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Skinny genes: How DNA shapes weight-loss success Add to ...

Worse, starving people eat very fast, outpacing the brain's ability to register that they have had enough. "It takes 20 to 40 minutes for brains to realize they're full," Dr. Sharma says. "But in 20 minutes you can do a lot of damage."

Spreading calorie intake more evenly throughout the day helps "keep the furnace burning all day long," he says.

Yet biology can be a flawed and unfair beast, and some people simply have a furnace that burns hotter and faster than others. "I have patients who live well - better than I do," Dr. Sharma says, "and they are still 300 pounds; and there are lots of thin people out there who have crappy diets and don't exercise at all, and no one labels them lazy."

Indeed, studies have found that there are those who can consume as many as 1,000 extra calories a day, but, thanks to the gift of inheritance, they just melt away. Dr. Scherer just happens to be one of the lucky ones.

The brain-belly connection: Hormones play a big role

Hormones play a major role in our mental motivation to eat or not to eat. Neurobiology research suggests that people who struggle with weight may well have genes that disrupt these substances, making food as addictive as a drug in some cases.

There's the neurotransmitter dopamine, which - with sex, drugs and food - can excite the brain's reward circuit with a shot of pleasure that reinforces the desire for more. There's the appetite suppressant leptin, which fat tissue secretes to signal the brain when you're full. There's insulin, the sugar-busting hormone that regulates energy and glucose metabolism. And there's ghrelin, the stomach's hunger hormone that can spark the brain's yearning to eat with the mere image of a Big Mac.

One 2008 study suggests that ghrelin levels rise with stress, making the stressed more likely to overeat. Another recent experiment found ghrelin powerful enough to make people crave more food even after a full meal. And at McGill's Montreal Neurological Institute, researchers have shown that ghrelin triggers certain brain regions to respond to visual food cues with an almost hedonistic desire, explaining why it's dangerous to shop for groceries on an empty stomach and how a steady stream of junk-food advertising can hinder weight loss.

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