In this procedure, surgeons restrict food intake by creating a smaller stomach pouch that bypasses large sections of the digestive system.

"When performed in high-volume centers and with a low rate of complications, gastric bypass provides sustained and meaningful weight loss, significant improvements in quality of life, improvement or resolution of obesity-associated co-morbidities, and extended life span. However, 5 percent to 15 percent of patients do not lose weight successfully, despite perceived precise surgical technique and regular follow-up," the researchers wrote.

The researchers, who analyzed data from more than 300 gastric bypass patients, defined poor weight loss as losing 40 percent or less of excess body weight after one year and good weight loss as losing more than 40 percent of excess body weight.

Before gastric bypass surgery, the 310 patients had an average body mass index (BMI) of 52. One year after surgery, the patients had an average BMI of 34 and had lost an average of 60 percent of excess body weight. However, 38 patients (12.3 percent) had poor weight loss.

After they adjusted for different factors, the researchers concluded that diabetes and having a larger size of stomach pouch after surgery were independently associated with poor weight loss.

The study authors noted that people with diabetes take insulin or other drugs that stimulate the production of fat and cholesterol.

"Other factors that may lead to weight gain in patients with diabetes include a 'protective' increase in caloric intake to treat episodes of hypoglycemia (low blood sugar), reduction of urinary glucose losses, and sodium and water retention that are a direct effect of insulin on the distal tubule in the kidney," they wrote.

The researchers added that proper stomach pouch size is a critical part of gastric bypass surgery, but many surgeons estimate pouch size using anatomical landmarks rather than using a sizing balloon.

"As the use of gastric bypass continues to grow, we believe it is critical to stress the importance of, and to teach the creation of, the small gastric pouch and to better understand the technique used for pouch creation," the study authors wrote.

"Changes in the use of diabetes medications may reduce the risk of poor weight loss among diabetics undergoing gastric bypass. Detailed attention to the creation of a small gastric pouch is essential for achieving the best results," they concluded.

The study was published in the September issue of the journal Archives of Surgery.

More information

The U.S. National Institute of Diabetes and Digestive and Kidney Diseases has more about bariatric surgery.