Scientists have isolated a hormone that makes us feel full when we eat, and they demonstrated its potential as a new weight-loss drug by injecting volunteers with the substance before a big buffet lunch.

The participants injected with the so-called “third helping hormone” ate one-third less than usual and resisted snacking for up to 12 hours, scientists reported.

The hormone infusion was “sort of a fake meal,” said the study’s senior author, Steven Bloom of the Imperial College of London. “The brain was fooled into thinking that it had already eaten.”

The experiment offers a preliminary but promising opportunity to develop a new drug to curb obesity, researchers said.

Still, some cautioned that treating overweight people with direct hormone injections has not been proven effective and may be dangerous.

Public health officials have been warning of an obesity epidemic in the United States, and the problem is spreading around the globe. The International Obesity Taskforce estimates 300 million people worldwide are obese.

In the United States, obesity is implicated in the deaths of 300,000 people annually from heart disease, stroke, diabetes and cancer.

The hormone experiment was reported in today’s issue of the journal Nature. Scientists in England and Oregon isolated the hormone PPY3-36 that is secreted by cells lining the intestines. Levels of the hormone rise in the blood after eating and remain high between meals.

Using mice, researchers first identified its effect on the hypothalamus, a part of the brain that regulates appetite.

Then, 12 people of normal weight 6 men and six women in their 20s were injected with extra hormone or a placebo saline solution. Two hours later, they were offered an Indian curry buffet lunch chicken or vegetarian, spicy or mild.

The volunteers who received the hormone boost ate one-third less than they did at previous meals.

However, other scientists in the study said the one-time experiment does not by itself prove that the injections can safely control appetite or reverse obesity. How the body might respond to elevated PPY3-36 levels over time is unclear.

“It would not make a suitable weight loss drug due to its potential effect on other important systems of the body,” said Oregon Health Science University neurobiologist Roger Cone. “Scientists still have a long way to go before the development of a drug that can help Americans fighting obesity.”