It's often considered a last resort for the severely overweight and obese, but gastric bypass surgery can be a lifesaver for one group of overweight patients: those with diabetes. Several recent studies have reported that the surgery not only reduces patients' risk of death - particularly from obesity-related diseases, including diabetes and coronary artery disease - but that in some patients with diabetes the surgery is practically a cure, resulting in normalization of blood sugar, often within days. That's part of the reason that gastric bypass is now the most commonly performed weight-loss surgery in the U.S., with nearly 140,000 procedures done each year.

But a new study introduces a curious wrinkle in the evidence. Led by Dr. Guilherme Campos, director of the Bariatric Surgery Program at the University of California, San Francisco, the study found that gastric-bypass patients with diabetes did not lose as much weight as other patients after the surgery. Of the 310 patients in the study, 92% of those without diabetes were able to lose more than 40% of their excess weight - statistically, that's considered a successful procedure - while only 79% of diabetes patients were able to drop that much weight after one year. In both cases, doctors used the same surgical technique, which involves permanently sectioning off a small pouch from the stomach and connecting it directly to the intestines, which forces patients to eat less and allows food to bypass the rest of the stomach and the upper part of the intestines.

So why the difference in weight loss between the two groups? Campos notes that although the procedure is the same from patient to patient, doctors currently do not use a standard size when creating the new stomach sac. Instead, surgeons use anatomical landmarks unique to each patient to determine the size of his or her new, smaller stomach. But because the stomach lining remains elastic and flexible, sometimes the small stapled-off pouches simply balloon back to a larger size, which explains why 5% to 15% of people who get gastric bypass surgery often experience little or no weight loss.

The other reason for the weight-loss disparity, Campos says, may have something to do with the medications that diabetes patients take to control blood sugar. "One of the known factors for why diabetics have trouble controlling their weight is the types of medications they take," says Campos. "Diabetes is a consequence of being overweight, but [another complication] is having to take medications that add to weight gain. It's a double-edged sword, and a vicious cycle." The solution, he points out, may be to rely on newer anti-diabetes drugs, such as the DPP-IV inhibitors (like Januvia, the first to receive FDA approval), that can help patients keep their blood sugar and weight under control. "We can have even better results in controlling diabetes after gastric bypass surgery if we change the way we manage diabetes until these patients can get off their drugs," he says.

But despite the fact that diabetes patients lost less weight after surgery than patients without the disease, 90% of the former group still saw their blood-sugar levels fall after the procedure. That meant they could also cut back significantly on the amount of medication they needed. According to Dr. Osama Hamdy, director of the Obesity Clinical Program at Joslin Diabetes Center, the chances may be even better for those patients who address their diabetes early on. "If you have had diabetes for a long time, your response to surgery may not be as good as that of people who have had diabetes for a short period of time," he says. "People who have had a longer duration of diabetes may reduce their medication, but they may never be able to stop them completely."

Campos' study did not stratify the diabetes patients in his study by how long they had been living with their disease. But researchers think the benefits of gastric bypass may indeed be greatest in those obese patients who are recently diagnosed with diabetes, since their bodies are more likely to revert back to normal sugar metabolism after surgery. That's because much of the post-bypass weight loss is spurred by a shift in the hormonal feedback loop that controls hunger and satiety. Production of certain weight-related hormones, such as ghrelin, or the hunger hormone, are directly reduced by the patient's physically smaller stomach (ghrelin is produced by glands in the stomach), leading to a reduction in food intake. Meanwhile, the smaller stomach more readily triggers hormones that signal satiety to the brain, sending the message that the body has taken in its fill of calories. But the longer this system has been overwhelmed with too much sugar and too many calories as occurs in diabetes, experts suspect, the more difficult it is to normalize the body's metabolic thresholds and molecular messages.

"I believe that the benefits of gastric bypass surgery outweigh any risk that a patient will have," says Hamdy. "If you look at the mortality in relation to obesity itself, especially if it occurs with diabetics, that is much, much higher than the risk of mortality from the surgery."

What Campos' study shows, however, is that patients should have realistic expectations of what the surgery can do for them: People without diabetes have the best chance of losing weight and reaping the heart and metabolic benefits of being slimmer; among diabetes patients, those undergoing surgery early on in their disease might fare better than those who wait longer. Both Campos and Hamdy remind patients that surgery is never the final answer - weight-loss maintenance continues long after you're discharged from the hospital, and can only be done the old-fashioned way, with a healthy diet and exercise plan. Surgery, after all, can only get you so far.

Related articles on Time.com:

• Gastric Bypass Lowers Risk of Death
• Studies Bring New Hope for Obese
• Study: The Best Exercise for Diabetes
• Study Doubts Knee Surgery Benefits
• Women + Fat = Diabetes

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