Every year, Americans spend $33 billion to lose weight, handing over hard-earned money to the purveyors of diet plans, pills, and products. But are we spending our money wisely? Here's the bottom line on what's good and what's bad about the diets and diet products making the news and the best-seller lists.

Low-Carb/High-Protein Diets

The Theory: Given carbohydrates, your body releases insulin, which facilitates turning carbs into fat. If you limit carbs, you limit the insulin response and, consequently, your body's ability to turn fuel into fat.

The Plans: High-protein diets increase protein consumption at the expense of carbohydrates. The most famous high-protein diet was devised by Robert Atkins, M.D., whose plan allows almost no carbohydrates at first and then permits a few more fruits and vegetables down the road. The Zone Diet, espoused by Barry Sears, Ph.D., is a more moderate high-protein prescription—it calls for 30 percent protein, 30 percent fat, and 40 percent carbohydrates. Protein Power, The Carbohydrate Addict's Diet, and Sugar Busters offer other variations on the high-protein theme.

The Science: The high-protein diet is perhaps the most controversial on the market. Anecdotal evidence abounds—chances are you know someone who's been on it, or you may have even tried it yourself. High-protein dieters often see the needle on the scale go down fairly quickly. Why? Water. "When you take carbs out of the diet, you lose water weight right away," explains Katherine Tallmadge, M.A., R.D., a weight-loss consultant and American Dietetic Association spokesperson in Washington, D.C., and author of Diet Simple. "I've heard reports of 5 to 10 pounds in a week. If you lose 5 pounds in a week, most of it has to be water, and losing that much water isn't safe." People on high-protein diets lose more water initially because protein requires more water to be metabolized (that's why sticking to a strict high-protein plan for a long time can cause kidney problems). The water weight comes right back when you start eating carbohydrates again.

No long-term independent studies have been conducted to test the efficacy of high-protein diets (although two large studies are now underway). "In the meantime, I can't recommend the Atkins diet because it's unhealthy for other reasons," says Bonnie Liebman, M.S., director of nutrition at the Center for Science in the Public Interest. "It's high in red meat and saturated fat and low in nutrient-dense fruits and vegetables." In fact, many of the plans—including the Atkins plan—declare such healthy foods as potatoes, corn, and many fruits totally off limits.

The Verdict: Focusing on protein can help cut down on refined carbohydrates like pasta, bread, and ordinary sugar—all foods many people find hard to resist. But some high-protein plans don't distinguish adequately between refined bagel-type carbs and nutritious broccoli-type carbs. If you limit fruits and vegetables, you're not getting the full complement of the nutrients and fiber your body needs.

Moreover, not all proteins are alike. Some (cheese, eggs, and bacon) are high in saturated fat, which has been shown to increase cholesterol and boost the risk of heart disease and certain kinds of cancers. But other forms of protein—such as lean meats, chicken, and fish—do have a place in a healthy diet.

The Theory: Portion control and a reduced-fat diet combined with a support system help dieters shed pounds.

The Plans: Plans like Weight Watchers, Jenny Craig, and eDiets combine lifestyle guidance, exercise recommendations, and ongoing advice and moral support with a balanced diet. With Weight Watchers and Jenny Craig, the guidance and support are provided at meetings—private weigh-ins, classes, and one-on-one or group sessions. With eDiets, it's all online. (Weight Watchers and Jenny Craig recently added their own online components.) These programs employ diet and exercise experts (although their qualifications vary) to advise participants, and dieters can thrive on one another's success. Each of the plans incorporates sensible eating guidelines based on the usda Food Guide Pyramid: heavy on grains, vegetables, and fruits; light on meat, sugar, and full-fat dairy. Jenny Craig gives dieters a feel for portion control by starting them on the company's brand of packaged food, and eDiets attaches specific quantities to both their recipes and other allowed food choices. Weight Watchers controls portions with a point system that takes calories, fat, and fiber into account.

The Science: There hasn't been third-party research specifically on plan participants, but the kind of diet these plans recommend isn't controversial. They're moderate- to low-fat plans that focus on a sensible, balanced diet. That, and the success stories of participants, forms the bulk of the argument in favor of these diets.

