Juice diets do prevent your body from going into a state called ketosis, he says. Ketosis means your body has no carbohydrates to burn for energy, so it has to burn stored fat or whatever else is available, he tells WebMD. "You feel bad, even smell bad. That's what makes you feel like hell during a [water-only] fast. But is that because the toxins are coming out? No! You're going into ketosis. It's known physiology."
Fasting indeed has a long-standing spiritual tradition. "Almost every religion has some type of fasting ritual -- Lent, Ramadan, Yom Kippur ... the Hindus and Buddhists fast, too," says James Dillard, MD, assistant clinical professor at Columbia University College of Physicians and Surgeons in New York City. He's author of Alternative Medicinefor Dummies.