After two years, adults who participated in Weight Watchers averaged a weight loss of about six pounds, while those who tried to do it on their own had an average loss of less than one pound.
At the same time, researchers are saying there isn't yet enough evidence for or against low-carb diets.
Both studies appear in the April 9 issue of the Journal of the American Medical Association, which focuses on obesity.
The fattening of America may be one of the biggest public health challenges of the 21st century. According to the authors of the low-carb study, the prevalence of obesity among Americans aged 20 to 74 years more than doubled between 1960 and 2000, from 13.4 percent to 30.9 percent.
The failure to thin down is not for lack of trying. During that time, about one-third of adults were attempting to lose weight, while another third were trying to maintain.
Low-carb diets have been at the top of the list of dieters in recent years. The Atkins Diet has appeared on The New York Times bestseller list continuously for more than five years, while other low-carb diet books also sell like hotcakes. Medical experts have expressed concern, however, that these diets could lead to abnormal metabolic functioning and serious long-term effects, especially for people with Type 2 diabetes, high blood pressure and cardiovascular disease.
The claims for and against low-carb regimens are largely unproven, which is why Dr. Dena Bravata of Stanford University and her colleagues undertook an exhaustive search of the medical literature for studies published on the subject between 1996 and February 2003.
There were big gaps in what they found. While 107 articles reported data on 3,268 participants, a minority of articles evaluated diets of 60 grams of carbohydrates per day or less and even fewer looked at the 20 grams/day diets, which are among the most popular. Furthermore, no study looked at diets of 60 grams/day or less in participants with a mean age older than 53 years, and only five studies looked at these diets for more than 90 days.
When weight loss did occur (and it did), it seemed to be due primarily to calorie restriction, not fewer carbohydrates. Diets that lasted longer also resulted in more weight loss. "Weight loss was principally a result of reduced calories and also a function of increased diet length," says Dr. Dena Bravata, lead author of the study and a social science research associate at Stanford University's Center for Primary Care and Outcomes Research in California.
"All dietary interventions get to essentially the same issue, and that is caloric restriction," says Duane Eichler, a professor of biochemistry and molecular biology at the University of South Florida in Tampa. "People argue different ways of doing that, but if you only take so much energy in and you expend more energy, you're going to lose weight. And if you take in more energy than you expend, then you're going to gain weight. That's how efficient the system is."
While Bravata and her colleagues found no significant adverse effects, most of the studies may not have lasted long enough for anything to show up. "There were very few diets over three months," Bravata says.
About 13 percent of women and 5 percent of men are consulting not books but formal, structured weight-loss programs, according to the authors of the second study. But like the low-carb diets, these programs had never been intensively studied.
"There's a tremendous shortage of sound, scientific evaluation of these commercial programs," says study author Stanley Heshka, a research associate at the New York Obesity Research Center at St Luke's/Roosevelt Hospital and an associate research scientist at Columbia University's Institute of Human Nutrition, both in New York City. "There had never been a rigorous, scientifically sound evaluation of a large-scale commercial weight-loss program."
Heshka and his colleagues decided to compare Weight Watchers, one of the best-known commercial weight-loss programs, with a do-it-yourself program. The study was funded by Weight Watchers, and one of the co-authors, Karen Miller-Kovach, is the vice president for research and development for the company.
They randomly assigned 423 overweight or obese adults to either group, with the self-help group receiving two 20-minute counseling sessions with a nutritionist as well as some dieting resources. The Weight Watchers group got a food, activity and behavioral modification plan, and was instructed to attend weekly meetings.
At the end of the first year, the self-help group lost and maintained 2.86 to 3.08 pounds. By the end of the second year, however, the weight was back to what it had been at the beginning of the study.
The Weight Watchers group lost 9.46 to 11 pounds by the end of the first year, and 5.94 to 6.6 pounds overall by the end of the second year. This group also lost about 1.77 inches off their waist circumference at the end of the first year and 0.98 inches at the end of the second year. Body mass index also decreased.
Participants who attended at least 78 percent of Weight Watchers' weekly meetings maintained a mean weight loss of 11 pounds.
Heshka is not able to say exactly why Weight Watchers was more successful. "We were not set up to dissect the components, but if I had to guess, it would be the structure," he says. "You have to go somewhere every week and get weighed. People are going to look at you and ask, 'How did you do?'"
Weight Watchers may also be more effective for people who are just a little bit overweight and are trying to forestall obesity. It could also be helpful for people who have a family history of diabetes and heart disease, situations in which even a small weight loss is helpful. "This is one of the things you can undertake for those people who have not yet really developed severe obesity and the health problems that go with it," Heshka says.
"We don't have a magic bullet," Eichler adds. "Two things that really work in weight loss, if you get the patient's participation, are dietary management and lifestyle changes, which means invoking exercise. The real issue is not to do cosmetic work but to try to promote health. That's really the bottom line."
For more on obesity, visit the American Obesity Association or the North American Association for the Study of Obesity. The American Dietetic Association has loads of information on eating right and losing weight.