The most important weight-loss message - you have to use more calories than you eat - hasn't changed in decades.

But dieting experts say science has some new, surprising things to say about the other half of the standard recommendation - exercise - and about which diet to use.

Researchers are also refining the behavioral tricks that can change the way people eat, not only to lose weight but also to keep it off. Modern technology is giving dieters new options, such as Internet- and cell-phone-based programs or scales that can transmit your weight from your home to your doctor or dietitian.

Thomas Wadden, director of the Center for Weight and Eating Disorders at the University of Pennsylvania, said the last decade has seen "pretty aggressive diet wars" among the proponents of low-fat, low-carb, or low-glycemic-index approaches.

After years of comparisons, Wadden and other experts said the verdict is clear: What you choose doesn't matter. Pick a diet you can stick to, including liquid diets or prepared meals. If you follow the rules, the results are all about the same. Wadden said he does recommend that, whatever the diet, patients reduce saturated fats and trans-fats for better health. You can pick a more aggressive approach while you're losing, eventually transitioning to a diet rich in vegetables, fruits, and lean protein for maintenance.

Gary Foster, director of Temple University's Center for Obesity Research, thinks diet gurus have scared people off with too much information. "We tell people so much stuff, it's paralyzing," he said.

He agreed that the specific diet doesn't matter. "Calories drive weight loss," he said.

To lose a pound a week, a dieter needs to eat 500 fewer calories a day. That's simple enough, but your maintenance needs come down as people get smaller. That explains why weight loss gets harder as you go. Think of your body as a house. It takes more energy to heat a big house than a small one. A woman who weighs 300 pounds maintains her weight on about 3,000 calories. (Because they have more muscle, men use a few more.) A woman who weighs 140 needs only 2,050 calories to stay there. Caloric needs drop further as we age and lose muscle mass.

Cheryl Marco, a registered dietitian who runs Thomas Jefferson University's weight management program, starts patients on a prepackaged diet that includes shakes and bars. Dieters have few options.

"Fewer choices work better," Marco said. "What works is the narrowing of stimuli." So her most successful long-term dieters eat meals that don't vary much from day to day.

Some people may just have to stay away from foods that trigger overeating.

"I am not of the belief . . . that we have the ability to learn to eat high-risk foods in moderation," Marco said. Your high-risk food may be ice cream. Someone else's may be fettuccine Alfredo.

John McAroy, 38, dropped 70 pounds (from 300 on a 6-foot-2 frame) between Thanksgiving and early February using Marco's method. He hasn't cheated much and hasn't missed having more choices. "You'd be surprised," he said. "I'm rarely hungry."

So what about exercise? Isn't it the answer?

No, the experts said.

Exercise is "the single best predictor of who keeps weight off and who doesn't," Foster said. "It has very little effect on weight loss."

Exercise can help people lose weight, but so slowly that most people lose interest, he said. It's easier for most people to cut 500 calories out of their diet a day than to burn an extra 500 calories. You have to walk five miles to do that.

What exercise does do is help people maintain muscle mass, which revs up metabolism a little and improves overall health. Still, adding five pounds of muscle takes a lot of work, and a pound of muscle burns only about 15 calories a day.

In one of the cruel twists of our evolutionary history - the one that shaped our bodies to withstand famine, not a world full of junk food - people who have lost a lot of weight burn fewer calories during exercise than people who have never dieted. "It's as if your body's gone green on you," Wadden said. To keep weight off, dieters need to exercise 225 to 300 minutes a week: about 32 to 43 minutes a day.

Foster's group is looking at whether getting a good night's sleep affects weight loss. People who weigh more tend to sleep less, but it's not yet known whether sleeping more helps people lose weight.

On the behavioral front, researchers say that "accountability" is a crucial factor in making diets work. People do best when they keep track of what they eat and how much they exercise. They need to weigh themselves frequently, ideally every day. It also helps a lot to report what they're doing to someone else. The gold standard is a face-to-face meeting, but results also improve with telephone and Internet reporting.

"It's really important to be accountable to somebody else," Foster said.

It's also important to act quickly when the numbers on the scale start moving up. Experts recommend having an action plan when weight rises by two to four pounds - perhaps returning to more intensive monitoring of food intake - and when dieters fall off the wagon.

"One of the major differences between the average-weight person and the overweight person is the ability to recover from overeating," Marco said.

Wadden says technology is offering new alternatives to people who need to lose weight. While not yet in wide use for weight control, scales that transmit weights to doctors or diet programs can build in accountability. Internet- and phone-based programs make it easier to calculate calories and energy usage.

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New science can help guide diets and exercise