By Victoria Griffiths in Boston
Published: April 1 2004 22:26 | Last Updated: April 1 2004 22:26

People's brains may be "hard-wired" shortly after birth for obesity, with faulty circuits erroneously telling the body to eat more, according to two separate research reports out on Friday.

"There's comfort here for some people that their overeating is not a lack of willpower," says Richard Simerly, a researcher at the Oregon Health and Science University and lead author of one of the reports in the publication Science. The findings support the notion that everyone has a predetermined "set-point" for their weight.

The "don't eat" signalling mechanism is controlled by leptin, a protein produced by fat. The more fat in the body, the more "don't-eat" signals are sent to the brain. Yet some mice, in laboratory experiments, are born with a resistance to leptin. The brain essentially becomes deaf to the "don't-eat" messages, and the mice eventually become obese. In humans, overweight people tend to have unusually high levels of leptin in their bodies, indicating a similar resistance to the protein.

Mr Simerly hopes the findings will help researchers identify and treat leptin-resistant babies early in life. "There may be a window of opportunity when we can rewire the brain to reduce the chance of obesity later in life," he said. He believes the work may help explain why underweight babies - particularly from other cultures such as India - may suddenly become obese when exposed to a western diet.

The research sheds light on the role of leptin, a little-understood protein that was isolated in 1995 by scientists at Rockefeller University. When people diet, their levels of leptin drop. Their metabolism slows and they feel hungry much of the time. This mechanism may be why most weight-loss programmes fail long-term.

Yet simply giving people doses of extra leptin does not help. Their brains only seem to become more resistant to the protein, and we need more and more "don't eat" signals to control our appetite.

Mr Simerly warns that leptin-resistance does not explain all obesity. "The causes of being overweight are very complex and this is likely just one contributor," he says. "People who have leptin resistance still need to lose weight for the sake of their own health. But this at least gives them reassurance that it is harder for them than for others. They have not been dealt the same biological hand."

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