Diet programs promise big weight-loss results. Science suggests otherwise.

A lot of weight-loss programs promise to revolutionize your life. "Lose 10 pounds in 10 days," they'll say. "Slim down and get healthy in two weeks!" Ads for these diets often feature dramatic before-and-after photos, women and men thumbing a pair of jeans that seem as if they could fit a person three times their current size.

But every time scientists study commercial diet schemes, they keep coming up with consistent results: on average, participants lose a small amount of weight. That's certainly helpful, but it's not quite the revolutionary shift these programs often claim.

A new systematic review in the Annals of Internal Medicine adds to the evidence here. The researchers found that Weight Watchers and Jenny Craig helped people lose, on average, about 3 to 5 percent of their body weight. Weight Watchers participants lost about eight pounds in 12 months, and Jenny Craig followers shed 15 pounds in a year.

What's more, those were the only two programs for which there was actually decent evidence. Kimberly Gudzune, an assistant professor of medicine at Johns Hopkins who co-authored the study, said many diet plans have zero or very little rigorous scientific evidence backing their promises.

"Weight Watchers and Jenny Craig are the two programs that have the evidence to support that they help people lose weight and keep it off," she explained. "For the other diet programs [there's an] absence of evidence. We don’t know whether they help people to lose weight and keep it off."

The science on weight-loss programs is flimsy

Warning: Results not typical. (Michael D Brown/Shutterstock)

So despite the presence of a multibillion-dollar weight-loss market, despite the claims of big results, commercial diets have little evidence behind them. For the Annals review, the researchers looked at more than 4,200 studies on the effectiveness of diets and discovered that only a few dozen were actually scientifically valid. Many studies were flawed or biased by design, or were too short-term to suggest anything meaningful about real-life weight loss.

This isn't the first review of the diet literature to come to this conclusion. Last November, researchers published a study examining the best available evidence about Atkins, Weight Watchers, and Zone diets. They demonstrated that all diets had roughly similar performance. Participants lost about five or seven pounds over the course of a year, on average, then regained some of that weight in the longer term.

The authors also found that — notwithstanding all the claims of superiority these programs make — a lot of the evidence on branded diets was so low-quality and limited in duration that it was hard to truly know which diet worked best.

Similarly, last September, a big JAMA review found that all diets — low-fat, low-carb — have about the same modest results, no matter their macronutrient compositions.

Even modest weight loss is a good thing

To be clear, even modest weight losses can help lower blood pressure and blood sugar, and prevent diabetes. Still, the results could well fall short of the desired weight loss of many overweight and obese dieters, particularly when compared with the price tag for involvement.

"Entering any weight-loss program, people come in with idea of losing large amounts of weight," Gudzune said. "It’s not impossible to do, but it’s the more unusual occurrence." She noted that in 2014, Weight Watchers cost about $43 a month, while Jenny Craig was $570 a month — the latter being more expensive because it involves food for the month.

Pricey diet programs aren't the only way to lose weight

Gudzune also emphasized that commercial programs aren't the only way to lose weight. "The National Weight Control Registry has been following people who have lost weight and successfully and kept it off for years," she said. "Over time, we learned a lot of things about habits that help promote weight loss maintenance: weighing yourself, keeping a modest calorie restriction, working out, tracking calories." None of these options comes with a price tag.

At Vox, when we talked to the leading obesity and weight-loss experts across North America, they also emphasized that what seems to work best is when people simply cut calories in a way they like and can sustain, and focus more on healthy eating than dieting.

Dr. Yoni Freedhoff, an Ottawa-based obesity doctor — who has worked with thousands of overweight and obese patients — says he tells people, "Ultimately you need to like the life you're living food-wise if you're going to keep living that way. It's crazy to think, with billions of people on the planet, that there's one approach that suits everybody." He added: "The best diet for one individual is the worst diet for another individual."

Watch: Weight-loss lies, debunked

Further reading

• Surprisingly simple tips from 20 experts about how to lose weight and keep it off
• The right way to count calories, according to weight loss experts
• We make exercise way too complicated. Here's how to get it right.
• The Bulletproof Diet is everything wrong with eating in America
• The Vox Anti-Detox Diet
• How we got so stupid about our diets

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