A passenger waits for a delayed flight at Heathrow airport's terminal four in London August 12, 2006.[Agencies]

Falling levels of a hormone called leptin that helps the brain resist tempting foods may explain why people who lose weight often have a hard time keeping it off, U.S. researchers said on Friday.

Restoring leptin to pre-diet levels may reverse this problem, they said, offering a way for weary dieters to finally win the weight battle.

"When you lose weight you've created about the perfect storm for regaining weight," said Michael Rosenbaum of Columbia University Medical Center in New York, whose research appears in the Journal of Clinical Investigation.

After weight loss Rosenbaum said the metabolism not only becomes more efficient, so the body needs fewer calories, but the brain becomes more vulnerable to tasty-looking treats.

"Areas of your brain involved in telling you not to eat seem to be less active. You are more responsive to food and you are less in control of it," he said in a telephone interview.

Leptin is a natural appetite suppressant secreted by fat cells in the body. Its discovery created a stir in the 1990s when researchers found leptin caused mice to eat less and lose weight. This rarely happens in humans.

Since then researchers have been looking the best way to use the hormone to help treat obesity.

In earlier studies, researchers found that when people lose weight, leptin levels fall as the body tries to protect its energy stores.

Rosenbaum investigated the impact of this loss of leptin on the brains of people who had lost weight, and whether replacing the hormone might help them keep off the weight.

He used an imaging technique known as functional magnetic resonance imaging that shows activity in the brain. The researchers studied six obese patients before and after going on a hospital-supervised diet that reduced their body weight by 10 percent.

People were shown pictures of food and non-food items, such as an apple or a yo-yo. The researchers found that after weight loss, areas in the brain responsible for regulating food intake were less active when people were shown food images. Areas in the brain responsible for emotion were more active.

When the researchers restored leptin to the levels before the dieting, these changes were largely reversed.

Similar results have been seen in people with a rare genetic condition in which their bodies do not make leptin.

Rosenbaum believes leptin could be a useful tool in helping people maintain weight loss. "The idea is there should be a whole new class of therapies to help us keep weight off after we have lost it," he said.

(Agencies via China Daily June 26, 2008)