If you've ever felt the crushing disappointment of getting back on a scale and finding your weight back where you started after several months on a successful diet, you already know:Losing weight is hard work, but keeping it off is harder.To find out why, Kevin Hall, a researcher at the National Institutes of Health, began following contestants on the TV show "The Biggest Loser,"a reality show in which overweight people compete to win cash by losing the most relative to their initial weight. A study he wrote in 2013 showed that after returning home, contestants were highly likely to begin slowly gaining back the weight they'd worked so hard to lose.A recent story in the New York Times looks into a more recent study by Hall, which appears to confirm his preliminary findings: Out of 14 contestants he studied, 13 have regained weight. Four contestants are heavier today than they were before the competition began.The reason' The body fights really hard against weight loss, especially in the months and years after you finish a grueling diet and exercise plan. And this happens, at least partially, because of a series of complex biological changes that occur when you diet.These findings aren't brand new. In fact, other researchers have been emphasizing some of Hall's main findings for years.University of Minnesota psychologist and eating researcher Traci Mann, for example, has said that a central problem of most weight loss plans is that they're designed simply to help you lose weight quickly, not to keep it off for longer.Plus, the very act of dietingrestricting the amount of calories you take in on a daily basisunleashes a series of shifts in the body that make it incredibly tough to keep the weight off. These are the changes on which Hall's study are based.Some of them include:1. Your hormones levels change, often making you feel hungrier no matter how much you eat. Losing weight affects the presence of several key hormones in the body. One of them is leptin, one of several hormones that control hunger.The Times reports that in the years after "The Biggest Loser," contestants reported near-constant feelings of hunger and cravings, which often led some to binge after they'd succeeded in sticking to a healthy eating regimen for several weeks.While all of the contestants started the show with normal levels of leptin, the levels of this hormone plummeted by the time the show ended, the Times reports. "By the seasons finale, they had almost no leptin at all, which would have made them ravenous all the time." And as they slowly began regaining weight, leptin levels rose a little, but got stuck at about half of what they'd been at the start of the showwhich would help explain why they felt such strong urges to eat."As you lose body fat, the amount of different hormones in your body changes," Mann told the Washington Post in 2015. "the hormones that help you feel full ... decrease. The hormones that make you feel hungry ... increase."2. Your metabolism slows down, and more of what you eat gets stored as fat. When you diet, you're effectively convincing your body that it is starving, Mann said. Your body responds by running as efficiently as it can (i.e. burning the fewest calories possible). This shift "would be good thing if you're starving to death," said Mann, "But it isn't a good thing if you're trying to lose weight."3. Your brain becomes more aware of food.In addition to all of the physiological changes that happen during a diet that make it tough to keep off weight, some people also apparently experience psychological changesthat could contribute to the struggle."Your brain becomes overly responsive to food, and especially to tasty looking food," said Mann, "so the thing you're trying to resist becomes harder to resist."A 2013 studyshowed food to people who had dieted and people who hadn't, and then compared their brain scans. Unlike the non-dieters, the dieters tended to show a heightened response in the areas of their brains linked with processing rewards, and a loweredresponse in the parts of their brains linked with a sense of control.Biological changes don't mean long term weight loss is impossibleNone of this means adoomsday scenario for people trying to lose weight.Instead, it points out the problems with focusing entirely on changing what you eat rather than taking into account other behavioral patterns.These include when you eat, how much you exercise, and whether or not your friends and family members support your health goals. These factors, often lumped together under the term"behavior modification," are the focus of programs like Weight Watchers, for example, and they work for thousands of people."Most of us know peoplefriends, family members, colleagueswho have lost weight and kept it off for years by changing the way they eat and boosting their physical activity," writes David H. Freedman, a consulting editor at Johns Hopkins Medicine International, in a post for the Columbia Journalism Review."They cant all be freaks of biology," says Freedman.SEE ALSO:Here's the best time of day to work out to lose weightRELATED:Yes, bacon has been linked to cancer AGAIN ' here's how bad processed meats actually are for youJoin the conversation about this storyNOW WATCH: The pros and cons of drinking protein shakes after a workout
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