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Which diet plans really pay off?

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Weight loss is big business. Two-thirds of the U.S. population is overweight or obese, and Americans were expected to spend $2.5 billion on commercial diet plans and services in 2014. More than 60 percent of U.S. adults have made a serious attempt to lose weight at some point in their lives, and 29 percent say they're currently on a diet.

But before sinking serious money into a weight-loss program that may involve spending hundreds of dollars on packaged meals, nutrition shakes, or counseling sessions, it pays to see which ones are most effective in the long run.

A new study by researchers at Johns Hopkins, published Monday in the Annals of Internal Medicine, reviewed clinical trials on some of the most popular commercial weight-loss programs, including Weight Watchers, Jenny Craig, and Nutrisystem, along with some like Medifast and OPTIFAST that promote more extreme calorie restrictions and meal replacements.

Out of 4,200 studies, they found only a few dozen met the scientific "gold standard" of reliability. Just a small number of diet plans were supported by data establishing that participants, on average, lost more weight after one year than people who were either dieting on their own, got printed health information, or received other nutrition education and counseling sessions.

One key thing the most successful plans had in common: social support. "They integrate teamwork. They really integrate a group approach," medical contributor Dr. Holly Phillips told "CBS This Morning." "The only diets that work are those that you stick with, and if you have some companionship along they way, you're more likely to do it."

Click through to see how the different diet plans stack up.

Weight Watchers

The oldest and best-known commercial diet plan -- and the biggest, with 45 percent market share -- Weight Watchers also has a proven track record in clinical trials.

The Johns Hopkins researchers found that "Weight Watchers participants consistently have greater weight loss" than people in who try dieting on their own, "and sustain it beyond 12 months." One year after they started the program, the study found Weight Watchers participants lost about 8 pounds -- 2.6 percent more weight than people in the control group.

Weight Watchers is what the researchers considered a "high intensity" program, meaning it requires attending at least 12 sessions a year. But it's also one of the lowest-cost options in the study.

An editorial accompanying the study in the Annals of Internal Medicine says, "Programs that help patients restrict calories with a structured approach to making healthier, real-world dietary choices, such as Weight Watchers, may fare better over the long term than programs that rely solely on prepackaged meals or supplements, but this would need to be confirmed in future studies."

Next: Another top-performing diet

Jenny Craig

Jenny Craig also fared well in the study overall. Participants lost a few more pounds than those on Weight Watchers, but spent more money to do so.

A review of clinical trials found that after one year, people who followed the Jenny Craig program lost at least 4.9 percent more weight compared with people in the control group or those getting weight-loss counseling.

"Participants on Jenny Craig, on average, lost at least 15 pounds at the 12-month mark," study author Dr. Kimberly Gudzune, of Johns Hopkins University School of Medicine, told CBS News.

Unlike Weight Watchers, the Jenny Craig plan requires participants to buy specially-packaged meals and snacks, making this plan more expensive.

Jenny Craig and Weight Watchers were the two diet plans the researchers suggested doctors recommend to patients who need to lose weight.

Next: How Nutrisystem compares

Nutrisystem

Nutrisystem also appears to be one of the more effective commercial weight loss programs reviewed in the study. However, the clinical trials on Nutrisystem lasted only 3 to 6 months, so its longer-term effectiveness is not as clear.

People on the Nutrisystem plan achieved at least 3.8 percent greater weight loss at 3 months than people dieting on their own or getting counseling. The researchers say "Nutrisystem shows promise, but the lack of long-term [randomized clinical trials] precludes definitive conclusions."

Next: Are extreme low-cal plans a good bet?

HMR, Medifast & Optifast

Health Management Resources (HMR), Medifast, and OPTIFAST are three "very-low calorie programs" included in the study. They rely on low-cal meal-replacement products like bars or shakes to promote faster short-term weight loss.

Participants consume just 800 to 1,000 calories a day on these plans, resulting in at least 4 percent more weight loss than people who got counseling alone. However, the benefits diminished by the 6-month mark, and researchers say more long-term studies are needed.

Next: How Atkins compares

Atkins

Programs based on the low-carb Atkins diet also helped people lose more weight than counseling alone -- but not as much as with some competing diet programs. Still, the approach "appears promising," the authors write.

Next: Some that didn't make the cut

Ornish, Zone & more

Emilia Stasiak

These popular diet plans were not included in the study. The researchers say they did not evaluate the Ornish diet because it is primarily focused on heart health rather than weight loss. The Zone diet was not reviewed because unlike other weight-loss plans in the study, Zone does not include any behavioral or social support.

Read: One thing to add to your diet if you want to lose weight