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This elaborate chemical dance is a brilliant system, capable of balancing food intake with what Jeffrey Friedman, MD, PhD, who led the team that discovered leptin, calls "an extraordinary level of precision." Unless, that is, there's a glitch in your particular system. One glitch, says Friedman, a medical researcher at the Howard Hughes Medical Institute at Rockefeller University, is that "for some reason, obese people appear to be resistant to leptin, which is probably part of the reason they're fat": The leptin message—to eat less—isn't getting through. Another problem isn't really a glitch as much as a very clever (too clever) refinement of the system. Lots of evidence now suggests that your genes determine your personal ballpark body weight, or set point, and that hunger is one of your body's tools to keep you right there. When you try to diverge from your set point by becoming thinner or fatter than nature decrees, your entire chemical system kicks into gear to pull you back.

When you diet, your metabolism slows down to conserve calories. At the same time, your hunger increases dramatically, regardless of your need to lose weight. "The 300-pounder who goes to 285 will feel very hungry," Cummings says. "The 100-pounder who goes to 95 will also feel very hungry."

One reason, he explains, is that when you lose weight, ghrelin levels not only spike before mealtimes but also rise overall. "You'll still have three spikes a day, plus the whole level shifts upward," Cummings says. "The troughs and peaks are all higher than before losing weight. And these changes appear durable—they don't seem to go away even if you stay below your natural point for more than six months. Over time, they erode willpower." As fat stores go down, so does leptin, and your brain gets an unrestrained hormonal signal to eat more. "The evidence suggests," Friedman says, "that people who have lost weight below their personal set point are hungry a lot of the time."

You can probably guess why all these systems kick into gear when you lose weight: To your genetic code, this restraint looks like starvation. "People have an illusion that they can consciously control their food intake," Friedman says. "That's true over the short term, but over the longer term the biological drive to eat enough to get your weight back to your individual level almost always overcomes your conscious control."

Outside Influences
Wait a minute—we're more than just animals, right? We've got highly evolved brains full of thoughts and emotions and plans for a bikini summer, and surely we can tell our bodies what to do. It also seems clear to our logical brains that there are other things besides hormones that prompt us to eat. The biggest factor, even die-hard biologists like Cummings and Friedman say, is the environment—everything from food smells and portion sizes to emotional connections to food dating back to childhood.

It's well documented, for instance, that an environment of many flavors prods us to eat more and keeps us hungry. Megan McCrory, PhD, a nutritional scientist at Tufts University, puts hunger and appetite into separate categories. "Hunger is a physiological feeling, while appetite is the desire to eat a certain food or foods," she explains, and that desire—that craving—gets turned on by our having lots of choices. "We want to try all of the great variety of flavors available," McCrory says. "And when we eat a little bit of everything, we tend not to keep the calories the same as when we're eating only one or two foods." She says that studies have shown that we eat 25 percent more, on average, in a single meal when more variety is available.

This so-called buffet binge may help explain why some restrictive diets, like Atkins, work—at first. How many meals of steak and eggs can you eat before you lose interest in eating them, and are driven to eat carbs?

An environment that constantly offers lots of carbs can also spur hunger. High-glycemic-index (GI) foods like potatoes and white bread break down quickly into glucose, which causes a spike in blood sugar levels, followed by insulin release, which pushes blood sugar down. Your body wants to bring blood sugar back up to normal—and what quicker way than with another cinnamon bun or bagel? "Of all the studies that have shown the effects of GI on appetite," says David Ludwig, MD, PhD, associate professor of pediatrics at Harvard Medical School and director of the obesity program at Children's Hospital Boston, "virtually all showed that high-glycemic-index meals make you hungrier sooner after eating. In our own study of obese teenagers, after a high-GI meal they wound up eating many more calories than if the previous meal was low-glycemic-index."

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