Scientists used clues from the Human Genome Project to find a hormone that seems to suppress appetite in rats.
Three hormones, leptin, melanocortin and ghrelin, are known to act on appetite and weight.
The discovery of each raised interest in weight-control strategies, but scientists say dozens of hormones are likely involved. Little is understood about the stomach and brain's complex system of regulating weight.
Researchers at Stanford University School of Medicine identified the latest hormone, obestatin, by studying human genome data for hormone receptors.
The hunger hormone ghrelin was found to have obestatin attached to it.
The team then found obestatin was present in rat stomach tissues and brains.
When obestatin was injected into rats, it suppressed food intake in the animals by about half compared to those not given the hormone, the team reports in the Nov. 11 issue of the journal Science.
Obestatin seems to act by putting a brake on ghrelin, say the researchers, who found it also slowed the movement of digested food from the stomach into the intestines.
The normal rats lost 20 per cent of their weight in eight days. Researchers plan to test its effect on obese rats next.
They also need to test if obestatin made the rats eat less by suppressing appetite directly or by making the rodents feel ill.
Johnson & Johnson Pharmaceutical Research & Development, LLC sponsored the research into the newly identified hormone. The company has licence rights on the discovery.