All animal use conformed to the European Community guidelines and was approved by the local ethics committee (Comité d'éthique en expérimentation animale N° 121) and the Ministry of Higher Education and Research (Ref #02285.03).

1. Animals and Diets

1. Rats
   1. Use male Wistar rats weighing 260 – 320 g, started on a high-fat diet (HFD) at 7 weeks of age.
2. Diet
   1. Induce a diet-induced obesity, with at least 12 preoperative weeks of the HFD. Give the rats free access to water and the HFD.
      NOTE: The HFD consists of a purified laboratory hypercaloric rodent diet containing 45% fat (see Table of Materials).
   2. Ensure that the rats are obese at the time of surgery, with a weight between 600 – 800 g.

2. Material

1. Surgical equipment
   1. Autoclave all surgical instruments before surgery.
   2. Lay out all the surgical instruments needed for the procedure (see Table of Materials).
   3. Perform the anastomosis under magnifying glasses, using conventional and microsurgical instruments.
   4. Ensure that the rats are positioned on the warming plate at a temperature of 37 °C and under a heat lamp. Position the isothermal electric board as needed.
2. Anesthesia and animal preparation
   1. Anesthetize the rat with an inhalation of 3% vol isoflurane in air for the induction of the anesthesia in the rodent box.
   2. Transfer the rat to the warming plate and maintain the anesthesia with the facemask at the same flow rate of 3%. Ensure that the anesthetization is complete by pinching the tail of the animal.
   3. Shave the anterior fur, corresponding to the laparotomy's incision zone, a ~3 cm-wide zone from the xiphoid to the animal pelvis, using an electric razor.
   4. Shave a 3 cm-wide zone on the back of the animal, centered on the neck of the animal. This step facilitates monopolar power conduction.
   5. Spray an alcohol-based disinfectant solution over the operation site.

3. Surgery

1. Laparotomy
   1. Apply alcohol-based local antiseptic on the shaved skin.
   2. Open the abdomen with a midline cutaneous incision from the xiphoid process of about 3 cm in length with a scalpel. Open the fascia and the peritoneum along the linea alba with a monopolar scalpel.
   3. Ensure that the peritoneal cavity is opened on the entire length of the cutaneous incision.
      Expose the peritoneal cavity with a self-retractor.
2. Greater curve and hiatal dissection
   1. Dissect the greater gastric curve from the junction between the glandular and nonglandular stomach to the left side of the abdominal esophagus with surgical scissors.
   2. Progressively coagulate the gastrosplenic vessels with the monopolar scalpel, in order to fully liberate the great gastric curve to the His angle. Then expose the hiatal area by sectioning of thin ligaments.
   3. Dissect the right side of the abdominal esophagus, preserving the left gastric artery and vagal nerve, using a manual dissector.
3. Gastric pouch confection
   1. Nonglandular stomach ablation
      1. Apply a 35 mm staple gun (see Table of Materials) at the junction between the glandular and nonglandular stomach to the left side of the gastroesophageal junction. Ensure that the nonglandular stomach is on the right side of the stapler.
      2. Fire the gun and then remove the resected nonglandular stomach.
   2. Gastric pouch
      1. Apply a second stapler using a thoraco-abdominal (TA) device (3 – 3.5 mm) (see Table of Materials), parallel to the first stapling, on the right side of the gastroesophageal junction to exclude the antrum and a part of the body of the stomach.
      2. Ensure that the result of the two staple lines is a gastric pouch about 0.5 cm wide.
4. OAGB
   1. Small bowel measurement
      1. Locate the duodenojejunal angle. The proximal jejunum is the small bowel portion located under the medial portion of the colon; it is the only nonmobile part of the small bowel in the rat.
      2. With a 5 cm-long tape, manually measure 35 cm from the duodenojejunal angle. Then perform a punctiform enterotomy on the antimesenteric side of the jejunum.
   2. Gastrojejunal anastomosis
      1. Perform a punctiform gastrotomy with the monopolar scalpel on the greater curve side of the gastric pouch, precisely between the two previous staple lines. If the gastric pouch is not totally empty, perform gentle manual pressure on the gastric pouch to empty it.
      2. Perform an end-to-side gastrojejunal anastomosis with nontraumatic forceps and a small needle-holder.
      3. Start the suture with two extremity stitches of 7-0 isotactic polypropylene (see Table of Materials), suturing the two extremities of the gastrotomy with the extremities of the enterotomy.
      4. Then, with the same threads used for the two extremity stitches, perform two running sutures on each side of the anastomosis. If needed, return the anastomosis to facilitate the maneuver. Ensure the quality of the anastomosis by placing close stitches, at least seven or eight on each side of the anastomosis.
      5. Take 1 mm-wide serosa in every stitch. Avoid taking any mucosa in the stitch in order to gently bury the enteric and gastric mucosa inside the suture.
         NOTE: The result of this is a terminolateral gastrojejunal anastomosis with a biliopancreatic limb on the left and an alimentary (AL)/CL on the right. Ensure that the anastomosis is permeable with gentle pressure on the gastric pouch.
5. Closure and immediate care
   1. Rinse the peritoneal cavity with 0.9% NaCl solution and manually replace the abdominal organs into their physiological positions.
   2. Close the abdominal wall with one running suture of braided polyester suture 2-0 (see Table of Materials) with a classical needle-holder.
   3. Close the skin with one running suture of polyglactin 910 4-0 (see Table of Materials).
   4. Perform two parietal injections of lidocaine 1% on each side of the laparotomy. Administer a total dose of 1 mL/kg of lidocaine 1% into the rectus abdominal muscle aponeurosis.

4. Postoperative Care

1. Perform an intramuscular injection of 20,000 units/kg penicillin G.
2. Perform a subcutaneous injection of 10 mL of isotonic polyionic solution potassium (2 g of KCl/L, 4 g of NaCl/L, and 50 g of glucose/L). In this injection, it is possible to mix with suitable analgesics such as buprenorphine (0.1mg/kg) and meloxicam (2mg/kg).
3. Place the rats in individual cages warmed up by a lamp until they are fully awake.
4. Frequently inspect the animals in order to administer postoperative treatment with suitable analgesia and hydration (isotonic polyionic solution).
5. Inject suitable analgesic, buprenorphine (0.1 mg/kg) and meloxicam (2 mg/kg), subcutaneously from the immediate postoperative period till day 4, every 12 h.
6. Keep the rats with free access to water during day 1. On day 2/3, give them free access to a liquid diet. On day 4, give them ad libitum access to a normal diet.