Diet, diet, on the wall ... which makes you the leanest of them all?

The best diet is the one you can stick to," says Michael Dansinger, director of the Diabetes Reversal Program at the Tufts-New England Medical Center in Boston.

He led one of the largest, longest studies pitting four diets--Atkins, Ornish, Weight Watchers, and The Zone--against each other. After a year, weight loss was about the same on each. (1)

And it wasn't much--5 to 7 pounds if you include people who dropped out (4 out of 10 participants) and 10 to 15 pounds if you don't. Mind you, these are people who started off with 220-pound bodies and 43-inch waists.

Of course, those numbers are averages. Many people lose and keep weight off for years. But it's still clear that a long-term leaner you isn't as easy as the diet books claim.

The Real Battle

Why is it so hard to achieve what the diet books call "weight loss that will last a lifetime"? Often it's not a lack of willpower, but that our bodies struggle to recapture those lost pounds.

"It's difficult for the body to accept being thinner than it was," says Robert Eckel, an obesity expert at the University of Colorado Health Sciences Center in Denver.

"Once you have put on extra fat, you tend to defend it," he explains. "If you shed pounds, a number of biological and psychological mechanisms help return your weight to where it was." Among them: (2)

* Appetite surges. "We have reasonably solid evidence that after people have lost weight, they're hungrier," says Eckel. And not just for any food.

"Their preference for high-fat and high-sugar foods goes up," he notes. More hunger plus a craving for calorie-dense foods is, of course, the last thing dieters need. "Both make you likely to eat more than you should if you want to keep the weight off," explains Eckel, who is also the president of the American Heart Association.

* Metabolism slows down. As far as your body knows, your weight loss means you're starving. To make your remaining fat stores last, you burn fewer calories to keep your heart, brain, and other organs working.

"Your basal metabolic rate falls with weight loss, so the laws of physics are working against you," says Eckel. Metabolic rate falls, in part, because it's closely linked to how much muscle and other lean tissue you have.

"With weight reduction, people lose not just fat mass, but a little bit of liver, heart, muscle, and other lean tissue." The only way to counter the slowdown in metabolism is to exercise more.

* Enzymes promote fat storage. Just when we want to burn fat, lipoprotein lipase (LPL), an enzyme that promotes fat storage, makes it tougher.

"When you lose weight, LPL goes up in fat tissue and falls in skeletal muscle, so you tend to store fat rather than burn it," explains Eckel.

But you don't actually gain weight unless you eat more. "LPL doesn't predict weight gain in the absence of extra calories," he adds.

* Less fat and more carbohydrates are burned. At any given time, the body burns a mixture of fat and carbs. Guess which one it prefers to burn when you've shed pounds? Not fat.

"After you lose weight, glucose oxidation goes up and fat oxidation goes down," notes Eckel.

Why are our bodies so bent on undoing all that dieting? They're trying to save us from famine.

"If you look at data from famines during World War I and World War II, it's clear that the fatter you were, the longer you lived," says Eckel.

"When Leningrad was heavily bombed during World War II, many people were killed not by bombs, but by food deprivation. And women unquestionably outlived men, because women brought more fatness to the table."

Our tendency to store fat caused no harm when people only lived to age 40 or 45, he adds. They weren't around long enough to get diabetes, heart disease, stroke, or cancer. "But now that people live to 75 or 80, obesity may get you before you get there."

Stick to It

"Adherence trumps everything," says diet researcher Michael Dansinger.

His study tested four diets against each other. Two were extreme (a low-carb Atkins and a low-fat Ornish) and two were less drastic (Weight Watchers and The Zone). (1) "Weight loss was virtually identical among the diets," says Dansinger. "What matters is whether you can stick to it."

By the end of one year, three out of four people on each diet had either slacked off or quit the study altogether. Those assigned to the Atkins and Ornish diets were most likely to drop out, suggesting that moderate diets have more staying power.

"The moderate-carb approach exemplified by the South Beach and Zone diets is alive and well, and evidence to support them continues to mount," says Dansinger.

Unlike the Ornish diet, which cuts all fats, and the Atkins diet, which cuts all carbs (at least initially), the less-extreme diets--like The Zone, Weight Watchers, and South Beach--cut only bad fats (saturated and trans) and bad carbs (refined grains and sugars).

"I don't think it makes a hoot of difference which diet you choose to lose weight in the short term," says Eckel. "But I wouldn't recommend an Atkins diet, because it teaches a nutrition lesson that's not worth learning in the long term."

The lesson--that you should load up your plate with unlimited quantities of bacon, steak, pork chops, cheese, cream, and other saturated fats--may not cause problems while you're losing weight. "It's not as bad as we thought, at least in the short term," says Eckel.

