BAKERSFIELD, Calif. -- More than 200,000 Americans underwent a gastric bypass operation last year to treat their obesity.

The surgery severely reduces the size of the stomach and therefore limits the amount of food patients can eat.

For many, the weight loss following gastric bypass is dramatic -- the average patient loses around 75 to 80 percent of his or her excess body weight after just one year. But for a minority of patients, the extra weight remains a problem.

Researchers studying more than 300 people who underwent gastric bypass found that 12 percent had poor weight loss -- shedding an average of 32 percent of their excess body weight. One participant lost just 11 pounds.

Doctors discovered that patients with diabetes and those whose stomachs were not reduced as much were three times as likely to have poor weight loss following their surgery.

They said the lesson for surgeons is to ensure the stomach is very small after the operation.

For patients, doctors said the best way to maximize weight loss is to follow doctor's orders regarding diet changes, vitamins and follow-up care.