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updated 4/7/2015 4:15:54 AM ET 2015-04-07T08:15:54

Among commercial weight-loss programs, Weight Watchers and Jenny Craig show the strongest evidence that they can help dieters keep weight off for at least 12 months, a new study suggests.

Researchers found that after one year, Jenny Craig participants lost an average of 4.9 percent more weight, and people enrolled in Weight Watchers lost an average of 2.6 percent more weight than people who either dieted on their own, were given printed advice about weight loss or received a few sessions of health education and behavioral counseling.

The study showed that for the majority of commercial weight-loss programs out there, researchers don't know whether or not they work, said lead author Dr. Kimberly Gudzune, a weight-loss specialist and an assistant professor of medicine at The Johns Hopkins University School of Medicine in Baltimore.

"It's important for the public and doctors to know which programs help people to lose weight, but also which ones help to keep it off," Gudzune said.

Weight-loss success

In this review study, the researchers looked for published studies on weight-loss programs that were rigorous, long-term randomized controlled trials, which are considered the highest-quality data to evaluate whether a program works.

The researchers included only studies that were at least 12 weeks long and were based on comprehensive weight-loss programs, meaning the programs emphasized nutrition and also offered behavioral counseling or social support, although they may or may not have focused on physical activity. [ 7 Biggest Diet Myths ]

Only 39 studies of 11 weight-loss programs met the researchers' criteria to be included in the review. The 11 programs evaluated included Weight Watchers, Jenny Craig, Nutrisystem, Health Management Resources (HMR), Medifast, Optifast, Atkins, Slimfast and three Internet-based offerings (The Biggest Loser Club, eDiets and Lose It!). Other programs that were not included had not done rigorous studies, the researchers said.

"I had hoped more programs had done more rigorous long-term trials in the 10 years since the last review study on this topic had been done," Gudzune said.

Among the 11 programs, only the people who participated in Jenny Craig or Weight Watchers achieved significant weight loss that was sustained for at least 12 months, Gudzune told Live Science.

The findings, which were published today (April 6) in the journal Annals of Internal Medicine, also revealed a few other programs with promising results early on, such as at three or six months, but they lacked research on whether the weight loss was sustained after one year on the program.

For example, the researchers found that participants in the Nutrisystem program achieved at least 3.8 percent greater weight loss after three months than people who were not enrolled, but the program lacked solid evidence of its long-term effectiveness.

Commercial weight-loss programs are regulated by the Federal Trade Commission, and although the agency encourages programs to collect data on participants' outcomes, it hasn't stipulated the type of studies programs should do, Gudzune said.

Achieving health benefits

As a physician who sees weight-loss patients, Gudzune said she wants more program options in her tool box when she is counseling people about programs that might meet their needs and budget, whether they are commercial programs, such as those evaluated in the study, or services offered through hospitals, clinics or dietitians.

It's not unreasonable for people who are interested in weight-loss programs to ask about the typical results that people have had in a program, in both the short and long term, so they can have realistic expectations, Gudzune said.

She said that although this review study found that participants in the best commercial programs had modest weight loss — of 3 to 5 percent after 12 months — from a health perspective, such weight loss is an important first goal to reach.

Obesity experts recommend a 3 to 5 percent weight loss as the first steps to achieve such key health benefits as lowering blood sugar levels, preventing diabetes and improving cholesterol, Gudzune said.

© 2012 LiveScience.com. All rights reserved.

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