Metabolic Surgery for Obese Type 2 Diabetes: Korean Multicenter Cohort Study

Obesity and type 2 diabetes mellitus (T2DM) are two of the most pressing public health challenges worldwide. Over the past four decades, the global prevalence of obesity has risen drastically, with more than 650 million adults classified as obese in 2016, according to the World Health Organization [1]. Alongside this increase, the number of people with T2DM has also grown rapidly, nearly tripling between 1980 and 2014, affecting more than 422 million individuals globally [2]. In addition to the significant individual burden, T2DM and obesity together constitute the concept of “diabesity,” reflecting the close interrelation between the two conditions [3]. In East Asia, including Korea, the prevalence of T2DM is also on the rise, with a growing need for effective therapeutic interventions [4].

Metabolic/bariatric surgery (MBS) is now recognized as one of the most effective treatments for both obesity and T2DM, offering substantial weight loss and metabolic improvements [5]. Current international guidelines recommend metabolic surgery as a treatment option for T2DM in patients with a body mass index (BMI) of 30 kg/m² or higher, especially in Asian populations where lower BMI thresholds are used to define obesity-related comorbidities [6, 7].

Multiple mechanisms have been proposed to explain the T2DM remission observed after bariatric surgery [8]. A key pathway involves increased secretion of glucagon-like peptide-1 (GLP-1), which boosts insulin secretion and suppresses glucagon. Additionally, surgery-induced changes in gut microbiota and bile acid metabolism may enhance insulin sensitivity and metabolic homeostasis. These effects highlight the role of MBS as a potent metabolic therapy beyond weight reduction.

Among MBS procedures, sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB) are the most widely performed [9]. SG is technically simpler and associated with fewer complications [10], whereas RYGB generally offers greater weight loss and better glycemic outcomes [11, 12]. Given these distinct profiles, the choice of surgical procedure for a patient with T2DM and obesity remains a topic of debate. Surgeons must weigh the risks and benefits of each procedure to select the optimal approach for individual patients. However, randomized controlled trials have reported conflicting results regarding procedure choice for diabesity. The SLEEVEPASS and SM-BOSS trials suggested no significant difference in long-term diabetes remission between RYGB and SG [13, 14], whereas the Oseberg trial demonstrated superior remission outcomes with RYGB [15]. These discrepancies underscore the ongoing debate and highlight the need for population-specific evidence in East Asian patients.

Furthermore, most studies focus on Western populations, with limited evidence on the Asian population who have unique genetic and phenotypic characteristics influencing obesity and T2DM outcomes [16]. As East Asian populations tend to develop T2DM at lower BMI levels than their Western counterparts, the applicability of existing evidence to Asian patients may be limited [17, 18].

This study sought to compare the effects of SG and RYGB on glucose control, weight loss, and the incidence of postoperative complications in Korean patients with obesity and T2DM. Therefore, we aimed to provide robust, population-specific evidence that can guide surgical decision-making for patients with combined T2DM and obesity in East Asia.

A total of 435 eligible patients were included in the final analysis. Of these, 271 (166 SG and 105 RYGB) had 1-year follow-up data, and 169 (105 SG and 64 RYGB) were assessed at 2 years (Fig. 1). Patient enrollment across the 14 participating centers ranged from 5 to 94 individuals.

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This study found that the type of surgical procedure was not significantly associated with T2DM remission at 1 or 2 years in Korean patients with BMI ≥ 30 kg/m². Even after IPTW adjustment, no significant difference in remission rates was observed. In multivariable analysis, diabetes duration and insulin use were significant predictors, while procedure type was not. These findings align with some prior studies but differ from others, highlighting ongoing debate over the metabolic superiority of RYGB vs. SG.

Several landmark randomized controlled trials (RCTs), including the SLEEVEPASS and SM-BOSS trials, have offered key insights into the metabolic benefits of RYGB and SG [13, 14]. Both the SLEEVEPASS and SM-BOSS trials primarily assessed long-term weight loss between procedures, with T2DM remission as a secondary outcome. At 5 years, neither study found a significant difference in remission rates, and the SLEEVEPASS trial confirmed this at 10 years [11]. However, as weight loss was the primary endpoint, these trials had limited power to detect differences in diabetes outcomes. Thus, the lack of significant differences in remission should not be viewed as definitive evidence of equal efficacy between procedures for T2DM.

The Oseberg trial, a RCT with T2DM remission as the primary endpoint, demonstrated that RYGB was significantly more effective than SG at both 1- and 3-year follow-ups [12, 15]. The result revealed stronger evidence to support the conclusion that RYGB may have superior benefits. A RCT in East Asia reported findings consistent with this trial, showing significantly higher remission rates with RYGB [23].

The findings of our study aligned with SLEEVEPASS and SM-BOSS trials showing no difference in diabetes remission but superior RYGB weight loss outcomes [13, 14]. However, our study had notable baseline imbalances between SG and RYGB groups prior to adjustment, particularly in T2DM severity parameters. Interestingly, patients in the RYGB group had significantly lower baseline BMI than those in the SG group, which may appear counterintuitive. This likely reflects the fact that all patients in this study underwent surgery primarily for the treatment of diabetes, rather than weight loss alone. In this real-world context, surgeons tended to preferentially select bypass procedures for patients with longer disease duration, greater severity, and higher rates of insulin use, even if their BMI was relatively lower. This practice pattern is consistent with prior reports, including a multinational Asian study demonstrating superior T2DM remission outcomes with bypass-type procedures compared to non-bypass procedures [24].