The Verdict: The individual guidance and moral support that are offered by structured-program diets can make difficult adjustments easier. But plans can get expensive, especially those that require you to buy their food products. "Weight Watchers has been a sensible diet for many years," says Liebman. "I'd recommend that over Jenny Craig because it's less expensive and doesn't rely on selling its own packaged food."

The Theory: Over-the-counter weight-loss supplements contain ingredients to increase metabolism and prevent the body from absorbing dietary fat.

The Products: Supplements like TrimSpa, Stacker 2, Dexatrim, Chitosan, and Blocker 3 have "active" ingredients like hydroxycitric acid (hca), glucosamine, chitosan, and conjugated linoleic acid (cla), most of which claim to burn or trap food's fat and prevent it from becoming your fat. In another class are supplements like Metabolife that contain ephedrine (also called ma huang), a close relative of methamphetamine, which comes from the herb ephedra.

The Science: "Because these are dietary supplements, they're virtually unregulated by the fda. And none of them work except ephedrine," says Steven Heymsfield, M.D., of the the New York Obesity Research Center. "But there's overwhelming evidence that ephedrine poses significant cardiac and stroke risk. I never recommend it." Ephedrine is an amphetamine-like stimulant that boosts metabolism and has been linked with many reports of collapse, permanent disability, and even death.

The Verdict: At best, weight-loss supplements are useless. At worst, they're dangerous. Stay away.

Prescription Drugs

The Theory: Prescription drugs prevent the body from absorbing fat, increase your sense of fullness, or suppress your appetite.

The Products: The big three are orlistat (sold as Xenical), sibutramine (sold as Meridia), and phentermine (sold as Fastin, Phentrol, and others).

The Science: Orlistat prevents fat from being absorbed by the body; sibutramine boosts neurotransmitters to lessen hunger pangs and increase a sense of fullness; and phentermine raises levels of the adrenaline-like neurotransmitter norepinephrine, which increases the rate of metabolism. Only the first two are fda-approved for long-term use. But phentermine remains the most prescribed—after the first three months, it has to be used off-label (meaning it has to be prescribed for something other than weight loss). Harvard University researchers note that most trials of these drugs also included low-fat and low-calorie diets, and only half of the total weight lost, which usually hovered in the 10 percent range, could be attributed to the drug alone. In general, prescription drugs are intended for morbidly obese patients, for whom even a 10 percent weight loss is significant.

The Verdict: "None of them are really effective," says Heymsfield. "People can expect to lose 5 to 10 percent of their initial weight; then they stop losing. If you go off the drug, you're going to gradually regain the weight. There's no magic bullet." Plus, each medication comes with its own litany of unpleasant side effects—everything from elevated pulse to oily stools.

Liquid Diets

The Theory: By substituting drinks for meals, you consume fewer calories.

The Products: Drinks like Slim Fast, Optifast, Satietrol, and Scan Diet are ultrafortified meals-in-a-shake. If you follow the instructions—which usually call for a combination of shakes (or, in a variation, bars) and limited meals—you can lose weight.

The Science: Meal-replacement shakes are fortified low-fat, low-calorie dairy products that contain essential vitamins, minerals, and fiber. While clinical and anecdotal trials demonstrate that these types of meal replacements can be useful weight-loss tools, they also show that staying on the plan can be difficult. Bear in mind that these products were originally developed for nutrient-deficient hospital patients unable to chew or swallow on their own.

The Verdict: "Most people who lose weight on these gain it right back because they know no middle ground," says Tallmadge. That's because meal-replacement drinks don't give dieters a good sense of how to control calories and portions on their own. "Although these products are loaded with vitamins and minerals," Tallmadge adds, "it's better to use real food to help you lose weight and take a daily multivitamin to fill any nutrition gaps."

Reduced-Fat Diets

The Theory: Eating high-fiber, low-fat fruits, vegetables, and grains helps you feel full and makes it easier to get the nutrients you need and consume fewer calories. If, when you're hungry, you eat whole plant foods until you're full, you'll lose weight.

