The promise: You'll drop pounds fast, kick-starting a long-term healthy-eating plan; you'll reset your metabolism; you'll clear out the "toxins," fats and other nasty things that are glomming you up.
The problem: When you eat only one kind of food, you fail to eat all the other kinds, so you miss out on protein, fats, fiber and other essential vitamins and nutrients.
The risks: After a few days, your calorie-starved body may break down muscle mass for energy. This can throw off your metabolism and make future calories (i.e., the ones you'll consume after the fast) harder to burn off. Consuming massive quantities of the foods that make up these diets—vegetables, fruit—can lead to uncomfortable bloating as well as severe diarrhea. Further, what are leaving your body aren't "toxins" (your body naturally flushes out excess nutrients, as well as chemical substances) but the effluvia of an out-of-whack gastrointestinal system that's both overloaded with water, simple carbs and fructose, and deprived of solid matter.
What's more, says Lisa Sasson, a registered dietician and a clinical associate professor of nutrition and food studies at New York University, for those who already have an unstable relationship with food, this kind of fasting usually leads to obsessive thoughts. ("At the end of this juice rainbow is a nice, rare steak. And curly fries.") Let's be clear: No nutritionist is going to rip a beet-apple tonic, cup of veggie soup or kale salad out of your hands. But most would also agree that a week's worth of these foods (in the absence of any others) is an impractical way to lose weight. (By the way, if you really want to "cleanse," Sasson suggests eating more whole grains and high-fiber foods, which act like a natural scrub brush to clear out the intestines.)