One Anastomosis Gastric Bypass (OAGB) is the most common metabolic and bariatric surgery (MBS) in Israel, recognized for its effectiveness in achieving sustainable weight loss and mitigating obesity-related diseases. The metabolic outcomes of OAGB are significantly influenced by the length of the biliopancreatic limb (BPL). The objective of this study is to determine whether tailoring the BPL length to the total small bowel length (TSBL) results in more effective weight loss compared to patients undergoing OAGB with a fixed BPL of 180 cm. Efficacy and safety of this approach will also be evaluated, ensuring it does not lead to long-term morbidity or negatively impact patients' quality of life.

Scientific Background and Rationale for the Study One-Anastomosis Gastric Bypass (OAGB) is the most common bariatric procedure in Israel [1]. Over the years, OAGB has been proven to be an effective surgery, yielding significant and sustained weight loss, improvement in obesity-related comorbidities, and a relatively low failure rate [2].

The procedure involves separating a long gastric pouch from the remaining stomach and creating an anastomosis between the pouch and a distant loop of the small intestine [3]. Since this procedure bypasses a relatively long intestinal segment (typically 150-200 cm), it results in malabsorption of carbohydrates, fats, proteins, and other nutrients. This malabsorption-related mechanism leads to greater weight loss compared to purely restrictive surgeries like sleeve gastrectomy [4].

The length of the bypassed segment, specifically the biliopancreatic limb (BPL), plays a critical role in determining the metabolic outcomes of OAGB. A longer BPL is associated with more significant weight loss and greater improvement in obesity-related diseases [5]. However, the ideal BPL length remains a topic of global debate. Some experts recommend measuring the total small bowel length (TSBL) or ensuring at least 250-300 cm of common channel (CC) to prevent pathological malabsorption in patients with a shorter bowel and to reduce the risk of weight regain in those with a longer bowel [3,6].

Currently, the most widely accepted approach is to standardize the BPL length at 150-200 cm, as studies have shown that this range minimizes the risk of nutritional deficiencies [7]. However, tailoring the BPL length to the patient's TSBL is an increasingly recognized strategy that may optimize outcomes by ensuring a more precise CC length, thereby reducing complications and improving long-term weight loss.

Study Objective To evaluate whether tailoring the BPL length to the TSBL results in more effective weight loss compared to a standard bypass length of 180 cm.

Study Endpoints BPL and TSBL lengths Weight progression during follow-up Postoperative nutritional complications Improvement in obesity-related comorbidities Study Participants Single-center cohort: 200 patients Multi-center cohort: 500 patients Participating centers: Ichilov Medical Center, Assuta Tel Aviv, Herzliya Medical Center Participant Recruitment and Informed Consent Process

Eligible participants (competent adults) will be invited to sign a digital informed consent form via the PM7 system. The process includes:

A face-to-face explanation Time for consideration An opportunity to ask questions The signed consent form will be stored digitally in PM7 and printed as needed. If digital signing is not possible, a paper form will be used. The process adheres to Regulation 169 and follows an updated consent protocol. Enrollment and consent will take place during the preoperative consultation at the bariatric clinic.

Inclusion Criteria Patients aged 18 and older scheduled for OAGB at Tel Aviv Sourasky Medical Center Exclusion Criteria Special populations - Patients under 18, pregnant women, individuals lacking decision-making capacity, etc.

Previous bariatric surgery Withdrawal Criteria TSBL < 450 cm, as this would prevent achieving a common channel length of at least 250-300 cm, increasing the risk of severe malabsorption and nutritional deficiencies (protein, vitamins, iron).

Study Methods and Design Randomization Process After enrollment and consent, participants will be randomly assigned to either the intervention or control group.

Surgical Interventions Intervention Group: The surgeon will manually measure TSBL intraoperatively and create a BPL equal to 40% of the TSBL. This ensures a minimum CC length of 250-300 cm, in line with standard OAGB recommendations.

Control Group: The surgeon will measure the TSBL but will create a fixed BPL of 180 cm, as per the current standard.

Postoperative Follow-Up Patients will undergo routine follow-up at 2 weeks, 1, 3, 6, and 12 months, and annually thereafter.

Data Collection and Privacy Identifiable patient information will be separated from the dataset by an authorized investigator.

The principal investigator will maintain the coding key in a password-protected file within the hospital network.

Data will remain within the hospital and not be shared externally. Statistical Analysis T-tests and chi-square tests will be used to compare baseline characteristics. Results will be reported in tables with standard deviation and p-values. Study Timeline and Duration Total study duration: 8 years from approval.

Inclusion Criteria:

• Adults (≥18 years) undergoing OAGB

Exclusion Criteria:

• Patients <18 years, pregnant women, or those lacking decision-making capacity Prior bariatric surgery Short bowel (<450 cm)