The Plans: Reduced-fat diets range from 10 percent of calories from fat (the plan by Dean Ornish, M.D., and the Pritikin diet) to the much-recommended 30 percent (American Dietetic Association, American Heart Association, and American Institute for Cancer Research). In general, these diets emphasize fiber-rich, nutrient-dense plant foods over foods high in saturated fats, like high-fat meats and full-fat dairy. The fat in these diets comes mainly from mono- and polyunsaturated vegetable, nut, and fish oils.

The Science: Ornish has performed controlled studies of his program, which includes not just a low-fat diet but regular exercise and stress-reduction techniques like meditation, to reverse heart disease. Although participants lost weight, it's impossible to pin the weight loss solely on the diet because other factors were involved. In any case, such low fat intake is a strict prescription for someone who just wants to lose a few pounds. But the basics of a moderately low fat diet are what virtually all mainstream health organizations recommend and are based on years of reputable studies.

The Verdict: Reduced-fat plans are consistent with a healthful diet. Vegetables, fruits, and whole grains, plus lean sources of protein like fish and poultry, are chock-full of the nutrients that can decrease your risk for heart disease, stroke, and some cancers. Scientists discover more benefits of these foods every day. Another plus: Because plant foods are high in fiber, they promote a feeling of fullness, which can curb overeating. Incorrectly interpreted, though, a low-fat diet can lull you into eating too many calories in the form of refined grains and sugar.

Recently, reduced-fat diets have been blamed for America's skyrocketing obesity rates. Told to limit their fat, the thinking goes, people turned to huge quantities of the wrong kinds of carbohydrates—refined grains and sugar. That may be so, but it's important not to blame the vegetable-centric reduced-fat diets that most health authorities recommend.

Of course, there's also the cabbage soup diet, the grapefruit diet, the juice fast, and countless other fad diets. To evaluate any diet, exercise common sense: If it sounds unbalanced, unhealthy, or just plain silly, it probably is. Once you cut through the gimmicks, promises, and shortcuts, you're left with the truth about weight loss: "To lose weight, you have to eat fewer calories," says CSPI's Liebman. There it is. To weigh less, consume fewer calories than you expend.

How many fewer calories? First, figure out how many calories you need to maintain your weight. For example, If you're a 35-year old, 5-foot 6-inch, 135-pound, moderately active woman, you need about 2,000 calories per day. (If you're not, try the Calorie Control Council's calculator at www.caloriecontrol.org/calcalcs.html.) One pound is about 3,500 calories, so a 500-calorie deficit a day will net you a pound-a-week loss.

Knowing you have to eat less is the easy part. The hard part is actually doing it. You'll increase your chances of success by taking a two-pronged approach: Pay attention to both what you eat and how much you eat. Build your diet on a foundation of low-calorie, nutrient-dense food and reasonable portion sizes. Even carrots have calories. The key question about a diet isn't whether it can stop calories from mattering—it can't. A good diet is one that helps you eat less food, healthfully.

Liebman makes it clear that there isn't just one "good" diet—you have some latitude. "There's a range of healthful diets, and the key is to find the one that helps you eat the right number of calories. For some people, it's easier to cut the bread and pasta and sweets. For others, it's easier to cut out fat. Work with whatever's easiest for you."

Whatever you choose, there are two constants: exercise and vegetables. "Vegetables are the most nutrient-dense, low-calorie foods you can eat," says Tallmadge. Liebman adds: "If you load up on fruits and vegetables, you won't have as much room for the calorie-dense foods." And exercise is a universal diet principle to shed pounds and keep them off. According to the National Weight Control Registry, a database of more than 3,000 people who have kept off at least 30 pounds for at least a year, 91 percent of participants exercise to keep their weight down. Exercise builds and maintains muscle, which helps the body burn more calories. Even those from a treat.

The bottom line: Eat modestly but well. Choose whole grains, vegetables, and fruits. Select protein from lean meats, poultry, or low-fat dairy, and consume some healthy fats like those from nuts and fish.
Then, just eat less.

"It's dull, but it's true," Liebman concludes. And the truth will make you thin.