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 R. 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 Thursday’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 consist of injections given before meals.

“We haven’t yet shown you get actual weight reduction. We’ve only shown you eat less,” Bloom said.

The findings could also point to a more natural treatment for obesity: Bloom said a high-fiber diet is 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.

The research is “a hopeful step in the right direction,” said obesity researcher Dr. David E. Cummings of the University of Washington in Seattle. “But there’s a fairly large difference between reducing food intake for one meal and actual weight loss.”

Dr. Rudolph L. Leibel, an obesity researcher at Columbia University in New York, said it is unlikely that PYY alone will turn out to be the “magic bullet” against obesity, but it may prove useful in combination with other drugs. He said the body has backup systems that kick in to resist the loss of body fat – the reason so many people who lose weight gain it back.

“We can’t fool Mother Nature yet,” he said.

Currently, there are only a few prescription weight-control drugs, and they produce only modest weight loss. They work by suppressing the appetite through a brain chemical or by blocking fat from being absorbed.

The British researchers recruited 24 people: 12 obese and 12 lean. Their food consumption was monitored twice, once after an intravenous dose of synthetic PYY and once after getting a harmless saline solution. Two hours later, they were given a large buffet lunch.

After the PYY dose, the obese ate 30 percent fewer calories than they did after the dummy solution. The lean people ate 31 percent less. The PYY continued to curb their appetites for 12 hours, with the obese eating 26 percent less and the lean eating about 34 percent less, based on food diaries they kept.

The PYY did not affect food consumption from 12 hours to 24 hours after the infusion.

Without the extra dose, PYY levels were lower in the obese participants than in the lean, but the researchers said it is not known whether that is a cause of obesity or a consequence.

“If it’s a consequence of obesity, it would explain why once people become overweight, it is very difficult for them to reverse it,” Bloom said. “They don’t feel as feel as full after food as normal people do as a result of their obesity.”

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