The condition was corrected with further surgery and no lasting effects, and the researchers and other experts said the problem is probably too rare to warrant cutting back on the weight-loss procedure.
In fact, the problem might suggest new ways of treating diabetics.

"If we can understand the molecular details here, we can bottle them," said Dr. David Cummings, a hormone researcher at the University of Washington. He wrote an editorial accompanying the federally funded study in Thursday's New England Journal of Medicine.
The possible complication stems from the stomach-bypass procedure known as the Roux-en-Y technique. A small pouch is stapled off from the rest of the stomach and then connected directly to the small intestine. The pouch can handle only small amounts of food.

This technique accounts for the vast majority of the roughly 140,000 gastric-bypass operations performed each year in the United States. The number has been rising at almost 50 percent a year in response to the obesity epidemic.
Researchers at the Mayo Clinic of Rochester, Minn., focused on six Roux-en-Y patients who developed severe low blood sugar, or neuroglycopenia. They suffered temporary confusion, passed out or developed tunnel vision after eating.

"For people who have it, they basically have to be baby-sat: They cannot be left alone, they can't drive," said one of the researchers, Dr. F. John Service.

The researchers suspect the effect happens like this: Barely digested food rushes right into the intestine. Its hormones then overstimulate the insulin-oozing beta cells of the pancreas. The excess of insulin -- the same hormone that fails in diabetics -- removes too much sugar from the blood.

To correct the condition, doctors had to remove most of the pancreas from the patients. But that put the patients in danger of developing diabetes, an illness that is often cured by gastric bypass surgery.

Dr. Neil Hutcher, a stomach-bypass surgeon who is president of the American Society for Bariatric Surgery, said he has never seen the possible complication in his roughly 3,000 operations.

"Do I think it can be called a substantial complication of gastric bypass at this time? Absolutely not. Do I think it's a reason to modify gastric bypass at this time? Absolutely not," he said.