Obese people who undergo gastric bypass surgery may lose weight quickly because levels of a hormone controlling appetite fall away, research shows.
The hormone, ghrelin, is released into the blood from the stomach and upper intestine and triggers hunger pangs.
It is one of two-dozen hormones thought to help regulate hunger.
The Archives of Surgery study, by Atlanta's Emory University, found ghrelin levels dropped by nearly one-third after gastric bypass surgery.
The researchers focused on a particular type of gastric bypass surgery known as the Roux-en-Y procedure.
This involves dividing the stomach to create a pouch that is attached to the small intestine.
It reduces the size of the stomach and bypasses parts of the gastric system that absorb food and may trigger releases of the hormone.
However, it is considered to be risky and is usually only offered to patients who are severely overweight and who have been unable to lose weight using other methods.
The researchers checked levels of the hormone in 48 obese patients, 34 of whom had undergone gastric bypass surgery.
Another eight obese patients had other types of weight-loss procedures that did not divide the stomach, and six non-obese patients undergoing surgery were monitored to provide a base level of the hormone.
None of these 14 patients showed any reduction in ghrelin levels.
'Prevent overeating'
Lead researcher Dr Edward Lin said: "This is the first time we have found that different types of weight-loss surgeries affect ghrelin levels in humans.
"When the stomach is divided, forming a small gastric pouch, the ghrelin levels are significantly reduced early after the surgery is performed.
"Other mechanisms can make you hungry, but ghrelin is one of the most potent hunger stimulants."
Dr Lin said patients with morbid obesity have lower baseline ghrelin levels compared with lean counterparts.
However, he added: "This study shows that surgically induced weight loss with RYGB in these patients appears to cause long-term, if not permanent, suppression of ghrelin secretions that is not associated with other weight-loss procedures.
"If you can find a way to control the release of ghrelin in the body, you can potentially prevent a person from overeating."
Dr Ian Campbell, of the UK National Obesity Forum, told BBC News Online that gastric bypass surgery was declining in popularity as doctors preferred to use safer laparoscopic surgery techniques.
However, he said: "It does not matter how it works, as long as it does, and for somebody who is morbidly obese, and for who nothing else has worked, surgery of this kind can be a lifesaver."