August 10 (HealthDay News) -- Leptin, a hormone that helps to control feelings of hunger, also appears to govern the desire to eat, British researchers report.

The finding could lead to new insights into obesity and how to treat the condition, the researchers said.

"This work shows that the rewarding properties of food have strong effects on brain areas concerned with liking and desire, and that the tendency for some people to overeat because they like food is influenced by specific hormones and chemicals in the brain," said lead researcher Paul C. Fletcher, a member of the Department of Psychiatry, University of Cambridge, Addenbrooke's Hospital.

Leptin is produced by fat cells and circulates in the bloodstream to reach the brain, where it acts to reduce hunger and increase the feeling of fullness, according to the report in the Aug. 9 issue of Science.

In the study, Fletcher's team studied two people with a rare genetic disorder and don't produce any leptin. They eat excessive amounts of food --even foods they don't especially like -- and are obese. However, when they were treated with leptin, they ate less and lost weight.

The researchers showed the two patients pictures of various foods while they recorded their brain activity. The scientists found that the pictures stimulated activity in the area of the brain called the striatal regions. These regions are associated with pleasant emotions and desires.

One region in particular, the nucleus accumbens, was highly responsive to pictures of foods that the patients liked, the researchers found.

But when the patients were treated with leptin, the pictures of foods produced a reduced response in the brain. And the response was activated mostly by foods the patients liked and only when they hadn't eaten and were hungry, the researchers said.

"Understanding how brain systems interact with hormones that signal hunger and energy stores will provide us with a more complete picture of factors controlling eating behavior and will hopefully take us beyond some of the prevailing and simplistic assumptions about why some people have difficulties in controlling how much they eat," Fletcher said.

"Such understanding will be a key step in the prevention and treatment of obesity. Importantly, the finding that the liking of food is biologically driven should encourage a more sympathetic attitude to people with weight problems," he added.

One nutrition expert doesn't think enough is known about how leptin works to change the way obese patients are currently treated to control their appetite.

"Such an extremely small study can't provide conclusive evidence, but it adds to the growing body of research in the area of leptin and appetite," said Connie Diekman, director of university nutrition at Washington University in St. Louis and president of the American Dietetic Association.

Studies continue to support the role of leptin in appetite regulation, but they have yet to determine the exact mechanism and how to modify the process, Diekman said. "Until more research is provided, the impact of research in the area of leptin doesn't change how patients are managed when it comes to appetite control," she added.

Another expert thinks the British study could lead to new obesity treatments.

"The reason we study leptin is because of the levels of obesity in society," said Michael A. Cowley, an assistant scientist in the Division of Neuroscience at Oregon Health and Science University. "This study is important, because it explains how and why people crave food."

Cowley thinks this knowledge will lead to new therapies for obesity. "We become obese, because we really like hamburgers or can't stop eating doughnuts. We become obese, because we obsess about food," he said.

This study shows that leptin can help control that obsession, so it can become a new therapeutic target, Cowley said. "This study shows that craving is modifiable by leptin," he said.

SOURCES: Paul C. Fletcher, Ph.D., Department of Psychiatry, University of Cambridge, Addenbrooke's Hospital, Cambridge, England; Michael A. Cowley, Ph.D., assistant scientist, Division of Neuroscience, Oregon Health and Science University, Beaverton; Connie Diekman, M.Ed., R.D., director university mutrition, Washington University, St. Louis, and president, American Dietetic Association; Aug. 9, 2007, Science