon-like Peptide 1 (GLP-1) and Peptide YY (PYY), which contribute to increased satiety and improved glycemic control (7, 8). In fact, GLP-1 paramount role in mediating the effects of RYGB is now well established (5), and GLP-1 receptor agonists are used for obesity and diabetes treatment (9, 10). In contrast, the putative role of the other incretin hormone, Glucose-dependent Insulinotropic Polypeptide (GIP) in mediating the effects of MBS is much less clear given the disparities found in GIP levels after MBS and conflicting results from both human and animal studies (11). Therefore, it has been assumed that if there was any GIP contribution for MBS outcomes, it was likely to be negligible. Still, in non-operated euglycemic individuals, GIP seems to have a more important role in β-cell function than GLP-1 (5). Moreover, there is evidence that the two hormones have comparable activity after sleeve gastrectomy whereas GLP-1 stands out as predominant after RYGB (5), even though, the incretin effect of endogenous GIP is weak or absent in people with T2D (12). More recently, tirzepatide, a GLP-1 receptor and GIP receptor (GIPR) co-agonist, demonstrated greater WL and antidiabetic efficacy compared to isolated GLP-1 receptor (GLP-1R) agonists (13), GIP was brought back into the spotlight and elicited a renewed interest in further understanding its role in glucose and energy homeostasis (14, 15).

While MBS efficacy has been widely investigated, the reasons for a suboptimal clinical response, in particular whether gut hormones also play a role in WR or T2D relapse is less well established (8). Therefore, the aim of this study was to gain further insights into the putative mechanisms leading to MBS suboptimal response through the evaluation of hormone profiles of individuals with optimal and suboptimal response to MBS due to WR or T2D relapse.

This study shows that long-term WR after RYGB is associated with elevated fasting glucose, insulin resistance, and a blunted postprandial GLP-1 response alongside an exaggerated GIP response. On the other hand, T2D relapse correlates with impaired insulin secretion but not with altered incretin dynamics.

In particular, although the participants have no medical history of T2D, the patients with higher %WR have higher fasting levels of glucose, insulin, C-peptide and insulin resistance, as well as lower insulin sensitivity. The significant correlation between the %WR with glucose postprandial excursion but not with insulin/C-peptide postprandial profiles suggests that individuals with higher %WR face an additional challenge to control glucose levels after meals, unsurprisingly given the well-known association of obesity with increased risk of dysglycemia (22). Nonetheless, when corrected for insulin sensitivity (through calculation of the disposition index), there is an impairment of insulin secretion is those with more significant WR/lesser WL, which might possibly suggestive of beta-cell exhaustion, while perfect aligning with the documented relative lack of GLP-1 and glucagon postprandially, both of which known to be powerful insulinotropic hormones (23, 24).

We found significantly higher glucose and lower insulin excursions in the T2D relapse subgroup when compared to the T2D remission subgroup. Furthermore, lower HOMA2-B, OGIS and IGI were present in the T2D relapse subgroup. All together, these support the hypothesis of β-cell exhaustion at the end road of insulin resistance, consistent with the pathological mechanisms underlying T2D (25, 26). In addition, a significantly higher insulin clearance was observed in patients with T2D relapse compared to patients with T2D remission. This could be explained by the fact that when T2D relapses, as a consequence of inadequate insulin secretion, there is a rise in glucose levels in the bloodstream. In the presence of normal insulin sensitivity, this could lead to the upregulation of insulin receptor expression primarily in the liver, responsible for the majority of the uptake of insulin and therefore its clearance. Indeed, this phenomenon of increased insulin clearance has been previously reported in patients with early onset T2D diabetes and normal insulin sensitivity (27). When focusing on hormone profiles, participants with higher % of WR present lower levels of fasting and postprandial levels of GLP-1. One of the key mechanisms contributing to successful initial WL following bariatric surgery is GLP-1 mediated. Previous studies have consistently demonstrated that postprandial GLP-1 levels rise after RYGB (28) and that patients who experience greater WL after MBS have higher levels of GLP-1 compared to those with suboptimal WL (8, 29–32). In addition, and in line with our results, higher postprandial GLP-1 levels have also been observed in individuals with sustained WL, compared to those who experienced WR (32, 33). However, these results were not consistently confirmed by all studies reporting long-term outcomes. In particular, Lompropoulos et al. compared patients with successful WL maintenance and patients with WR 7 years post-surgery and found no significant differences in post-prandial GLP-1 levels (34).

