Given the number of weight-loss plans out there, surprisingly few are actually backed by science — or at least the kind of science that can tell us something about the long-term value of the plans. A new study rigorously analyzes the major weight-loss plans and pinpoints which plans are truly helpful.

Johns Hopkins University researchers reviewed 32 major weight-loss plans, like Jenny Craig, Weight Watchers, and Atkins. Just 11 plans (across 39 studies) had the kind of rigorous randomized controlled trials that are needed to find differences between a given treatment and no treatment. They focused on long-term studies lasting from 12 weeks to 12 months.

What to keep in mind when looking for a weight-loss plan.

Also included in the study were NutriSystem, HMR, Medifast, OPTIFAST, SlimFast, and the Internet-based Biggest Loser Club, eDiets and Lose It!

The two most valuable plans for weight loss were Weight Watchers and Jenny Craig. In contrast to other plans, their participants lost a significant amount of weight compared to people who weren’t on the plans. Atkins and related low-carb diets also showed promise at both six and 12 months.

Very low-calorie diets didn’t seem to do much at all over the long term, and carry higher risks of side effects, like gallstones, the authors warn.

The results are important for people to keep in mind as they choose a diet plan — many boast incredible results in short amounts of time, but they may not live up to their promise long-term, which is what really matters.

“Primary care doctors need to know what programs have rigorous trials showing that they work, but they haven’t had much evidence to rely on,” said study author Kimberly Gudzune in a news release. “Our review should give clinicians a better idea of what programs they might consider for their patients.”

If you’re in the market for a weight-loss program, consider those that have the most research support, rather than the ones that make the most extreme promises up front.

Diet and exercise are still the tried-and-true keys to weight loss, but if you do want to try a commercial plan, pick one that works with your lifestyle and weight-loss goals. And always remember that losing weight quickly isn’t always the best approach — maintaining the weight loss healthily and over the long term is the better strategy.

The research is published in Annals of Internal Medicine.