"They don't have to reach their ideal weight in order to make some pretty significant health improvements," Dr. Joan F. Carroll of the University of North Texas Health Science Center in Fort Worth, one of the study's authors, told.

In the operation Carroll and her team are investigating, known medically as laparoscopic gastric banding surgery, an elastic band is placed around the stomach, restricting the amount of food the stomach can comfortably hold. Another procedure, surgical gastric bypass, has been shown to help reduce the body's resistance to insulin - often a prelude to full-blown diabetes - before major weight loss has taken place, but less is known about how lap band surgery affects insulin resistance.

To investigate, Carroll and her team have been following 37 lap band patients for up to one year. Those followed for six months have lost 23 kilograms (51 pounds), on average, while average weight loss for those who have been followed for a year is 34 kg (75 pounds).

Their level of insulin resistance had fallen by 60 percent after six months, she told, even though the patients remained clinically obese.

Given that resistance to insulin greatly increases the risk of developing type 2 diabetes, which has a number of other health consequences including heart disease and even amputations, "over the long term it's really a benefit for all the body systems," Carroll said.