A large retrospective analysis of more than 665,000 bariatric procedures presented at the ASMBS 2025 Annual Scientific Meeting shows a clear correlation between preoperative body mass index (BMI) and postoperative complications. Patients in the highest BMI category (≥50 kg/m²) experienced the greatest increase in both overall and serious adverse events compared with those in lower BMI ranges.
Background and Rationale
Bariatric surgery—most commonly sleeve gastrectomy and Roux-en-Y gastric bypass—remains the most effective treatment for severe obesity and its associated comorbidities. However, obesity itself poses perioperative risk, and patients with extreme obesity often raise concerns among surgeons about heightened complication rates. Though obesity is recognized as a general risk factor for surgical morbidity, granular data quantifying risk across BMI categories have been lacking.
Study Design and Methods
Data Source and Patient Selection
Investigators conducted a retrospective review of the Metabolic and Bariatric Surgery Accreditation and Quality Improvement Program (MBSAQIP) database, which compiles detailed perioperative data from accredited centers across the United States and Canada. Patients aged 18 years or older with a preoperative BMI of 30 kg/m² or higher who underwent primary sleeve gastrectomy or Roux-en-Y gastric bypass between January 1, 2020, and December 31, 2023, were included.
BMI Stratification
Preoperative BMIs were categorized into five clinically meaningful groups:
• Class I obesity: 30.0–34.9 kg/m²
• Class II obesity: 35.0–39.9 kg/m²
• Class III obesity: 40.0–44.9 kg/m²
• Severe obesity: 45.0–49.9 kg/m²
• Super obesity: ≥50.0 kg/m²
Outcomes Measured
The primary outcomes were the rates of any postoperative complications and serious occurrences within 30 days of surgery. Serious occurrences were defined as events requiring invasive intervention, prolonging hospitalization, causing permanent harm, or resulting in death.
Statistical Analysis
Complication rates were calculated for each BMI category. Linear regression analysis quantified the change in complication rates per BMI category increment. Subgroup analyses compared sleeve gastrectomy versus Roux-en-Y gastric bypass outcomes.
Principal Findings
Overall Complication Rates
Among 665,047 patients, the overall complication rate increased from 4.1% in the Class I obesity group to 6.8% in the super obesity group—a relative increase of approximately 66%. Linear regression showed an average rise of 5.56 percentage points in overall complications when comparing the lowest to highest BMI categories.
Serious Occurrences
Serious occurrence rates rose from 1.8% in the Class I group to 3.5% in the super obesity group, representing a near doubling. The absolute increase was 3.21 percentage points across BMI categories.
Surgical Procedure Subgroups
• Sleeve Gastrectomy: Among 412,500 sleeve gastrectomies, the trend of increasing complications with higher BMI mirrored the overall cohort, though absolute rates were slightly lower than in gastric bypass patients.
• Roux-en-Y Gastric Bypass: Of the 252,547 bypass patients, complication and serious occurrence rates were consistently higher across all BMI groups, but the incremental risk per BMI category was comparable.
Key Inflection Point at BMI 50
Dr. John Scott, co-author and Division Chief for Minimal Access and Bariatric Surgery at Prisma Health, highlighted that the most pronounced risk escalation occurs once BMI exceeds 50 kg/m². “This inflection point suggests that patients with super obesity warrant enhanced preoperative optimization and postoperative surveillance,” he said.
Clinical Implications
Preoperative Counseling and Shared Decision-Making
The granular data detailing complication risk by BMI category equip surgeons to provide more precise risk assessments during preoperative counseling. “Patients considering bariatric surgery often focus on weight-loss outcomes, but understanding their individualized perioperative risk empowers them to make fully informed decisions,” explained Dr. Scott. Surgeons can now communicate quantitative risk increments rather than general statements about “higher risk.”
Optimizing Preoperative Health
Given the identified risk gradient, multidisciplinary preoperative interventions—nutritional optimization, supervised weight loss programs, management of comorbidities such as diabetes and obstructive sleep apnea—may mitigate perioperative complications, especially in patients approaching or exceeding a BMI of 50.
Procedure Selection and Staging Strategies
In select patients with super obesity, a staged approach may be considered: an initial sleeve gastrectomy to induce moderate weight loss, followed by conversion to a more complex bypass procedure after BMI reduction. This strategy may reduce surgical stress and complication rates. However, prospective studies are needed to confirm the safety and efficacy of staging in this population.
Enhanced Postoperative Monitoring
Higher-risk patients could benefit from extended inpatient monitoring, early mobilization protocols, and vigilant surveillance for complications such as anastomotic leaks and thromboembolic events. Institutions may consider pathway modifications—such as delaying discharge by one to two additional days—to ensure early detection and management of adverse events.
Expert Commentary
Dr. Richard Peterson, Professor of Surgery at UT Health San Antonio and ASMBS President-elect, who was not involved in the study, praised the contribution: “This analysis provides the most comprehensive risk stratification we’ve seen based on BMI. It reinforces that bariatric surgery remains safe, even in super-obese patients, with overall low complication rates. But it also underscores the need for targeted perioperative strategies in those with highest BMI.”
Limitations and Future Directions
Retrospective Design and Registry Data
As with all registry-based studies, the analysis is subject to potential inaccuracies in data entry and variability in center-level practices. Furthermore, the MBSAQIP 30-day follow-up window may undercount late complications.
Unmeasured Confounders
Variables such as socioeconomic status, insurance type, and functional capacity were not captured but may influence outcomes. Prospective studies incorporating these factors could refine risk models further.
Weight Loss Trajectory Analysis
Future research might evaluate how preoperative weight loss—medical or surgical—modifies complication risk. Understanding the interplay between rapid preoperative weight loss and perioperative safety could guide patient preparation.
Long-Term Outcomes
While perioperative safety is paramount, long-term metrics—sustained weight loss, resolution of comorbidities, and quality of life—also define procedural success. Studies correlating BMI-related perioperative risk with longitudinal benefits will help balance risk–reward considerations.
Conclusion
This landmark study from the ASMBS 2025 meeting affirms that while higher BMI is associated with a measurable increase in bariatric surgery complications, the absolute risks remain relatively low. Patients with BMI ≥50 kg/m² face the greatest perioperative challenges, highlighting the importance of tailored preoperative optimization, surgical planning, and postoperative care. Armed with these data, surgeons and patients can engage in more nuanced shared decision-making, ensuring that the benefits of weight-loss surgery are delivered safely to those who need it most.
References
- American Society for Metabolic and Bariatric Surgery 2025 Annual Scientific Meeting Abstracts.
- MBSAQIP Participating Centers.
- Scott J, et al. Relationship Between Preoperative BMI and 30-Day Postoperative Complications in Bariatric Surgery. ASMBS 2025.
- Peterson RM, Commentary on Bariatric Surgery Risk Stratification. ASMBS Perspectives, June 2025.
Disclosure
Dr. Scott reports no conflicts of interest. Dr. Peterson is President-elect of ASMBS. The MBSAQIP database is funded by the U.S. Centers for Medicare & Medicaid Services and the American Society for Metabolic and Bariatric Surgery.
READ MORE: Paper-Based Mass Spectrometry Device Detects Asymptomatic Malaria with Unprecedented Accuracy
