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."
What is moderation? In essence, it means eating only as much food as your body needs. You should feel satisfied at the end of a meal, but not stuffed. For many of us, moderation means eating less than we do now. But it doesn’t mean eliminating the foods you love. Eating bacon for breakfast once a week, for example, could be considered moderation if you follow it with a healthy lunch and dinner—but not if you follow it with a box of donuts and a sausage pizza.
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.