Only two of 32 major commercial weight-loss programs evaluated in a study can scientifically show participants actually lose weight over a year. People who want to use a commercial program to shed pounds should choose Weight Watchers or Jenny Craig, the results indicate. The programs led to average weight loss of 3 percent and 5 percent, respectively, across multiple trials that lasted a year or more.

Participants in Jenny Craig lost an average of 15 pounds while Weight Watchers enrollees lost eight pounds in the course of a year.

The study's results are significant because Americans spend an annual $2.5 billion on weight loss, and researchers say popularity of the programs is increasing as obesity grows as a public health problem. Nearly 70 percent of adults in the U.S. are overweight, and part of the Affordable Care Act now covers obesity counseling and screening.

“There’s a ton of commercial weight loss programs out there, and most of them are not evidence-based,” said Dr. Christina Wee, an obesity specialist at Harvard Medical School and Beth Israel Deaconess Medical Center who wrote an editorial about the study in Annals of Interior Medicine.

Dr. Kimberly Gudzune, a weight specialist at Johns Hopkins University, and her team reviewed 4,200 studies of 32 commercial weight-loss programs published in scientific journals or supplied to the group by those programs for the analysis. They looked for randomized, controlled trials designed to reduce bias through comparison with a control group, and found only about a third of the programs had been studied to this degree of rigor. Even fewer had trials that lasted for a year or longer. Gudzune said the lack of evidence may be a symptom of the fact the American Medical Association only recently recognized obesity as a disease in 2013, and researchers are just starting to define more carefully its diagnosis and treatment.

Several of the nation's most popular weight loss programs -- including SlimFast, the Biggest Loser and Optifast -- did not have enough evidence to support any long-term weight loss claims, though some did help participants to quickly lose a few pounds. Participants in Nutrisystem, for example, lost about 12 pounds after six months but there was no long-term data available to assess impact beyond that point. Weight Watchers is the most popular weight loss program in the U.S. and makes up 45 percent of the industry’s sales, but Nutrisystem ranks second with 14 percent of sales and Jenny Craig comes in third at 13 percent, according to an IBISWorld report cited by the researchers.

Adherents to the Atkins diet lost 7 percent of their body weight after six months in the program but only about 2 percent of body weight after one year, though that difference may be partly due to the fact the control group in the yearlong study received behavioral counseling, which may have led to a lower baseline.

The team also studied “very low-calorie” programs that provide meal replacement plans of 800 to 1,000 calories a day for participants, including Health Management Resources (HMR). While some people boasted 13 percent weight loss after six months of following these plans, there was no evidence to show the weight stayed off, and they seemed more susceptible to side effects than those in other types of programs -- more than half of enrollees in HMR suffered from constipation.

Even the pounds lost by participants on Jenny Craig and Weight Watchers were still only considered a modest success by weight management experts. The Centers for Disease Control and Prevention said people start to see improvement in their health such as reduced blood pressure and lower cholesterol after losing 5-10 percent of total body weight.

Wee said Jenny Craig and Weight Watchers are successful programs because they provide a clear set of rules and a strong sense of social support along with dietary advice and an exercise regimen. Still, she argues visiting a specialist who can design a personalized diet and exercise routine and checks in with patients every week or two is the best approach to weight control. The only problem with this model is that insurance plans rarely cover it.

“So the question is -- when you're counseling a patient and trying to recommend treatment, what can you offer them when you cannot offer the thing that's actually been shown to be effective?” she asked. “The commercial weight loss programs could fill that void.”

Gudzune said her services in weight loss management are in greater demand than ever, and she hopes these results can serves as a guide to discussions between doctors and their patients about a sensible weight loss strategy. “I think it’s a common thing that providers struggle with -- not everyone has a lot of training in weight management so where do we send our patients for weight loss if that's something they're interested in?” she asked.

She also pointed out participants in clinical trials often receive free access to weight loss programs as a condition of their contributions to the research. In reality, many people who start a program drop out after a few weeks or months because the cost of enrolling is so high. It costs $570 a month on average to complete Jenny Craig, which includes meals while Weight Watchers costs $43 per month but does not include food.