US researchers have developed a vaccine which prevents weight gain in rats, offering clues about human treatments.
Many treatments for obesity have so far proved unsuccessful
The vaccine prompts the body to produce antibodies against ghrelin, a hormone that stimulates hunger and weight gain.
Vaccinated rats put on less weight while eating the same amount as those which did not have the jab.
A UK obesity expert said the Proceedings of the National Academy of Sciences study was interesting, but it might not be safe for people.
Researchers at the Scripps Research Institute in California developed three synthetic vaccines that recognise different parts of the ghrelin hormone.
When injected into rats, two of the vaccines were found to bind to the active form of ghrelin, inducing antibodies against the hormone and blocking its action.
The vaccinated rats gained less body weight - specifically body fat - and the reduction was associated with the levels of antibody present.
The reduction in weight gain occurred despite the rats eating and drinking normally, indicating that the inhibition of ghrelin was having an effect on the animals' metabolism.
The researchers said the results demonstrated that active immunisation against ghrelin could be used to control weight gain and accumulation of fat tissue in mammals.
Study leader Professor Kim Janda said: "We believe that the active form of ghrelin is what we are blocking but we can't say for certain that's the only effect.
"Ghrelin slows down the metabolism so the vaccine is affecting that and also fat storage. This is a promising bit of science."
Professor Janda said the ultimate goal was to develop a vaccine to promote weight loss in humans although it may not be an active vaccine because it is more difficult to control.
"Whether active immunisation against ghrelin would help prevent the development of obesity because of energy-dense, palatable, high-fat Western diets or would facilitate weight loss once obesity is established is uncertain," he added.
Professor Stephen Bloom, professor of investigative science at Imperial College London, said big pharmaceutical companies had been looking for ways to block the action of ghrelin for years but it had not yet proved effective.
"What is surprising about this study is that by producing antibodies to block ghrelin it works as well as it does.
"This is an important proof of principle but there are problems. For example if you develop antibodies against something you can't get rid of them very easily, so if there's anything bad you can't undo it.
"And since ghrelin is present in the brain you might start an immune system response against the brain. It's not necessarily a very safe thing to do - I wouldn't want to be a volunteer."
But he added: "Maybe we can find a safer way do to it and in that way the study is useful."