Gastric bypass, in which the stomach is surgically shrunk to allow less food to be eaten, carries extra benefits such as a better life quality, decreased risk for obesity-related illnesses and a longer life.

Although gastric bypass surgery is believed to be a sure bet for losing a great amount of weight, not all people who undergo the procedure experience tremendous success.

A new study done by the researchers at the UC San Francisco found two factors these unfortunate people have in common: diabetes and a larger stomach pouch.

They examined data from 310 people with a starting body mass index of 52 (a BMI over 40 is considered morbidly obese)who had undergone the gastric bypass surgery. After a 12-month follow-up, most of the participants lost an average of 60% of their excess body weight, and had an average BMI of 34. However, 38 people (12.3%) were in the poor weight-loss category, losing less than 40% of excess body weight.
After adjusting for factors such as age, sex, race and insurance status, the researchers found diabetes and a larger stomach pouch to be linked to the smaller weight loss.

Although bypass surgery has been shown to cure diabetes or at least decrease symptoms, patients taking insulin may be at a disadvantage, since it may promote fat and cholesterol production. When suffering from low blood sugar episodes, diabetes patients may be eating more and therefore gaining more weight. Those with the disease may also need to eat more when experiencing low blood sugar episodes, leading to weight gain.

The size of the stomach pouch may be an important factor too. The researchers report that the pouch sizes vary but also that most surgeons estimate the pouch size solely on anatomical landmarks rather than a sizing balloon.