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Hank: Maybe this has happened to you. You invite a friend out to lunch because frankly it's been too long since you had a decent burrito, but they politely decline. They say they're on a cleanse. Turns out they are consuming nothing but juice for the next three days because they want to give their digestive system a kind of hard reset.

There are even juice cleanse or juice fast programs out there, the basic idea is you pay a certain amount per day -- like sixty bucks -- and you’re provided with a set of juices to drink in a particular order. For the next few days -- usually 3, but sometimes up to 10 -- you eat no solid food. It’s all juice, all the time, though the programs also encourage you to drink water. If you’re lucky, you’re getting about 1200 calories a day -- and with some programs, much less than that. Some people also choose to supplement their fasts with laxatives and colonics, basically enemas.

Different programs promise different benefits, but most claim the same basic thing: that a cleanse will clear out your toxins and “reset” your digestive system. They’re also often considered a quick way to lose weight. Except, none of these things are actually based on science.

So, let’s explore how juice cleanses supposedly work, and compare that with we know about the human body. It’s rare to find a juice fast program that explains what they mean by toxins, or specifies what any of them are. But the generally accepted definition of toxin is that it’s a kind of poison produced biologically. And your body does produce harmful byproducts, like the ammonia that ends up in your urine. But that’s the thing -- it ends up in your urine, because it’s filtered out, first by your liver and eventually your kidneys. Your body has built-in mechanisms to get rid of the bad stuff.

Drinking juice for a few days isn’t going to make your liver or kidneys suddenly go into overdrive and clear out anything left over. Those organs are supposed to be constantly working -- if they aren’t, that’s what we call liver or kidney failure. These programs also tend to present another image: that there’s all this extra gunk stuck to the sides of your intestines, and the best way to get rid of it is to give yourself diarrhea for a few days, and then maybe finish that off by flushing everything with water.

But the human plumbing system doesn’t work like the one you find in an old house. Instead, waste is constantly moving through a healthy digestive system. There’s nothing that’s still hanging out in your intestines from that time you ate a handful of watermelon seeds when you were five, or even that double cheeseburger you had last month. Food normally takes 24-72 hours to be completely digested, depending on what it’s made of and how often you have bowel movements. Your body absorbs whatever it can, and the rest comes out. It doesn’t get stuck to the walls along the way.

But what about the weight loss? Some of these programs do claim to help you lose weight. Others emphasize that a cleanse should be for your health, not for weight loss, but also say that cleansing will do things like “put you on a path to healthier habits.” And people who do juice fasts do almost always lose weight, but most of it is in the form of water weight. But despite what it sounds like, this water weight isn’t just the weight you lose from peeing out a bunch of water -- it actually comes from glycogen in your muscles

Glycogen is a form of stored sugar that your body uses when it’s running low on energy -- like when it’s getting way fewer calories than usual, all in the form of juice. As soon as you start eating normally again, your body will replace those glycogen stores and you’ll bounce right back to wherever you were.

On the other hand, for most healthy people, a juice fast probably won’t hurt them. They almost certainly won’t be getting all the calories and nutrients they need, but most of these programs aren’t long enough to do lasting damage. So, if your friend wants to drink nothing but juice for a few days, odds are they’ll be fine. But they aren’t detoxifying their body or rebooting their metabolism. They’re just... drinking a lot of usually very expensive juice.

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