In contrast to GLP-1 postprandial levels, which have been consistently reported to increase after RYGB, reports regarding the circulating levels of GIP following MBS are fewer and results have been inconsistent (28). Gao et al., in a meta-analysis, showed that changes in fasting GIP levels after RYGB are influenced by diabetes status. Specifically, individuals with T2D tended to show a more marked reduction in fasting GIP levels compared to those without T2D. In contrast, studies conducted in populations including both individuals with and without T2D often did not observe significant changes (35). In addition, short-term WL was not a significant predictor of fasting GIP reduction after RYGB (35). Sima E et al. also reported comparable levels of GIP during an oral glucose tolerance test between patients with and without optimal WL five years post-RYGB. On the other hand, Santo M et al. with a follow-up ranging from 27 to 59 months found that postprandial GIP levels were significantly different in patients with WR compared to those who experienced optimal WL maintenance (31, 32). The present study demonstrates that participants with greater WR after RYGB have higher GIP excursion following the intake of a mixed meal, despite having similar baseline levels. Nevertheless, it remains unclear whether these altered incretin dynamics are a cause or consequence of WR. It is possible that impaired GLP-1 signaling reduces satiety, leading to increased caloric intake and subsequent WR, while higher GIP levels may exacerbate this effect by promoting fat deposition (36, 37). Alternatively, WR itself might induce changes in incretin secretion. Both fasting and postprandial GLP-1 levels are lower in individuals with obesity compared to normal-weight (38), while GIP levels have been reported to be higher in people with obesity in both basal (39) and stimulated states (40). Therefore, the results related to GIP and GLP-1 excursion observed in our study could be simply explained by the differences in BMI between the groups. Noteworthy is the fact that, in our study, no significant correlations were observed between %WR and fasting GIP and GLP-1 levels, while it is the response to the mixed meal that is distinct, which suggests that the BMI is unlikely to be the sole reason for the differences observed. Diets with high fat content were also reported to elicit greater postprandial GIP excursions (41). However, in our study the meal tolerance test was performed with a standardized mixed meal and all subjects had exactly the same meal challenge.

The discrepancies in the results across studies regarding the relation between postprandial GIP and GLP-1 levels and WR may be attributed to differences in the criteria used to define suboptimal outcomes, as well as the inclusion of patients with T2D in some analyses. People with T2D are known to exhibit distinct gastrointestinal hormone profiles (42). To minimize this bias regarding weight trajectories, our study excluded participants with a history of T2D from the %WR analysis and defined TWL and WR as non-categorical continuous variables, without committing any artificial or arbitrary cut-offs. Additionally, the divergences that were observed may suggest the involvement of other mechanisms in long-term weight maintenance after MBS.

Individuals with higher %WR exhibited higher fasting glucagon levels, along with higher glucose and insulin levels, likely reflecting their impaired insulin sensitivity. However, a contrasting pattern emerged when analyzing postprandial glucagon levels, as patients with greater %WR showed lower postprandial glucagon levels. While glucagon had not previously been directly correlated with WR after MBS, previous studies suggested that glucagon may contribute to WL by suppression of appetite and activation of energy expenditure and thermogenesis (43). The discrepancy between fasting and postprandial glucagon levels correlation with %WR can be explained by the complex regulation of glucagon secretion and action after MBS (44).

There was an upsurge of GIP focused research since the extended-release GLP-1R and GIPR co-agonist tirzepatide became available for T2D and obesity treatment (13). However, the mechanism by which weight reduction is achieved is not totally clear, nor to what extent is the agonism of the GIP receptor is responsible for the observed results (45), particularly since the insulinotropic effect of GIP is known to be severely hampered in patients with T2D (12, 46).

