It's true that you'll probably drop a few pounds while juice cleaning, but it's not sustainable weight loss. You're likely consuming fewer calories per day than usual, and given all the fluids you're knocking back, you'll be urinating often.
"When the body hangs on to extra toxins, it stores them in fats and other tissues and also retains more water," says Dr. Elson Haas, author of The Detox Diet. "So when detoxing, you'll pee more than usual." Once you go back to Whole Foods, you'll probably gain most of the weight back. But remember, you're cleansing to give your body an edge on eliminating toxins, not to instantly help your pants fit better.