Two-stage surgery for high risk obese patients could drastically improve their health, surgeons say.
Obesity linked to a range of illnesses
Doctors first removed part of the stomach and then, in a separate operation, inserted a bypass in the intestines.
The first stage allowed significant weight loss so the second stage could go ahead.
A study of 75 patients was presented to the Society of American Gastrointestinal Endoscopic Surgeons.
The morbidly obese patients, aged from 23 to 72, first had a laparoscopic sleeve gastrectomy. This removes a large part of the stomach.
They were later given a gastric bypass, which involves constructing a pouch and bypassing a small segment of the intestines.
University of Pittsburgh researchers found this reduced the average body mass index (BMI) of patients by 19 points to 49 points after six months.
A person's BMI is calculated by dividing weight in kilograms by height in metres squared. A BMI of over 30 is considered obese.
The risk from the gastric bypass, or Roux-en-Y, operation itself also fell significantly in all the patients.
Life threatening conditions associated to obesity, such as diabetes, obstructive sleep apnoea, hypertension and asthma, were improved or resolved in all the patients.
Professor Philip Schauer, assistant professor of surgery and co-director of the minimally invasive surgery centre at the University of Pittsburgh, said obese patients were often prevented from having surgery.
But by first performing the removal of part of the stomach, it then became safe to carry out the gastric bypass.
He said: "Our study found that by performing this less drastic surgery first, allowing the patient to lose a substantial amount of weight, and then performing the laparoscopic Roux-en-Y, mortality was greatly reduced."
The UK's National Hospital For Obesity Surgery said large scale trials would be needed to confirm the findings.
David Allison, hospital manager, said doctors in the UK were unlikely to alter the way they practise on the basis of one set of findings and full clinical trials would be needed.
He added: "Surgery should always be a last resort. Even surgical options won't provide a miracle cure. The patient has to be motivated."