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Many weight loss plans claim to hold the key to success for dieters looking for a new way to take off those pounds for good. Two studies show, however, that the type of diet program you choose is less important to success than your resolve in sticking with it.

Putting diet plans to the test

One of these studies, published in the Journal of the American Medical Association, compared four popular diets—the Atkins low-carb diet, the Ornish low-fat diet, the Zone and Weight Watchers. Dr. Michael Dansinger, who conducted the research at Tufts-New England Medical Center in Boston, found that although each diet plan focuses on a different eating approach, they all end up reducing calories.

At the start of the research, the Tufts team randomly assigned 160 volunteers, 27 to 42 years of age, to follow one of the four plans. After one year, researchers determined the weight loss among the 40 participants in each plan averaged 4.7 to 7 pounds. This average factored in the large number of people who quit the diets before the year was over.

Sticking to the diets proved to be a challenge. About half the subjects who were on the Atkins and Ornish plans and about 35 percent of the Zone and Weight Watchers dieters dropped out before the year was up. Those who made it through the full year averaged a weight loss of 8.5 to 14.5 pounds. The biggest factor in how much weight was lost was not the type of diet plan but how long and how closely participants followed whatever plan they were on.

Those averages represent wide variation among the dieters. Some lost 20 pounds and some actually gained weight on the same diet. The encouraging news about the research, according to Dansinger, is that with so many options, people should be able to find a diet plan they can stick with, which is the most important factor in losing weight. Dansinger recommends trying different diets until you find one that works for you. “Date the diets until you find a life partner.”

Little evidence to support diet plans' benefit

A second, unrelated study, published in the Annals of Internal Medicine, highlighted the dearth of evidence that any commercial weight loss program actually works. Researchers from the University of Pennsylvania School of Medicine surveyed the scientific literature for research on nine of the most advertised diet programs, including LA Weight Loss, Jenny Craig, Optifast and Weight Watchers.

In “Systematic Review: An Evaluation of Major Commercial Weight Loss Programs in the United States,” authors Dr. Adam Tsai and Thomas Wadden assessed more than 1,500 abstracts looking for those that met their criteria for review. They included only studies that were conducted on adults in the U.S., had 10 or more participants and lasted at least 12 weeks.

They discovered almost no published scientific evidence to substantiate the most popular programs' effectiveness with the exception of three studies of Weight Watchers. One of those studies found about a 5 percent reduction in body weight over three to six months.

Studies from programs such as Optifast seemed to promise better results, a loss of 15 to 25 percent of body weight during the initial treatment period, but they are expensive and require close medical supervision. Patients on Optifast who were followed for a year maintained an average 8 to 9 percent weight loss, but as many as 56 percent of the participants dropped out of the program.

Internet-based and self-help programs fared even worse in the review, and the authors state that there's minimal evidence that would support their use. The benefit is accessibility and reasonable cost.

Find a diet that fits

There are proponents for all of these plans. The authors do not state that commercial weight loss programs do not work, they are just citing the lack of scientific evidence of their effectiveness. All of the plans have success stories, but the gold standard of proof—large-scale, long-term clinical trials—is more expensive than most of the commercial plans are willing to invest.

“It all comes down to whether you can find an approach that works for you,” Wadden says. “If you reduce calories, you lose weight. The future is helping people find the right program that fits best for them given their preferences.”

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