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."
Some still consider fasting -- in any form -- to be "out there." "When I review diets that are not based on science, the question I ask myself is: Would I feed them to my family? In this case, the answer is a clear no," says Susan Roberts, PhD, chief of the Energy Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging and a professor of nutrition at Tufts University in Boston.