Roux-en-Y gastric bypass (RYGB) delivers metabolic benefits for many patients with type 2 diabetes who are obese. The source of those benefits was a topic of debate on Saturday, June 26, during the Scientific Sessions.

Debate—Are the Metabolic Benefits of Roux-en-Y Gastric Bypass Due to Weight Loss Alone? can be viewed by registered meeting attendees at ADA2021.org through September 29, 2021. If you haven’t registered for the Virtual 81st Scientific Sessions, register today to access all of the valuable meeting content.

Samuel Klein, MD, Center for Human Nutrition, Washington University School of Medicine, argued that the benefits of RYGB are due to weight loss alone. Dr. Klein said studies show that RYGB has unique effects on the patterns of metabolic responses in patients. However, he said, the therapeutic effects that are induced by gastric bypass surgery on the metabolic response to mixed-meal ingestion, 24-hour glucose and insulin profiles, liver, adipose tissue, skeletal muscle sensitivity, and beta cell function are primarily due to weight loss itself.

Dr. Klein noted a paper from the 1990s that featured a patient with type 2 diabetes who was scheduled for gastric bypass, but during the procedure was found to have a stomach full of food. Surgeons stopped the procedure and requested the patient follow the same diet as if he had undergone a successful bypass. Blood glucose levels initially fell rapidly, in similar fashion to patients who had undergone gastric bypass surgery, but then rose to preoperative levels when the patient was unable to maintain the diet about a month post-procedure.

“This shows that when you reduce calories, you don’t need medications to treat diabetes if you have a marked reduction in calorie intake, and people who have RYGB surgery frequently consume only a few hundred calories a day for days after the procedure,” Dr. Klein said.

Blandine Laferrère, MD, PhD, Professor of Medicine, Columbia University, argued that the metabolic effects of RYGB extend beyond weight loss. While the surgery results in significant weight loss and is a useful tool to help people with severe obesity and type 2 diabetes, RYGB likely has more mechanisms that bring improvements in metabolism and contribute to decreased cardiovascular risk, she said.

Dr. Laferrère noted several studies, including some that she and her colleagues conducted, illustrating these benefits. A rodent study, for example, showed that gluconeogenesis is a key factor for metabolic changes post-RYGB, but not after gastric banding.

“Overall, the Roux limb has an enormous bioenergetic demand, and we see a response to that, and that resulted in whole-body glycemic control,” Dr. Laferrère said. “Now, whether this applies to humans has not been tested, but this is a very interesting mechanism.”

Other metabolic benefits of RYGB include reduced postprandial glycemic excursions resulting from smaller meal sizes, she said.

Dr. Laferrère also highlighted research showing that RYGB patients had lower total cholesterol, increased high-density lipoprotein (HDL) cholesterol, and lower non-HDL cholesterol compared to patients who had undergone laparoscopic adjustable gastric banding (LAGB) when the groups were matched with 20% weight loss at one year post-surgery. The LAGB group had slight increases or no change in total cholesterol and non-HDL cholesterol and a small decrease in HDL cholesterol, she said.