LONDON — An anti-obesity vaccine that significantly slowed weight gain and cut body fat in tests on animals will be unveiled today.

Mature male rats that received the jab ate normally yet gained less weight and had less body fat, suggesting the vaccine directly affects the body’s metabolism and energy use.

The vaccine, described by an American team in the Proceedings of the National Academy of Sciences, may be especially important to stop “yo-yo dieting,” the cycle of repeated loss and regain of weight.The vaccine acts against ghrelin, a naturally occurring hormone that helps to regulate energy balance in the body. This approach is called immunopharmacotherapy and is also being tested to treat drug addiction.

A professor at the Scripps Research Institute at La Jolla, Calif., and the report’s senior author, Kim Janda, said: “Ghrelin first has to move from the bloodstream into the brain where, over long periods, it stimulates the retention of … fat. 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.”

Mr. Janda told the Daily Telegraph: “We could speed quickly into human trials, maybe in a year, but we are going to be more cautious,” referring to the recent trial at Northwick Park Hospital that went wrong.

The study’s lead author, Eric Zorrilla, said a vaccine against ghrelin “is particularly compelling in terms of the well-documented problems of human dieting. When you diet, the body responds as if it was starving and produces ghrelin to slow down fat metabolism … As a result, many people end up regaining the weight they lost and more,” he said.

According to the World Health Organization, about 1 billion people worldwide are overweight or obese.