Weight-loss enthusiasts banking on "The Dukan Diet" book (Hodder & Stoughton, 2010) already a hit in France and set to be released here in the United States on April 19 to shed pounds for good may want to think again. Health experts say the diet touted by French doctor and nutritionist Dr. Pierre Dukan is no better than fad diets of times past.

The Dukan, Atkins and South Beach diets all promote high-protein, low-carb ways of eating , said David Levitsky, professor of psychology and nutritional sciences at Cornell University in New York.

"They all have their little gimmicks in it, but basically what they're saying is that changing the composition of the foods that we eat has some miracle effect on metabolism," Levitsky told MyHealthNewsDaily.

The Dukan Diet has four phases the first lets dieters eat as much protein as they want, the second lets dieters eat protein and vegetables, and the third lets dieters add starches, fruit, cheese and bread. The last phase of the diet allows dieters to eat anything they want as long as they eat three teaspoons of oat bran every day, take the stairs whenever possible and set aside one day of the week to eat only protein.

The Atkins diet is similar its first stage calls for limited amounts of carbohydrates with liberal amounts of meat, fish and eggs, cheese, salad vegetables, butter and oil. Dieters add on more carbohydrate-rich foods until they find the optimum balance where they are not gaining weight from what they are eating.

The South Beach diet also encourages low consumption of carbohydrates at the beginning of the diet, and then allows the dieter to gradually add fruits, vegetables and grains into his or her diet.

How the diets work

These diets usually lead to weight loss at the beginning because American diets are composed so heavily of carbohydrates, so giving them up simply leads to eating less, Levitsky said.

"Sixty percent of our calories come from carbohydrates that's the American diet," he said. "They're one of the cheapest forms of calories, so most meals have a large carbohydrate background that we add the protein and vegetables. It's pretty easy to reduce that aspect of the diet."

However, studies have shown that when you reduce the components on your plate, you don't usually compensate by eating other foods, he said. That means you end up eating less, and you won't even necessarily feel hungry because you've eaten the minimum amount of food to feel satiated, Levitsky said.

Reducing carbohydrates also leads to a decrease in sodium, which spurs an increase in water loss. Water loss is most evident in the face, because there are a lot of fluids under our sensitive facial skin, he said.

"You step on the scale and it seems like you're losing weight, but you're actually losing water content so it looks like you're losing more," Levitsky said. "When scientists do the study to examine what tissue you are losing, they find that it's water."

In the early 1980s, fat was demonized, so people tended to overeat carbohydrates. Now, it's carbohydrates that get a bad rap, said Stella Volpe, a registered dietitian and professor and chair of the Department of Nutrition Sciences at Drexel University in Philadelphia.

But healthy amounts of carbohydrates aren't bad for you, Volpe said.

"The deal is this we need carbohydrates, we need fat, and we need protein," Volpe told MyHealthNewsDaily.

The optimum 2,000-calorie diet should be 50 percent whole carbohydrates (preferably from whole grains), 30 percent good (monounsaturated) fats and 20 percent protein, she said. The Mediterranean diet , which is high in good fats, fish and fruits and vegetables, is a good diet model to strive for, Volpe said.

Eating a wide variation of foods within food groups such as a number of different vegetables is also essential for warding off nutrient deficiencies , Levitsky said.

The right way to diet

To lose weight healthfully, don't think in terms of going on a diet, which implies a drastic, temporary lifestyle change. Instead, think of making small lifestyle changes that will last, Volpe said. Two keys to losing weight healthfully are to take your time and to exercise portion control.

"That will decrease caloric intake immediately if they can really, really evaluate the portions and make changes, even if it's just a 20 percent reduction in their portions," she said.

People also shouldn't be concerned with losing 5 to 10 pounds in a week or two, she said. Losing 1 to 2 pounds a week is a healthy amount of weight to lose, especially because that means more fat and less muscle and water is being lost.

"The slower the weight loss, the more prolonged it is," Volpe said. "The likelihood of them keeping it [the weight] off is strong."

It's also easier to keep the weight off if a person makes small, stepwise changes like swapping out regular soda for calorie-free seltzer water, she said.

People who have managed to lose a significant amount of weight should also not be in the mindset that once they've accomplished their goal, they can go back to eating the same amount of food they were before they lost the weight, she said.

"If you are a 200-pound person, maintaining your 200 pounds takes a lot more calories than if you were 150 pounds," Volpe said. "So they need to re-evaluate and think, 'I'm not that 200-pound person anymore.' To maintain body weight, it will take less calories."

Pass it on: Fad diets such as Dukan, Atkins and South Beach all promote a high-protein, low-carbohydrate way of eating. But those diets only seem to work because they promote water loss and the dieter eats less than he or she normally would, health experts say.

Follow MyHealthNewsDaily staff writer Amanda Chan on Twitter @AmandaLChan.