Eating takes a lot of work. That “food coma” many lapse into after the big Thanksgiving meal is largely due not to the turkey, but to overeating. The caloric onslaught unleashes a cascade that sends a rush of blood to the gastrointestinal tract to help us digest, leaving less blood circulating elsewhere.

It’s estimated that up to 10 percent of our calories go toward the digestion and absorption of the foods we eat.

So what’s a busy person to do if the main goal is to stay alert and energetic and avoid all that sluggishness?

Juicing has become a popular answer. Juice bars — not the frozen ones in your fridge, but the establishments that sell all kinds of fruit and vegetable concoctions — have opened up like mad around the country. Celebrities have helped fuel the rage, raving about how juice cleanses have given them more energy, “detoxed” their systems and cured whatever was ailing them.

However, it’s juice cleanses, especially those lasting for more than a day or so, that worry me the most. People often go on these to rid their systems of “toxic waste” or to clean out their colons. They also do it to lose some weight to fit into an outfit they want to wear or to look better for an upcoming event. Sometimes bodybuilders want to do it for extra energy or to give their muscles the shrink-wrapped look that comes with losing some body fat.

The raw-food movement has also spurred an interest in juicing. It’s a way to process food without cooking it. You may not want to eat raw leafy greens, but putting them into a nuclear-powered blender that extracts every bit of liquid they possess is a lot easier and takes less time than cooking kale until its tender. The raw-food folks also feel that not heating food preserves more of the nutrients.

But there’s a catch to all this juicing. You can do it for only so long before the benefits become a detriment to your health. Let me explain.

As a means of detoxifying the system, a “juice cleanse” isn’t much different than doing a prep for a colonoscopy (though it usually tastes better!). Your colon needs to be “at rest” for a colonoscopy, so you are on liquids for a day or so and then have a laxative drink that ensures your colon will be as clean as a whistle when your doctor scopes you. When you do a juice cleanse, it gives your colon a similar kind of rest. After all, you’re not eating anything solid, and you’re drinking only liquids extracted from different combinations of fruits and vegetables, sometimes with herbs or spices such as ginger root added in.

This can make you feel better because you’re taking a break from the entire food thing. It takes energy to digest any food. Hence, it may be a little less about the juice than about the lack of work your colon has to do that gives you the vitality. People claim they also sleep better when doing a juice cleanse, and I don’t doubt it. Their bodies are going to sleep with fairly empty stomachs because juices don’t have a reason to hang out in the stomach to be broken down. That blender did the job pretty well, so shortly after the juice hits the stomach, out it goes.

For most healthy people, a day or so of a juice fast won’t be a big deal. It may also resensitize their taste buds, returning some acuity lost from eating foods too intensely flavored with salt, fat and sugar.

There’s something missing in this formula, though: protein. Your body needs protein every day, so the longer you stay on a juice cleanse, the more lean muscle you’ll lose. You’ll see the pounds drop, but it’s not the quality of weight loss you want. The goal should be losing fat weight, not muscle weight. When you don’t get any protein in your daily diet, you can’t help but lose some muscle, and when you lose muscle, it decreases your metabolic rate a little because it takes a lot of calories to maintain muscle. Men need more calories than women do because men tend to have more muscle. Adding some protein powder to the juice would help, but better still is just to eat a healthy, balanced diet with plenty of fiber from fruits and vegetables.

The other down side of juicing isn’t what you end up drinking, but what you don’t. That pulp the juicer leaves behind isn’t bad stuff at all. Indeed, it’s loaded with vitamins and minerals—and fiber. When I ask my patients what they do with the pulp left over after the juicing, they usually tell me they throw it out. At the very least, they could use the vegetable pulp in soups and the fruit pulp in smoothies.

Juice extractors have existed for decades. Some fancy juicers (usually the expensive ones that claim to do everything but tuck you in at night) take the whole fruit or vegetable and puree the entire thing into a slurry so there’s no “waste.” That’s fine; just remember—your digestive tract IS a juicer. It just works more slowly. That’s okay. We don’t need to be in a hurry here. I actually trust the digestive tract more than any mechanical juicer. By the time your GI tract gets through with the fruits and vegetables, you can trust that whatever’s left over truly needs to go. And since it’s largely fiber, it’ll even help you go.

Keith-Thomas Ayoob is director, nutrition clinic, Children’s Evaluation and Rehabilitation Center, Albert Einstein College of Medicine. He blogs at The Doctor’s Tablet.

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