The commercial weight loss industry has ballooned to an estimated worth of $2.5 billion, but few scientifically sound studies exist that prove the efficacy of these programs, a review of 4,200 related research papers reveals.

The report, published Monday in the journal Annals of Internal Medicine, found that among 32 popular commercial weight loss programs available in the U.S., only 11 have been studied using a randomized controlled trial, the model thought to be the gold standard for scientific research. And of those 11 diets, only two— Weight Watchers and Jenny Craig— indicated that people who used the programs lost more weight after 12 months than people following their own diet.

Researchers conducted the study as a follow-up to a 2005 systematic review of commercial and proprietary weight loss programs, which was based on results from three randomized controlled trials, and suggested that only Weight Watchers resulted in modest weight loss. The new study of 11 diets was based on 39 trials, all of which ran for at least 12 weeks.

“Losing weight and keeping it off is a top concern for most Americans,” lead study author Kimberly Gudzune, assistant medicine professor at Johns Hopkins University, told FoxNews.com. “I’m a physician myself, so I thought it was really important to give patients and their physicians guidance on programs to show which can really help them lose weight.”

Two-thirds of American adults are overweight or obese, conditions that increase the chance for high blood pressure and type 2 diabetes. About 63 percent of U.S. adults have attempted to lose weight at some point in their lives, while 29 percent currently report trying to lose weight, according to a 2011 Gallup poll. That poll also indicated that Americans looking to shed pounds are more likely to try dieting than they are to exercise.

A lack of regulation and research

No federal legislation requires commercial weight loss companies to provide scientific evidence that their programs work, but the Federal Trade Commission (FTC) recommended in 1999 that these companies conduct studies to do so. One reason for a lack of study and regulation, Gudzune said, is up until late 2013, the American Medical Association hadn’t even treated obesity as a medical condition.

“It was viewed as an individual problem and an issue of willpower,” she said, “and over the past 10 to 20 years, we’ve learned so much about the factors not just on a personal level, but that the environment and physiologic factors can contribute to this picture. It’s more complicated than we thought.”

Christine Santori, a registered dietician and program manager for the Center for Weight Management at North Shore-LIJ Syosset Hospital, in Syosset, New York, said it’s the attrition rates that stand in the way of good research in this arena.

For example, the Johns Hopkins team’s review of the Nutrisystem diet, a meal replacement program, helped participants lose 5.7 percent more weight compared to counseling and education. But that weight loss was studied only over a period of six months; no research has analyzed whether Nutrisystem helps patients sustain their weight beyond that time frame.

“Most of our patients come in with a long list of prior weight loss attempts, unsuccessful in most of them,” Santori, who was not involved in the Johns Hopkins study, told FoxNews.com. “What I hear over and over again, is, ‘I don’t know how to do it.’ Most of the time it’s about behavioral strategies and the changes they should be making. It’s a lot of program solving. It’s setting them up for long-term success.”

'Modest' weight loss

To sift through research, Gudzune and her team used Medline, the biggest database for medical literature, the Cochrane Database of Systematic Reviews, and the websites of the weight loss programs. They pinpointed plans with a behavioral and social support element, be it an online forum or a coach, as well as those that emphasized nutrition.

While Weight Watchers and Jenny Craig were the only two diets studied with a randomized, controlled trial that showed participants lost weight over one year, those losses were modest. In 12 months’ time, on Jenny Craig, participants lost an average of 15 pounds, while those people on Weight Watchers lost an average of 12 pounds.

“Both seem small and I think would be surprising to many patients— those usually translate into a 3 to 5 percent weight loss for the starting weight for most of the patients in these trials,” Gudzune said.

The average weight loss for Jenny Craig and Weight Watchers participants was modest, but guidelines from the American Heart Association, the American College of Cardiology and the Obesity Society show that sustained weight loss of that amount can lead to improvements in blood sugar and blood pressure.

“That may not be the end goal for many patients, but it’s an important place to start,” Gudzune said.

The big picture

Mitchell S. Roslin, chief of obesity surgery at Lenox Hill Hospital in New York City, who wasn’t involved in the research, said he wasn’t surprised by the study’s findings, and that they confirm what doctors and experts in the weight loss industry already know.  

“I think that the bottom line is commercial diets may work for people who are health conscious, but they’re not going to work for people who are less health conscious and need it the most,” Roslin said. “We keep on missing the big point.”

“Yesterday, I saw three kids between 18 and 23— all disabled from obesity,” he said. “They’re never going to have a job. I don’t think sending them to Weight Watchers is really going to work.”

Although researchers didn’t compare marketing tactics against diet results, Gudzune said companies that include a label showing the average amount of weight loss on any given diet— rather than using patient testimonials like before and after photos— would help give potential customers a better idea of what they’re walking into “because it is a financial commitment for consumers, and then they would know exactly what they’re paying for.”

Other diets studied in the review include low-calorie meal replacement plans— HMR, Medifast and OPTIFAST—in addition to five self-directed programs—Atkins, SlimFast, and Internet-based programs Biggest Loser Club, eDiets and Lose It! The program prices ranged from $0 for Web-based weight loss apps to between $570 and $682 per month for programs that require customers to purchase replacement meals.