More than a billion people are overweight or obese, creating a pressing public health problem because excessive weight is linked to an increased risk of diabetes, heart disease, strokes and certain cancers.
But scientists from Britain, the United States and Canada have discovered that a hormone in the body called PYY3-36 that tells the brain when the body is full could form the basis of new weight-loss treatments.
"The discovery that PYY3-36 suppresses appetite could be of huge benefit to those struggling with weight problems," said researcher Stephen Bloom of Imperial College London.
"It does cause an inhibition of appetite and reduces food quite significantly," he added in an interview.
Bloom said it may be possible to identify foods that cause the release of more PYY3-36 so appetite could be limited naturally. Another option would be injections of the hormone, similar to the way diabetics take insulin, which could provide a safe long-term treatment for obesity.
In the experiments, scientists isolated the short-term 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.
First, researchers identified the hormone's effect on certain neurons in the brains of mice associated with appetite and weight control.
Then, 12 people of normal weight at Imperial College were injected with extra hormone or a placebo saline solution for 90 minutes. Two hours later, they were offered a buffet lunch.
Results show the subjects who received the hormone boost ate one-third less. They reported feeling less hungry for another 12 hours and did not make up the "missing" calories by snacking.
"This is a naturally occurring hormone that affects centers in your brain. It stops you eating because it makes you feel less hungry. It is part of your natural physiology," said Dr. Caroline Small, a member of the research team whose findings are published in the scientific journal Nature.
The doses given to the volunteers mimicked the levels that occur naturally in the body after a meal and produced no side effects. People just felt less hungry.
However, other scientists in the study said the experiment does not by itself prove that injections of the "third helping hormone" 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."
Before the hormone can be developed into an anti-obesity treatment, scientists must first show that it produces the same satiety feeling and reduces appetite in obese people. Researchers also must work out better ways of delivering it into the body and determine how it is broken down.
The volunteers in the study received intravenous infusions of PYY3-36, but the researchers think an injection could be produced.
Small said the scientists are conducting further studies and comparing the effects of the hormone infusions on normal-weight and obese people.
"There is a huge epidemic of obesity and treatments are very much needed," she said.
"You need to reduce the amount of food people eat or increase the amount of energy they expend. This would reduce the amount of food people consume by making them satiated. It is enhancing those natural signals in the brain."
Weight loss schemes and food supplements are a $40 billion industry. Yet scientists still are untangling the complex chemical signals controlling appetite - how the body tells the brain it needs energy and how the brain tells the body how much to eat.
Its part of a chemical system called the adipostat, which continually adjusts food intake and energy expenditure like a thermostat.
Some hormones, like insulin, act over the long-term to regulate body fat stores. Other short-term hormones like PPY3-36 are released to trigger hunger pangs or communicate satiation.
Our metabolism evolved millions of years ago to store energy when prehuman ancestors hunted and gathered nourishment, but often returned home empty-handed. Today we live in a caloric extravaganza, but our adipostat - along with a sedentary lifestyle and other factors - hasn't adjusted.
"Ordinary folk feel hungry more often than they should," Bloom said "We have too much hunger for a civilized society where too much food is about. We now need a hunger regulator to stay at a normal weight."