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The hormone that tells us we are full also regulates our desire for certain foods, researchers said on Thursday, in a finding that sheds light on why people gain weight and could lead to new treatments for obesity.

The study showed that patients with a rare genetic disorder who lacked the hormone called leptin ate less after receiving injections of the hormone, said I.S. Farooqi, a researcher at Cambridge University who led the study.

Previous research has shown the hormone does not help people with normal leptin levels lose weight, but scientists still do not completely understand how it works, Farooqi said.

"By studying patients who have no leptin and then treating them with leptin, we can tell what it is doing," Farooqi said in a telephone interview. "It gives a clear look at how leptin operates in the brain."

In the study, published in the journal Science, researchers searched for "circuits" in the brain that signal when a person is hungry or full and found that they were linked to areas involved in determining the enjoyment of food.

To see how the hormone worked, the researchers showed the patients pictures of different types of food, ranging from tasty fare like chocolate cake and pizza to blander choices such as cauliflower and broccoli.

The patients with the genetic disorder -- of which there are about a dozen known cases in the world -- liked all types of food, ate excessively and were obese, the researchers said.

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Using magnetic resonance imaging technology, the researchers tracked the patients' brain activity as they responded to the pictures and pinpointed several key areas that play an important role when it comes to a desire for food.

After the patients received leptin injections, the areas that had previously shown activity all the time at the sight of food were only active if the people had not eaten the night before, which was a normal response, Farooqi said.

It showed desire for food is driven by biology -- not greed -- which causes overeating and obesity, Farooqi said.

Knowing how leptin, which is produced by fat cells, triggers different parts of the brain could lead to new drugs that target obesity and help dangerously overweight people take pounds off.

"If you find those molecules that leptin triggers then you can manipulate or target them with drugs to treat obesity," Farooqi said. "The first step is to work out what leptin does and how it does it."

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