Oprah, GOOP, and Bey all swear by 'em–but we have the final word on whether they actually work

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Celebrities like Oprah, Gwyneth Paltrow and Queen Bey herself have been praising the liquid-diet gods for years. So when we learned that Lily James, Miss Goody Glass-Shoes herself in the new live-action movie adaptation of Cinderella, went on a liquid diet to squeeze into her corset, we weren't shocked-a pretty starlet on a juice-y, soup-y, cayenne-pepper-and-weirdness fast isn't exactly breaking news. (Psst: Beyoncé even has her own cleanse program now.)

And we've definitely seen enough fad diets roll through to know that detoxes aren't always nice to your bod. But how bad is an all-liquid diet, really? Pretty awful, it turns out. We've got the skinny on why they're so bad for you.

They Deprive You of Nutrients (Which Generally Makes Life Awful)

An all-juice detox might knock out your daily helping of produce, but they're typically missing the fibers and nutrients you could only get from real food-leading to pretty terrifying consequences.

"Liquid diets are usually not going to provide you everything your body requires," says Jaime Mass, R.D., president of Jaime Mass Nutritionals. This could lead to a whole menu of awful: vitamin deficiencies, muscle breakdowns, low energy, thinning hair, lack of focus, dizziness, nausea, headaches and-surprise-a sucky mood.

And if that doesn't terrify you into a Hard No, know that not all juices-not even the seemingly healthy ones-are your BFF: some juices have the carbs of a 20 oz soda, and some smoothies equate to eating two candy bars.

"Now imagine drinking that four to six times a day," says Mass. "When you consume sugar in large quantities, the body will bring fluid into the gut to balance it out, which can lead to stomach upset, bloating, pain, and diarrhea."

They Mess with Your Natural Body Systems

Liquid diets often tout their "cleansing" properties, but the truth is that your body already has systems in place to do all the cleansing it needs.

"Your liver and other organs naturally remove so-called waste from your body," says David Grotto, R.D, David Grotto, R.D., founder of a nutrition consulting company in Illinois. "Eating whole grains, produce, healthy fats, low fat dairy, and lean protein keeps these organs and your body's elimination process in top condition."

But liquid diets-and almost all other detox plans-put your body into starvation mode, forcing your body to feed off of itself, hanging onto calories and burning into lean muscle for energy, says Mass. The loss of lean muscle can mess with your ability to burn calories, and interfere with your heart and muscle performance. Translation: if you're a gym-goer or an athlete, you could end up seriously injured. (Try the Best Pre- and Post-Workout Snacks for Every Workout Instead.)

They Can Worsen Pre-Existing Conditions

For most healthy people, a liquid detox won't lead to long-term health problems-but according to Dawn Jackson Blatner, an American Dietetic Association spokesperson, a detox could be downright dangerous for those with existing issues.

"For someone who has conditions such as diabetes, heart disease, kidney disease, digestive issues, or women who are pregnant, children or teens, and elderly, these extreme changes to their diet can mean anything from dizziness to fainting to coma since the diets affect electrolyte and blood sugar balance," says Blatner.

…And-Big Finish-You'll Gain it All Back, Anyway

Fact: Oprah's weight ballooned after returning to normal food. (Also fact: if it happened to Oprah, it could happen to you.) All of our experts agree: while a liquid diet could help you lose weight quickly, you're only losing muscle weight (which is dangerous) and water weight (which easily regained).

Over the course of a liquid diet, your metabolism will have slowed down-making you even more likely to regain the weight, and fast, says Grotto. This could negatively affect your body and your self-esteem. (And Low Confidence Affects Your Workout Performance too.)

"Liquid diets for weight loss usually leave the dieter feeling like a failure, when they were in fact not set up for success," says Mass.

Conclusion: Okay for Now; Bad for Forever

In the end, fake-real-Cinderella had it kinda right-liquid diets might work for quick fixes (like, oh, filming a major motion picture?), but as a long-term health plan, it just doesn't stick.

"These diets don't teach us anything about healthy eating, portion control, meal timing, food shopping, how to eat healthfully at restaurants, or what healthy weight control is," Mass says. "They foster disordered eating behaviors and lead us to believe that fast weight loss is good-and that could not be further from the truth." (By the way, these are The 8 Worst Weight Loss Diets in History.)

A liquid fast for a few days may jumpstart your diet, but you should definitely loop your doctor in. In fact, do yourself one better: instead of placing restrictions on your diet, experts like Blatner say to keep it well-rounded and think farther out.

"The most effective 'detox' is drinking plenty of water, getting enough sleep, and eating a balanced diet of fruits, vegetables, whole grains, lean proteins, and healthy fats so that our own detox organs can keep us healthy," says Blatner. "What you do for a few days can't ever make up for how we live and take care of our body the other 365 days!"