Background <p>Endoscopic sleeve gastroplasty (ESG) is a minimally invasive procedure that is less effective for weight loss. Single anastomosis sleeve ileal bypass (SASI) combines sleeve gastrectomy with an ileal bypass to maximize weight loss. We compared the postoperative results of ESG vs. SASI at 6 months to evaluate weight reduction and nutritional outcomes.</p> Methods <p>This retrospective study included 21 patients. Body weight and levels of serum iron, calcium, zinc, vitamin B12, and folate were measured preoperatively and at 1, 3, and 6 months postoperatively. The SASI group received postoperative supplementation of vitamin B12, folate, calcium, and ferritin, whereas the ESG group did not. Statistical analyses assessed changes within and between groups.</p> Results <p>At 6 months post-surgery, weight loss was greater in the SASI group. Although iron levels in both groups declined at 1 month post-surgery, they returned to baseline levels at 6 months. At 3 and 6 months post-surgery, the ESG group had a slight decrease in calcium levels, whereas the SASI group showed a significant decrease in zinc levels. No vitamin B12 or folate deficiencies were observed. At 6 months post-surgery, changes in micronutrient levels were comparable between the groups. The operative time and hospital stay were longer in the SASI group.</p> Conclusion <p>SASI achieved greater weight loss at 6 months post-surgery than ESG but required a longer operative time and hospitalization. With appropriate supplementation, short-term nutritional outcomes were similar in both groups. Long-term monitoring for iron and B12 deficiency is required following SASI.</p>