Testimonials about substantial weight loss continue, but whether these regimens are effective long-term is still a matter of debate.

Yes. Data from the best programs using liquid protein diets show that up to 78 percent of patients can safely lose at least 40 pounds. With traditional diets, generally only about 5 percent of patients lose that much, and only 25 percent lose 20 pounds or more.

Opponents of liquid diets say it is preferable to lose weight by eating properly. But most people on liquid protein diets have failed repeatedly on traditional diets.

In fact, such "failures" are often our most successful patients, especially if they've been referred by a doctor for medical reasons, such as hypertension. While on supplemented fasting, patients can have a honeymoon from food. They have time to think about it in an abstract way. They learn to separate psychological hunger from physiological hunger.

Keeping the weight off is a challenge. Studies show that those who keep it off tend to be those who stay in a program for at least 20 weeks. Of the patients who do remain that long, about 55 percent of men and 35 percent of women will be within 20 pounds of their goal weight 18 months later. But most who stay 10 weeks or less gain back almost all their weight.

Not all liquid diets are the same. To be successful, they should be nutritionally complete, medically supervised and include behavior modification methods. Our patients are in a support group from the beginning, which helps maintain their commitment. They see a doctor weekly while both fasting and resuming solid foods. -- Loren Wissner Greene, MD Clinical assistant professor of medicine, New York University; co-medical director, NYU and Cabrini Optifast Programs

No. Although liquid protein diets now have a good safety record, I'm not in favor of them because these diets don't get to the root causes of obesity. Instead, they focus on weight loss. Effectiveness isn't weight loss -- it's maintenance. There's no doubt that people can lose weight on these diets, but what's the use of losing weight if you can't keep it off?

A lot of people come to us after they've tried liquid protein diets, lost weight and then regained it. Overweight people need a complete lifestyle change; they don't do that on liquid protein diets. These programs claim to help modify behavior, but what they're trying to modify is eating behavior -- hard to do if people aren't on solid food. Patients can be given lectures about food, but there's little chance to work with them once they go back to solid food. Many leave these programs at that point because they've lost as much as they wanted to, and it costs money to continue. We have a program that tries to change patients' whole approach to food. That needs to be the focus, but it's not where these programs make their money. They make money on the formula. -- Xavier Pi-Sunyer, MD Director of the Obesity Research Center of St. Luke's-Roosevelt Hospital