WASHINGTON - A vaccine that slows down a key hunger hormone kept rats from gaining weight, even when they over ate, U.S. researchers reported on Monday.

The team at The Scripps Research Institute in California cautioned that such a vaccine is a long way from being tested in human volunteers, and that it may not work in people.

But the study shed light on how hunger and weight gain work, they reported in this week's issue of the Proceedings of the National Academy of Sciences.

The vaccine targeted ghrelin, a hormone discovered in 1999 that helps control appetite in animals and people.

"It appears that active vaccination against ghrelin is one avenue that can slow weight gain and fat build-up in the body," said Kim Janda, a chemistry professor who helped direct the study.

"The study shows our vaccine slows weight gain and decreases stored fat in rats," Janda added.

"While food intake was unchanged in all testing groups, those who were given the most effective vaccines gained the least amount of weight."

The vaccine also appears to help control whether the body stores fat or burns it off, Janda said in a statement.

"To have an impact on appetite and weight gain, ghrelin first has to move from the bloodstream into the brain, where, over long periods, it stimulates the retention of a level of stored energy as fat," Janda said.

"Our study is the first published evidence proving that preventing ghrelin from reaching the central nervous system can produce a desired reduction in weight gain."

Janda hopes the vaccine might help people avoid the weight loss and weight gain seen in "yo-yo" dieting.

"We're not claiming that our study answers the question of obesity treatment once and for all," Janda said.

"What we are saying, and what our study confirms, is that this looks like a serious workable solution to the problem."

The researchers vaccinated mature male rats that were then provided an unlimited amount of low-energy, low-fat food. Unvaccinated rats gained more weight than vaccinated rats fed the same food.

"Whether active immunization against ghrelin would help prevent the development of obesity caused by ... high-fat 'Western' diets, or would facilitate weight loss once obesity is established" remains uncertain, the researchers wrote.