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Problems With Diet Studies continued...

Why don't diets work? Mann says there are two issues. The first is that it's just plain hard for people to change their eating behaviors. And the second reason is that even if you do succeed at a diet, the rule of diminishing returns comes into play.

"When you keep to a reduced-calorie diet, your body makes metabolic adjustments that make it harder and harder for you to lose weight," Mann says. "Your body becomes very efficient, and you have to eat less and less to continue to lose weight. If you had the will to go on a diet, the fact that it steadily becomes less and less effective makes it even harder to stick to it."

That's true, agrees Madelyn Fernstrom, PhD, founder and director of the Weight Management Center at the University of Pittsburgh and diet and nutrition editor for NBC's Today show.

But Fernstrom worries that people will get the idea from this that their diets don't matter. And she's worried even more that the Mann report will discourage people from trying to lose weight.

"'Diets don't work' is only half the story," Fernstrom tells WebMD. "Lifestyle change will work if you have realistic expectations, good support, and choose a plan that you can stick with -- a plan that will give you moderate change over a long time."

That doesn't mean weight loss is easy. There's a myth, Fernstrom says, that normal-weight people can eat anything they want and don't need a strict exercise regimen. But that's true for only a very small number of people. Most people who have a healthy weight have to work at it.

"It is really hard to lose weight, and it is even harder to keep it off," Fernstrom says. "You can't cry about this. You must maintain hope. We just have to develop better strategies to keep people on track."

How to Lose Weight for Good

The basic problem is that people think diets are something you do for a little while before going back to your old lifestyle, says obesity expert Rob M. van Dam, PhD, assistant professor of medicine at Harvard Medical School.

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