While IPTW and multivariable analyses mitigated confounding effects, residual confounding cannot be ruled out. Before adjustment, SG appeared to show higher complete T2DM remission rates; however, after IPTW, this finding reversed, with RYGB showing higher remission rates, although without statistical significance. This striking reversal directly illustrates the impact of baseline imbalances and underscores that the apparent procedure effect is strongly confounded by diabetes severity. Importantly, this shift itself is clinically meaningful, suggesting that if groups were better matched for baseline severity, a significant advantage of RYGB might have been observed. Therefore, the absence of a statistically significant difference should be interpreted with caution, acknowledging the retrospective design and the inherent challenges of fully accounting for disease severity.

Another factor explaining comparable diabetes remission rates between procedures may be SG’s intrinsic efficacy in improving glycemic control. Both procedures promote glycemic control through mechanisms like increased GLP-1 secretion, gut microbiota changes, and bile acid metabolism [25‐27]. Thus, the differences in diabetes remission between RYGB and SG may exist but may not be large enough to reach statistical significance in a relatively small sample size study [28].

The evidence comparing RYGB and SG impact on T2DM remission remains complex and debated. A notable limitation of aforementioned studies is inadequate accounting for baseline T2DM severity. T2DM severity significantly influences remission likelihood after bariatric surgery—patients with mild T2DM achieve remission regardless of procedure, while severe diabetes patients may only improve or remain unchanged [29‐31]. This highlights the necessity of stratifying patients by diabetic severity when evaluating comparative metabolic effectiveness of RYGB and SG. A study combining SLEEVEPASS and SM-BOSS data addressed this by categorizing patients using the Individualized Metabolic Surgery (IMS) score, finding no significant differences in remission rates between procedures even after severity stratification. However, authors acknowledged potential underpowering in smaller subgroups [32]. Therefore, prospective trials stratifying patients by diabetic severity are needed.

This study has several limitations. First, its retrospective design makes it subject to inherent selection bias and residual confounding despite statistical adjustments. We attempted to mitigate confounding by applying IPTW with comprehensive covariates but the possibility of unmeasured confounding remains. Therefore, the findings should be interpreted with caution. In addition, covariate balance after weighting was not perfect. Balance was achieved (standardized mean differences < 0.1) for many variables including DM duration, C-peptide, and HbA1c in the 1-year cohort, but in the 2-year cohort the smaller sample size and larger baseline differences limited the ability of weighting to fully eliminate imbalance. Still, key variables such as BMI and C-peptide achieved good balance, supporting comparability for the main metabolic outcomes. Second, the sample size and follow-up duration were limited, which may have reduced the statistical power to detect modest but clinically meaningful differences. Thus, the non-significant findings after IPTW adjustment should be viewed in light of potential Type II error. Third, attrition was considerable at both 1 and 2 years, with follow-up rates of 62.3% (271/435) and 38.9% (169/435), respectively. This raises the possibility of attrition bias. However, a comparison of baseline characteristics between completers and non-completers showed no major differences other than a higher proportion of female patients at 1 year and shorter diabetes duration among those lost at 2 years (Supplementary Table 8), suggesting that attrition bias is unlikely to have substantially influenced the overall findings. Another limitation is the definition of complete remission. We used a stricter criterion (HbA1c < 6.0% and fasting glucose < 100 mg/dL without medications) based on the standardized outcomes reporting recommendations [22], whereas many studies adopt the ADA definition (HbA1c < 6.5% without therapy) [33]. This heterogeneity in remission definitions may reduce direct comparability with other reports. Furthermore, our study did not include an analysis of relapse, and the 2-year follow-up horizon is insufficient to determine the long-term durability of remission. Longer-term studies are needed to evaluate relapse and sustained glycemic outcomes after SG and RYGB. Lastly, the RYGB group included both conventional and resectional gastric bypass. In Korea, resectional gastric bypass is occasionally performed after patients are informed about the risk of remnant gastric cancer and opt for this procedure. To clarify potential heterogeneity, we conducted a subgroup analysis (Supplementary Table 9). Patients in the resectional group had longer diabetes duration, higher preoperative HbA1c, longer operative time, and longer hospital stay. At 1 year, BMI and %TWL were comparable between groups, but diabetes and dyslipidemia remission rates were lower in the resectional group, likely reflecting their more severe baseline diabetes rather than an intrinsic difference in procedure efficacy. Given the small number of resectional cases (n = 16), direct comparisons between the two subtypes should be interpreted with caution.

In conclusion, this study did not demonstrate a statistically significant difference in T2DM remission rates between SG and RYGB at 1 and 2 years after IPTW adjustment in Korean patients. Diabetes duration and insulin use emerged as stronger predictors of remission than procedure type. However, the limited sample size, considerable attrition, and baseline imbalance in T2DM severity mean that these findings should be interpreted with caution. Under comparable baseline conditions, RYGB may provide greater metabolic benefits, a hypothesis that requires confirmation in larger, stratified randomized controlled trials.