Scientists have discovered a hormone that suppresses appetite, raising hopes of new treatments in the fight against obesity, according to a study published today.

The hormone, named obestatin, halved food intake in rats and resulted in the animals losing a fifth of their body weight.

Neville Rigby, the director of policy at the International Obesity Taskforce, welcomed the discovery as another example of the fact that there was more to obesity than most people think. "It helps understand that it isn't simply, as people would have it, a question of obesity being a problem of sloth or gluttony. There are clearly mechanisms at work in the body which differentiate why one person becomes obese while another person seems to be unaffected. As we understand more of the science of obesity, we have more sympathy for the people affected."

Anyone with a body mass index of greater than 30 is classed as obese, and the World Health Organisation estimates there are some 300 million such adults worldwide. In the UK, more than a fifth of the adult population is obese and a further half of men and a third of women are classified as overweight. Obesity is a major risk factor linked to heart disease, diabetes and premature death.

Finding ways to combat this obesity epidemic has been a priority for many health researchers. Part of the work revolves around understanding the hormones that regulate body weight and food intake.

"This new research is a new piece of the puzzle in the complex system of bodyweight regulation," said Katrina Kelner, deputy editor of life sciences at Science, where the study is published today.

"We knew before that a hormone called ghrelin that was produced into the gut and then secreted into the bloodstream, stimulates eating. The new work shows us that a new hormone, aptly called obestatin, is encoded by the same gene but exerts opposing effects - it inhibits food intake. This is a completely unexpected finding and it's really extraordinary to think the hormone had been sitting there in plain sight until these authors discovered it."

Aaron Hsueh of the Stanford University school of medicine, who led the team that made the obestatin discovery, said the work came out of a larger project to understand the role of hormones in human physiology.

He had been using the results of the human genome project to create a database of hormone receptors for which there were no known partner hormones. He then identified the ones that seemed most important biologically - the ones that have been conserved through evolution across many species.

The hunt led him to the gene that makes ghrelin, where he found DNA instructions for an unexpected hormone tacked on to the end.

Professor Hsueh set out to make the hormone, which he later named obestatin. "We purified this hormone in rats' stomachs and tested its biological activity," he said. "To our surprise, we found that treatment with obestatin actually suppresses food intake. The food intake [dropped] by more than 50%. Bodyweight is more like 20% down. So the same gene codes for two hormones and these two hormones have opposing actions in bodyweight regulation. "

Prof Hsueh said his discovery had lots of potential uses. "Obestatin itself could have potential as an appetite-suppressing drug because one can use this small peptide by injection," he said. "The identification of the receptor for obestatin can also allow us to screen for new drugs that can also suppress appetite."

He added, however, that people with obesity should not expect drugs too soon. "Whether it's going to be effective or not it's too early to say," he said.

Obesity researchers have been here before with another hormone - leptin - which signals to the brain to stop eating. In 1995 scientists discovered that, in mice, leptin had a near-miraculous effect of reducing bodyweight by nearly a third. For a while it was hailed as a precursor a wonder drug. But it never lived up to its promise in humans.

Matthias Tschöp, an obesity specialist at the University of Cincinnati, said there were several unanswered questions on obestatin. "We don't know if obestatin administration to rats or mice causes some sort of illness or nausea, leading to a decrease in food intake," he said.

And the hormone might have properties which are yet to be discovered. "It could be that obestatin has a major effect on food intake but, at the same time, does something to energy expenditure that, at the end, balances things out," Professor Tschöp said.

Many hormones are known to affect eating patterns inadvertently: rexin, for example, makes people feel more aware of their surroundings and, as a result, eat more.

"It demonstrates nicely how a hormone that also influences food intake also has other biological functions and, at the end of the say, also influence food intake or bodyweight," Prof Tschöp said. "It could be that mother nature just put [obestatin] in place to regulate appetite or something whose primary function is something else."

Mr Rigby cautioned against thinking that obestatin was the whole story. "Each time we think we've found another of the key hormones, yet one more comes along," he said. "The more you know, the more there is to know. It's a big hill to climb in terms of accumulating all the knowledge we need to address obesity from the biological perspective."

Footnotes

Hormone

Chemical messenger of the body. Hormones feed the brain information on the state of the body and allow the brain to respond. Ghrelin instructs the body to eat, while leptin and obestatin tell the brain that a person is full.

Body mass index

Common measure of a person's weight relative to their height. Above 25 is overweight; above 30 is obese.

Human genome project

Mapping project started in the late-90s and completed in 2001: it sequenced human DNA and identified the location of the 30,000 genes which code for all the proteins needed for human life.

Hormone receptors

The proteins on cells that respond to the actions of specific hormones. Each hormone has its own type of receptor in the location where it is supposed to work.

Small peptide

A short strand of protein, the make-up of many hormones. Promising in terms of treatments because they are easy to make and deliver to patients.