Giving overweight people an extra dose of a hormone that tells the brain when to stop eating appears to curb their appetites by nearly a third, pointing to a potential new treatment for obesity.

The researchers at Imperial College London had previously shown that the hormone could suppress the appetites of lean people. But they worried it would not work in obese people because studies of another appetite-suppressing hormone, leptin, had proved disappointing.

“We had to do a second trial to see if obese people were as sensitive” to the hormone, said researcher Dr. Stephen Bloom. The answer: “Yes, they are. They’re just as sensitive as thin people.”

In the small study, both obese and lean people ate about 30 percent less from a buffet lunch after they were given a dose of the hormone, PYY3-36, to trick the brain into thinking they had already eaten. The research also showed lower natural levels of PYY in the obese, which may explain why they are hungrier and overeat, Bloom said.

The findings are published in today’s New England Journal of Medicine.

Bloom said long-term use of the hormone would have to be studied before it could be developed into a treatment for obesity that would likely consist of injections given before meals.

The findings could also point to a more natural treatment for obesity: Bloom said a high-fiber diet was believed to boost the body’s production of PYY.

The PYY hormone, one of a number of hormones that stimulate or suppress hunger, is released by the gut as you eat. It tells the part of the brain that controls appetite when you are full.

Sixty percent of Americans are obese or overweight, and obesity contributes to about 300,000 deaths a year, according to government estimates.