Four popular diet plans faced off in a new study. Which one won: Atkins (search), Ornish (search), Weight Watchers (search), or The Zone (search)?

These were the rules: 40 overweight or obese adults aged 27 to 42 were randomly assigned to the Atkins, Ornish, Weight Watchers, or Zone programs. For two months, a doctor and a dietitian coached each player on their game plan. Then they were on their own until a weigh-in at the end of a year of dieting.

Who lost the most pounds? Those who followed their diets most closely, no matter which of the four plans they followed. Those who didn't follow their program — 42 percent of the participants — didn't lose much weight.

The findings, as reported in the Jan. 5 issue of The Journal of the American Medical Association:

—On average, those on the Atkins plan lost 4.6 pounds. Fifty-three percent of participants stuck with the Atkins plan for one year.

—On average, those on the Weight Watchers plan lost 6.6 pounds. Sixty-five percent of participants stuck with the Weight Watchers plan for one year.

—On average, those on the Zone plan lost 7.1 pounds. Sixty-five percent of participants stuck with the Zone plan for one year.

—On average, those on the Ornish plan lost 7.3 pounds. Fifty percent of participants stuck with the Ornish plan for one year.

The weight loss differences between the plans were not considered significant, meaning the small differences seen between the various plans easily could have been due to chance. So does this mean that diet plans don't work?

No, says study leader Michael L. Dansinger, MD, director of obesity research for the atherosclerosis research lab at Tufts-New England Medical Center, Boston. It means that the plan you like the best is the plan that works the best.

"I would suspect that most of the popular diet books in the bookstore are likely to produce weight loss if you follow the plan closely, since almost all plans are similar to the diets we studied, or to a cross between two of them," Dansinger tells WebMD. "Most eating strategies will work well if you stick closely. The key is finding a plan you can stick to."

Finding a diet plan is like finding true love. Dansinger recommends playing the field until you find Ms. or Mr. Right.

"Date the diets until you can find a life partner," Dansinger says. "The best way is to try a number of them and give each a fair chance. There is a whole spectrum of options out there. The main finding of our study is that we need to find a way to improve adherence rates to the various diets that are available. The best way might be to be open minded about all of the options rather than focusing on finding the same 'best one' for everybody."

In an editorial accompanying the Dansinger study, Robert H. Eckel, MD, notes that the key to weight loss is a focus on not just food quantity, but food quality. Eckel is a professor of medicine at the University of Colorado Health Science Center and president-elect of the American Heart Association.

His three-way prescription for weight loss – or better still, obesity prevention:

—Eat a diet high in fruits and vegetables, whole grains, and fish.

—Get regular exercise.

—Eat just a little less, and keep it up.

This is what Eckel calls "the low-fad diet."

"Losing weight is not easy. So why be desperate to lose weight quickly on these fad diets?" Eckel tells WebMD. "Let's go about weight loss slowly. Let's modify our diets by trying to eat a little less each day. Unfortunately, that is too simple — and too difficult — for most people."

Thin Evidence on How Well Diets Work

Why can't doctors agree on which diet plans work and which don't? The reason may be thin evidence on how well the plans work, says Thomas A. Wadden, PhD, director of the weight and eating disorders program at the University of Pennsylvania.

"There is a very limited amount of data available," Wadden tells WebMD. "Clearly we would like to see more data about the efficacy of these programs."

Wadden and colleague Adam Gilden Tsai, MD, looked for published evidence on how well each ofnine popular diets work. The diets included Weight Watchers, Jenny Craig, LA Weight Loss, the HMR Weight Management Plan,the Optifast program, the Medifast/Take Shape for Life program, eDiets.com, the Take Off Pounds Sensibly (TOPS) program, and Overeaters Anonymous.

The bottom line, as reported in the Jan. 4 issue of Annals of Internal Medicine: Wadden's team found little published proof that eight of the nine diets really work. Only Weight Watchers had three published studies evaluating how well it works. Weight Watchers participants lost some 5 percent of their weight in six months. After two years, the average weight loss was about three pounds.

"I think the interpretation of the Weight Watchers data depends on whether you are an optimist or a pessimist," Wadden says. "I think it is good — about a 10 pound weight loss for most people, the kind of loss that may prevent diabetes. Doctors can now recommend Weight Watchers to patients and know they will get modest weight loss and pay a modest cost to get it."

Of course, absence of evidence isn't exactly evidence of absence. No popular diet plan lacks success stories or some plausible rationale for why it might work. What scientists consider the gold standard for proof is a large-scale, long-term clinical trial. The cost of such trials is more than many diet programs are able to afford.

The study findings are no surprise to nutritionist Leslie Bonci, MPH, RD, director of sports nutrition at the University of Pittsburgh Medical Center and team nutritionist for the Pittsburgh Steelers. Bonci saw an advance copy of the Wadden study.

"Weight Watchers has kept in touch with the changing needs of consumers, but also keeps the support group aspect that is so very important to many people," Bonci tells WebMD. "Their combination of support and their way of accounting for and monitoring a person's diet gives Weight Watchers the longevity it enjoys."

Different Programs for Different People

Does the Wadden study mean everyone resolved to diet should join Weight Watchers? No, Bonci says. Different programs in different places work for different people.

Despite the dearth of published studies, Wadden and Tsai collected a lot of data on weight loss programs. Some of their findings:

—There's a lot more data available on Weight Watchers than on Jenny Craig or LA Weight Loss. But Wadden says he's talked with patients who've had success with all of them. "If 100 people go to these programs, we just don't know how many will lose weight," he says.

—Very low-calorie programs entail health risks. Optifast and HMR require medical supervision. Medifast does not, and that worries Wadden. "Medifast was of concern to Dr. Tsai and myself because people may end up going on a very low-calorie diet and not having the medical supervision they should have," he says. "Anybody on a diet that gives them fewer than 800 calories a day should be under medical supervision."

—There's very little data on whether Internet-based diets work. But because they are available to so many people, Wadden is optimistic that they can be a big help. "At this point, Internet-based diets look pretty modest in terms of weight loss," he says. "But these Internet programs can potentially be of great benefit. They could be available to millions of people who don't have any other programs in their neighborhoods."

—Self-help programs such as TOPS and Overeaters Anonymous may work well for people to whom their very different approaches appeal. "The great thing about these is you may not lose much weight, but you won't lose much money, either," Wadden says. "These programs offer a place for those who can't afford commercial weight loss programs."

"It all comes down to whether you can find an approach that works for you," Wadden says. "There is no magic here. If you reduce your calories, you lose weight. The future is helping people find the right program that fits best for them given their preferences."

By Daniel J. DeNoon, reviewed by Michael W. Smith, MD

SOURCES: Dansinger, M. The Journal of the American Medical Association, Jan. 5, 2005; vol 293: pp 43-53. Eckel, R. TheJournal of the American Medical Association, Jan. 5, 2005; vol 293: pp 96-97. Tsai, A. and Wadden, T. Annals of Internal Medicine, Jan. 4, 2005; vol 142: pp 56-66.Michael L. Dansinger, MD, director of obesity research, atherosclerosis research laboratory, Tufts-New England Medical Center, Boston.Robert Eckel, professor of medicine, University of Colorado Health Science Center; president-elect, American Heart Association. Thomas A. Wadden, PhD, director, weight and eating disorders program and professor of psychology, University of Pennsylvania. Leslie Bonci, MPH, RD, director of sports nutrition, University of Pittsburgh Medical Center.