SASI developed from the transit bipartition concept. The operation first creates a long sleeve-shaped stomach, then connects the lower stomach to a distal portion of small bowel without completely dividing the bowel.
01 — ANATOMY
What changes during SASI bypass?
A sleeve gastrectomy is created
The wide outer portion of the stomach is removed, leaving a narrower tube. This part resembles a conventional sleeve gastrectomy.
A point in the ileum is selected
The surgeon measures the small bowel and selects a distal segment. Limb length and anastomosis size are technical variables that may affect outcomes and complications.
The stomach is connected to the ileum
A gastro-ileal anastomosis is formed between the lower stomach and the selected bowel segment. The bowel is not fully divided into separate limbs.
02 — MECHANISM
Food can follow two routes.
The defining feature is bipartition: part of a meal continues through the pylorus and duodenum, while another part passes through the new connection and reaches the ileum earlier.
Preserved natural route
Stomach → pylorus → duodenum → jejunum → ileum
Food continues to mix with bile and pancreatic secretions through the natural upper pathway.New gastro-ileal route
Stomach → anastomosis → ileum
Part of the meal reaches the distal small bowel sooner.Weight and glucose effects cannot be explained by “restriction” or “malabsorption” alone. Smaller stomach volume, altered gastric emptying, bile-acid signalling, gut hormones, satiety, and nutrient flow may all contribute — and responses vary between people.
03 — DIFFERENCES
How SASI differs from other bariatric procedures
Sleeve gastrectomy
Changes stomach size and physiology without creating a bowel connection. SASI adds a gastro-ileal route to the sleeve.
Roux-en-Y gastric bypass
Creates a small gastric pouch and two bowel connections with separated limbs. Food does not pass through the pylorus.
OAGB / mini gastric bypass
Uses one gastro-jejunal connection and bypasses pyloric food passage. SASI retains a functioning natural pathway alongside the new route.
SADI-S
Divides the duodenum and connects it to the ileum. SASI does not divide the bowel and connects the stomach directly to the ileum.
04 — PROCEDURE CHOICE
Why might one person be offered SASI and another a different operation?
Reflux history, endoscopy, diabetes duration, previous abdominal surgery, eating patterns, medicines, nutrient deficiencies, pregnancy plans, and access to follow-up may all change the balance. Meeting general bariatric surgery criteria does not automatically make SASI the best option.
05 — EVIDENCE LIMITS
What do we still not know?
Recent systematic reviews report encouraging weight and metabolic outcomes. Some pooled comparisons favour SASI over sleeve for selected outcomes and find no statistically significant differences from OAGB in others. However, follow-up is often short, techniques vary, and randomised evidence is limited. Better long-term data are needed for nutrition, bile reflux, revision, and durability.
Sources: Ataya et al., 2024; Oliveira et al., 2024. Evidence search updated August 2026.