Enteromedics is now enrolling patients for a larger, placebo-controlled trial of 300 patients in the United States and Australia. Two-thirds of the participants will have the device turned on immediately following implantation, while it will remain off in the other third for several months. Scientists will compare weight loss and other factors, and follow all patients for four years to assess long-term effects.

"I'm not sure it will ever have the same weight loss as gastric bypass would have," says Noel Williams, director of the Bariatric Surgery Program at the University of Pennsylvania School of Medicine, in Philadelphia, who is not involved in the trials. "But a certain population of patients, those with BMIs that are not as high [as candidates for gastric bypass], might do well with it."

Proof of principle for the vagus nerve as a target for obesity came accidently. In the 1950s, surgeons often severed all or part of the nerve as a treatment for ulcers. (The vagus nerve also controls the release of gastric juices.) Many people who underwent the procedure reported feeling less hungry and lost weight. But the effect was short lived: after two or three months, the body adjusted, and patients returned to their old eating habits. Knudson and his team believe that intermittently blocking the nerve can prevent the nervous system and digestive organs from compensating for the change.

Experts warn that testing of this device is still in the early stages. It is just one of dozens of medical devices being developed to target the gastrointestinal system in obese patients, including new versions of lap bands, devices that take up space in the duodenum, and devices that alter gastric motility, says Lee Kaplan, director of the weight center at Massachusetts General Hospital. In fact, some devices that stimulate rather than block the vagus nerve are also under development, although they're in an earlier stage of research. "At this point, no one can say with any certainty which device is most likely to be successful," says Kaplan.

Previous efforts to use electrical stimulation to treat obesity have had mixed results. A device that targeted electrical stimulation directly to the stomach helped some people but not others. "The problem was that research was never done to figure out what was the mechanism of action," says Scott Shikora, chief of bariatric and minimally invasive surgery at Tufts Medical Center, in Boston. Shikora, who is overseeing one arm of Enteromedics' new trial, says that he is optimistic about the new device.

One of the problems with electrical stimulation therapies is in determining the type of electrical signals to use. Knudson says that VBLOC therapy gets around this problem by using high-frequency signals to completely block the nerve. However, it's not yet clear what the broad impact will be. So far, more than 100 patients have undergone the procedure, and no deaths or serious adverse events linked to the device have been reported. But the company says that data on milder side effects has not yet been reported.