After the First Sip
Your brain's hunger signals are answered with a dump of pure fruit-juice sugar. And don't get any ideas—veggie-based cleanses aren't any healthier.
The sweet stuff prompts the pancreas to squirt out insulin, which moves sugar—now in your blood in the form of glucose—into your cells. (Search: How sugar hurts your health)
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After 30 Minutes
As your cells suck up the glucose, your blood sugar level can start to plummet and you may feel dizzy.
Meanwhile, lacking enough calories, your body is operating off its supply of glycogen, a form of short-term energy stored in the liver and muscles.
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After Two Days
With each shot of juice, your insulin levels skyrocket, then crash. Your glycogen stores are pretty much gone, leaving your tank on empty—and you feeling weak and listless. (Related: 5 Super Sugary Drinks to Avoid)
Since you're getting only about half the calories you need, your body draws on two long-term power sources: triglycerides, a type of energy stored in fat cells (woo-hoo!), and protein, taken straight from your muscles (oops). You begin to lose muscle mass, even if you're still exercising every day.
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After Three Days
Your brain is not happy. It enters into semi-starvation mode and gobbles ketones, fuel that comes from the breakdown of fat. Ketones work, but they're like low-grade gasoline; as a result, you may feel unfocused or irritable. (Any "mental clarity" is likely due to a strong placebo effect.)
Sans a fresh protein infusion, your brain is also lacking amino acids, the raw materials that neurotransmitters need to maintain your mood. If you're prone to depression, you may start feeling blue.
The proteins in your shrinking muscles break down into ammonia and uric acid, unwelcome chemicals that invade your bloodstream. Now your kidneys are busy detoxing your detox.
Stay near the bathroom: The juice's high carbohydrate load causes a surfeit of water to enter the intestines. That extra H2O in your gut means you're apt to get diarrhea.
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After Four Days
With no food to digest, your small intestine feels ignored. Its villi—the rows of tiny fibers that move food elements into the blood—start to atrophy. Your diarrhea may get worse, leading to dehydration... and there goes your rosy glow.
On the Eighth Day
Solid food! But uh-oh—you've lost muscle. Even if you go back to your regular eating habits, you now have less muscle mass to burn those calories; instead, the calories are more likely to be turned into fat. (Hence, one reason yo-yo dieting makes it harder to lose weight: Your reduced muscle-to-fat ratio messes up your metabolism and makes calories much harder to work off.)
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