Gastric bypass surgery is a type of bariatric (or weight-loss) surgery that can help patients with obesity lose weight through metabolic and hormonal adaptation by reducing the size of their stomach and re-routing the small intestine. It’s one of the most well studied and widely known weight-loss surgeries in the United States.

The procedure is known as “Roux-en-Y” surgery, which describes the new shape the small intestine takes on after surgeons reroute and reposition it.

During the procedure, surgeons section off a small pouch of the patient’s upper stomach (near the esophagus) to hold food. Surgeons then reroute part of the small intestine, connecting it to the bottom of the newly created stomach pouch. As a result, when a gastric bypass surgery patient eats, the food enters the small stomach pouch, then moves into the small intestine, bypassing the majority of the stomach and a portion of the small intestine, resulting in a favorable alteration of the hormones responsible for weight gain.

Gastric bypass surgery patients lose weight because having a smaller stomach pouch limits how much they can eat at one time. Additionally, the procedure has a positive effect on a patient’s hunger and satiety signals (more on that below), which also encourages them to consume less. However, the digestive tract’s new configuration may prevent the body from absorbing some nutrients, which, in rare cases, can result in malnutrition. As a result, patients may need to take nutritional supplements for the rest of their lives.

The procedure is safe and routine, but there is a low risk of complications, as with any surgical procedure. There is also a small risk of regaining weight. Patients who continue to consume large amounts of food despite feeling full may stretch out their stomachs over time.

Patients with obesity who undergo gastric bypass surgery typically lose between 65% and 70% of their excess body weight over time.

Gastric bypass surgery is a surgical procedure that can help patients with obesity lose weight by restricting the amount of food they can consume. It can be a safe, effective way for people with obesity to lose a significant amount of excess body weight in the long term.

During gastric bypass surgery, surgeons change how food moves through the digestive system. They section off a small stomach pouch to hold food, separating it from the rest of the stomach, thereby limiting the volume of food it can hold.

Surgeons also cut, move, and reattach a section of the small intestine so that food moving through the digestive system bypasses certain areas. The combination of these actions is designed to prevent people who have had gastric bypass surgery from overeating. It also has a positive effect on the hormones that affect hunger and satiety.

Gastric bypass surgery is typically performed as a laparoscopic procedure: Surgeons insert their tools and a camera through small incisions in the patient’s abdomen. (The patient will be under general anesthesia.)

First, surgeons create an egg-sized pouch near the top of the stomach by sectioning off an area that can hold 20 or 30 milliliters of food or liquid. They staple together the edges of this pouch, as well as those of the larger portion of stomach that has been bypassed by the procedure.

Next, surgeons cut through the small intestine—below the juncture where the stomach and small intestine meet. They connect the lower end of the small intestine to the bottom of the newly formed stomach pouch with stitches and staples. This creates a new pathway for food to travel through, from the stomach pouch to the lower portion of the small intestine.

After that, surgeons connect the detached end of the small intestine to the section of the small intestine that now travels out of the new stomach pouch. Important fluids that aid in digestion, including digestive enzymes and bile, travel through this section of the small intestine. Because the fluids encounter food in the intestines several feet farther along in the digestive process than usual, they play a less active role in digestion.

This change to the digestive process limits the amount of ghrelin (the “hunger hormone”) that the larger, lower portion of the stomach secretes. With less ghrelin, a patient with obesity may feel less hungry and eat less. The procedure also increases the levels of two hormones that increase feelings of “fullness,” glucagon-like peptide 1 (GLP-1) and peptide YY (PYY).

Patients usually stay in the hospital overnight after gastric bypass surgery. They need to consume a liquid diet for the first two weeks after the procedure. Over a few weeks, they can transition to softer foods and, eventually, a diet that includes more solid healthy foods.

Gastric bypass surgery is a procedure used to treat obesity. Patients with a body mass index (BMI) in certain ranges may be eligible for the procedure, helping them to achieve long-term weight loss.

Doctors calculate a person’s BMI by using their height and weight. Obesity is indicated by a BMI of 30 or higher.

Patients with obesity who may be eligible for gastric bypass surgery include:

• Those with a BMI of 40 or higher
• Those with a BMI of 35 or higher with at least one chronic health condition associated with obesity, including type 2 diabetes or high blood pressure
• Those with a BMI between 30 and 35 with diabetes or one or more chronic health conditions related to obesity who have been unable to sustain weight loss through non-surgical efforts

Patients do not need to specifically try or fail anti-obesity medications to be eligible for bariatric surgery, including gastric bypass.

Gastric bypass surgery can be a safe and effective way to help patients with obesity lose—and keep off—weight. Additionally, this procedure enables patients to lose the greatest amount of excess body weight—even more than through gastric sleeve surgery (sleeve gastronomy), the most widely performed weight-loss surgery in the U.S.

The procedure can often reverse or reduce the symptoms of patients with type 2 diabetes. Gastric bypass surgery may also help patients with sleep apnea and acid reflux, or gastroesophageal reflux syndrome (GERD), experience fewer symptoms.

Research has shown that patients who undergo Roux-en-Y surgery lose more excess body weight in the long term (about 10 years) than those who have undergone other bariatric surgical procedures, including sleeve gastrectomy and gastric band surgery.

When patients with GERD need bariatric surgery, doctors usually recommend gastric bypass surgery because it may help to improve GERD symptoms. By comparison, sleeve gastrectomy often causes or exacerbates GERD.

The following risks are associated with gastric bypass surgery:

• Bleeding or infection. After gastric bypass surgery, patients may experience bleeding or infection, which are risks that arise after any type of surgery. Bleeding may also occur at staple sites.

• Fluid leakage. Gastrointestinal fluid may leak from the stomach after surgery. Depending on the location of the leak and the volume of fluid that escapes, this complication may be life-threatening.

• Malnutrition. Patients may experience malnutrition or vitamin deficiencies because their bodies absorb fewer nutrients from food. Although it’s uncommon for a patient to have a deficiency of iron, vitamin B12, vitamin D, or folate, doctors recommend that patients take multivitamins for the rest of their lives.

• Dumping syndrome. Patients are more likely to experience a phenomenon known as “dumping syndrome” than those who have undergone other types of weight-loss surgery. Dumping syndrome occurs in about 15% of patients when they consume foods or beverages containing a large amount of sugar or other simple carbohydrates too quickly, which may accelerate the passage of food through the stomach and into the small intestine, resulting in abdominal pain and nausea.

• Gallstones. Having gastric bypass surgery may increase the risk of gallstones (a hardened deposit in the gallbladder that may cause pain), which may arise when a person loses weight quickly.

• Hernias or ulcers. The procedure may increase the risk of a hernia (bulging of an organ or tissue through an abnormal opening) at a surgical incision site or an ulcer (or sore) at the site where the stomach pouch and small intestine meet.

• Obstructions or fistulas. Sometimes, gastric bypass patients develop an obstruction in their intestines. They may develop a fistula (abnormal connection) between the new stomach pouch and the larger stomach.

Because of these potential complications, follow-up visits are recommended.

This article was medically reviewed by Neil Floch, MD, a Yale Medicine bariatric surgeon and director of Bariatric Surgery at Greenwich Hospital, and John Morton, MD, MPH, MHA, medical director of Bariatric Surgery for the Yale New Haven Health System.