But over years, a meat-laden diet may raise the risk of colon or prostate cancer. (3,4) And if people drop off an Atkins diet but keep piling on the fatty meats and cheeses, their artery walls may start to look like salami.

"With the overweight patients in my clinic, I always begin with quality before quantity," says Eckel. "First I make sure they're eating a heart-healthy diet, and then we talk about calories."

The Bottom Line

There are no magic bullets to keep you slim and trim. But here are a few rules of thumb to help you watch your weight:

* Fats. Fats are calorie-dense, but it's tough to avoid them over the long haul. Instead, minimize bad (saturated and trans) fats in meats, dairy foods, and fatty sweets. But leave room for some good (unsaturated) fats in foods like salad dressing, nuts, and oils.

Just don't expect your waist to shrink if you replace some of those bad fats with a 400-calorie bagel or 500-calorie bowl of pasta. "We encourage people to eat more fruits and vegetables, not more starches," says Dansinger.

* Refined carbs. It's clear that the carbs that need cutting are refined flour and sugar. "People have underestimated the damage they do, and low-carb diets brought that to the forefront," says Dansinger.

Whole grains are a better bet because their fiber may help stave off hunger. And (luckily) you can't find whole-grain versions of Mrs. Fields cookies, Cinnabons, Starbucks scones, and hundreds of other tempting carbs.

But whole grains aren't celery. They may have as many calories as refined grains, so you can't dig in as though there were no tomorrow.

* Protein. Getting enough protein may keep dieters from losing lean tissue, according to some studies. (5)

"There's some data that protein may preserve muscle, so it's prudent to keep it up," says Eckel. Protein may help by supplying amino acids, the building blocks of muscle protein, or by preventing muscle protein breakdown.

Some studies also find that protein may keep dieters satisfied. "In adults, protein has a higher satiating power per calorie than carbohydrate or fat," says diet researcher Arne Astrup of The Royal Veterinary and Agricultural University in Denmark.

How much is enough? He suggests up to 25 percent of calories, which translates into about 95 grams of protein a day in a 1,500-calorie diet. Most people can get there by cutting back on carbs and fats--but not protein--when they cut calories.

* Sweetened beverages. "Don't drink your calories," says one of Weight Watchers "Weight Loss Do's and Don'ts." That's because our bodies don't notice the liquid calories we consume, so they leave us hungry for more food.

For example, Astrup's research team supplied overweight people with free beverages (and a few other foods) sweetened with either sugar or artificial sweeteners. After 10 weeks, the sugar eaters gained 3 1/2 pounds and the other group lost 2 pounds. (6)

"Calories from beverages are less satisfying than calories from solid foods, and it's easier to over-consume calories from drinks than from solids," says Astrup.

* Alcohol. Do people consume more calories if they drink alcohol with a meal than if they drink, say, soda pop? The evidence is mixed.

"It's not clear if alcohol makes us less restrained about eating or if we just don't compensate for liquid calories," says Astrup.

Either way, it would be unwise to forget about the calories in wine, beer, and mixed drinks.

* Exercise. Experts recommend 60 to 90 minutes a day of exercise to keep weight off. That's how much activity people report to the National Weight Control Registry, which tracks dieters who say they have kept off at least 10 percent of their weight for at least a year. (7)

And don't forget strength training. The more muscle you build, the more calories your body burns, even while you're sitting in traffic.

* Sleep. Getting too little sleep can boost your appetite by lowering leptin and raising ghrelin levels. (8) Leptin, a hormone released by fat cells, signals the brain to stop eating. Ghrelin, a hormone that's made in the stomach, pushes you to eat more.

What's more, young men who were allowed to sleep for only four hours a night had a 30 percent greater appetite for high-carb foods (like cookies, ice cream, bread, pasta, and chips) than when they were allowed to sleep for 10 hours a night. (8)

And the brain interprets a drop in leptin as a sign that you're starving, so it ratchets down your metabolic rate.

As if that weren't enough, too little sleep may make insulin less effective at clearing glucose from the blood, which could raise the risk of diabetes and heart disease. When researchers limited healthy 20-year-olds to four hours of sleep for six straight nights, it brought them to a nearly diabetic state. (9) (See Nutrition Action, Jul./Aug. 2005, p. 10.)

(1) J. Am. Med. Assoc. 293: 43, 2005.

(2) J. Am. Med. Assoc. 293: 96, 2005.

(3) J. Nat. Cancer Inst. 97: 906, 2005.

(4) Cancer Res. 65: 11779, 2005.

(5) Am. J Clin. Nutr. 83: 260, 2006.

(6) Am. J. Clin Nutr. 76: 721, 2002.

(7) Am. J. Clin. Nutr. 82: 222S, 2005.

(8) Ann. Intern. Med. 151: 846, 2004.

(9) J. Appl. Physiol. 99: 2008, 2005.