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For people struggling to lose weight, a new study offers some insight that helps explain why it's so difficult for dieters to keep weight off.

As NPR Allison Aubrey reports, success isn't just a matter of willpower. Researchers now say that some dieters are overwhelmed by strong biological signals that tell them to keep eating.

ALLISON AUBREY, BYLINE: We all know how to read a gas gauge, right, telling us that our cars are full of gas or running on empty? Well, our bodies have an internal gauge of sorts, too. Hormones send signals to our brains meant to clue us in on how much energy or food we need.

Dr. Louis Aronne, who directs the comprehensive weight control program at Weill Cornell Medical College, says the problem is this internal system isn't always so precise. And in the case of dieters, the gauge can sometimes be way off. This has been demonstrated elegantly in a new study published in the New England Journal of Medicine. It documents the outcome of a weight loss trial involving 50 overweight and obese volunteers, who were put on a very low-calorie diet, eating only about 550 calories a day. Now, not surprisingly, over 10 weeks, most of them shed many, many pounds - about 10 percent of their body weight.

DR. LOUIS ARONNE: Think about what happened here. The gas gauge, instead of going down 10 percent, they lost 10 percent of their bodyweight. And instead of the gas gauge going down 10 percent, the gas gauge went down 65 percent. So, they think that they're going to run out of gas very, very soon.

AUBREY: And how did their bodies respond when levels of appetite-regulating hormones plummet after dieting? Well, instead of being satisfied with less food, the dieters began to want more.

ARONNE: What this tells us is that a lack of willpower is not the cause of weight regain in people who regain weight. It's that they may have a more powerful mechanism that pushes their weight back up.

AUBREY: The mechanism isn't just the fluctuating hormones. It's also metabolism. The study found that the metabolic rate of the dieters remained low one year after the low-calorie diet ended, making it harder to burn off those calories.

Tim Church, of the Pennington Biomedical Research Center, says this study may show that dieters face a double-whammy of obstacles. But he says it does not solve the whole weight loss puzzle.

DR. TIM CHURCH: One thing I keep asking myself is, there are a lot of people out there who do lose weight and do keep it off for many years, so why are they different from the people who don't?

AUBREY: From genes to hormones to metabolic rates, there's a lot of variation from one person to the next. And Church says his hope is that further research will push towards more individualized weight loss strategies.

Allison Aubrey, NPR News, Washington.

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