"There’s nothing wrong with going on a juice fast for a few days,” said Dr. James Dillard, assistant clinical professor at Columbia University College of Physicians and Surgeons in New York City, on WebMD. “But it’s not a great way to lose weight, because you’ll gain it all back — you yo-yo. It’s just like the Atkins diet. The weight you lose is water weight.” And Dr. Braunstein (of Cedars-Sinai) says this type of deprivation can also result in dizziness, nausea, constipation, fatigue and irritability.

Additionally, if you do this to your body enough, you could permanently lower your metabolism — as if it’s not tough enough to lose weight as it is. New York Times writer Judith Newman tried a juice cleanse and wrote about her experience: “This kind of cleansing puts a lot of stress on your body,” she wrote. “Your body wants and expects food. And as with most crash diets, which is really what this is, your body thinks it’s starving. It doesn’t know it’s going to get more food. So it lowers your metabolism, and if you do this enough, it can lower your metabolism permanently.”