More recently, the triple agonist of GLP-1, GIP, and glucagon receptors (GCGR), retatrutide, was also developed. Retatrutide has shown impressive WL results of up to 24.2% after 48 weeks of treatment (47), which appear to be even more substantial compared to tirzepatide, despite there are no head-to-head clinical trials comparing tirzepatide and retatrutide. The addition of GCGR agonism is hypothesized to lead to increased energy expenditure and decreased calorie intake, though the precise role of GCGR stimulation in retatrutide’s WL mechanism remains unclear (48).

In people with history of T2D before RYGB, we found no significant differences in the gut hormone profiles that were measured following a MMTT. Our results suggest that these hormone profiles do not seem to be influenced by T2D remission nor relapse following MBS. Thus, other mechanisms are likely to predominate at regulating glycemic status once excessive body weight is no longer in the equation.

Previous studies identified that preoperative insulin therapy, HbA1c levels and T2D duration are key factors in predicting T2D remission (49–51). In the current study, although none of the participants were on insulin therapy before surgery, the T2D relapse group exhibited higher preoperative HbA1c levels and longer T2D duration, suggesting a more pronounced β-cell dysfunction and reduced pancreatic reserve, which may have contributed to the T2D relapse.

Although not significant, our T2D relapse group did not lose as much % of body weight as our T2D remission group at nadir. While most studies suggest that the initial WL following RYGB is related with long-term T2D-related outcomes (16, 52, 53), a recent study found that WL had an impact on T2D remission in the first year, but did not influence the relapse rate in subsequent years (54). Taken together, these findings highlight stressing the importance of implementing dietary and lifestyle modifications following the surgical intervention in order to optimize the WL and consequent metabolic benefits, as early weight reduction may improve β-cell function and insulin sensitivity—both critical for achieving and sustaining T2D remission.

Although this study provides valuable insights, some limitations should be acknowledged. First, its cross-sectional design does not allow a confirmation of a causal relationships between hormone profiles and clinical surgical outcomes. The strict inclusion criteria for cohort 2 (patients with T2D relapse who were not receiving GLP-1 receptor agonists or insulin therapy; matched groups for pre-operative age, pre-operative BMI, time since surgery, and post-operative WL trajectories) were applied to avoid introducing bias into our results. Consequently, only 10 patients from our 10-year cohort qualified for the T2D relapse group. While this relatively small sample size, may limit the interpretation of the results, the detailed characterization of participants and the significant differences observed in their glucose and insulin profiles allowed us to investigate the impact of incretin hormones on these distinct metabolic profiles. Therefore, despite this limitation, the study provides valuable insights and supports the robustness of the findings. Additionally, although the type of RYGB procedure was not randomly assigned, the balanced distribution of surgical techniques reduces the potential for confounding. The influence of diabetes medications on meal test results, especially in cohort 2, should also be considered. To minimize this effect, only participants treated with metformin were included. Plasma glucose levels were measured using a glucometer, which has lower precision compared to laboratory-based plasma glucose measurements. However, since no part of the study protocol relied on real-time highly accurate glucose values, the method used for glucose measurement, although less precise, did not appear to raise significant concerns regarding the study’s overall findings.

In conclusion, we aimed to understand whether the enteropancreatic hormone profile of patients subjected to RYGB could be responsible for the long-term suboptimal response to the surgery, both regarding WR or T2D relapse. Although our results point towards a potential role of both GLP-1 and GIP in WR, this warrants further research. In contrast, the enteropancreatic hormone profile does not seem to be influenced by T2D remission nor relapse following BMS, and other mechanisms seem to be at play, including the natural history of disease progression. Overall, our results highlight a possible link between altered incretin responses and WR after RYGB, and support further prospective trials targeting GLP-1/GIP pathways to improve long-term metabolic outcomes.