It's not going to 'detox' your body.
In a lot of ways, the fervor some people have for juice-based diets and “cleanses” is comparable to support for Donald Trump. No matter how much the media ridicules the object of their affection, these people will go on believing what they want to believe.
And while most of the media ridicule is warranted, there can be benefits associated with drinking your produce. You just have to squint hard and ignore the hype.
But first, the bad: If the juices or smoothies you’re drinking contain fruit, they’re probably doing more harm than good. A massive Harvard School of Public Health study, published in 2013, laid things out clearly when it showed that people who ate whole fruits enjoyed a reduced risk for type-2 diabetes, while people who drank fruit juice had an elevated risk for the disease.
“The problem with fruit juices or smoothies comes down to fiber,” says Robert Lustig, a professor of pediatric endocrinology at the University of California, San Francisco.
Lustig has published numerous studies (and one bestselling book) about sugar and how the human body responds to it. He explains that whole fruit contains two types of fiber—soluble and insoluble—which together, when chewed and swallowed, form a kind of gel that coats the upper part of your digestive system. Like a hair-catcher placed over a drain, this gel prevents the sugars in fruit from slipping into your blood stream en masse and overloading your liver. Instead, the sugars hang together until they reach a portion of your digestive tract called the jejunum. “What’s in the jejunum?” Lustig asks. “Bacteria, also known as your microbiome.” He says these health-promoting bacteria feast on those fruit sugars, and so prevent 25 percent or more of them from being absorbed into your liver and bloodstream.
When you juice or blend your fruits, none of this happens. “When you strain fruit into juice, you’re removing all the insoluble fiber,” he says. If you blend it instead, “the insoluble fiber gets sheared to smithereens.” In either case, the effect is similar: The digestion-helping gel doesn’t form, and your liver is bombed with fruit sugar, he says.
Also bad: blending or juicing fruits makes it easy to overconsume them. “If you eat an orange or two, you get full,” says Barry Popkin, a professor of nutrition at the University of North Carolina. “But if you drink a glass of orange juice, you could be drinking the amount of sugar in four to six oranges—or more than a Coke—and you don’t feel full afterward.” Blending fruits into smoothies carries the same risks, he says. And there’s a lot of research to back him up on the idea that drinking a food is much more likely to result in overconsumption than eating the same food.
Yes, all this holds even for cult-y health fruits like acai and camu. In fact, those may be even worse for you. “Acai and a lot of these exotic fruits have much more sugar than an orange or common fruit,” Popkin says. Whatever antioxidants or nutrients they may have, the sugar you’re swallowing far outweighs their benefits. Both he and Lustig also say that adding healthy fats—stuff like yogurt or avocado—or other ingredients won’t affect the issues of too-quick sugar absorption.
If all of this makes you scratch your head—because you know you’ve seen studies showing that juices and smoothies can be beneficial—it’s true that those studies exist. One, published just this year, found that a three-day, 1,300-calorie, juice-only diet protocol helped people lose nearly four pounds, and also produced beneficial changes in their gut bacteria and nitric oxide levels. (Nitric oxide improves arterial elasticity and blood flow.)
“These were drastic changes after just three days,” says Susanne Henning, an adjunct professor in the Department of Human Nutrition at UCLA and author of that study. But Henning says her study did not compare the juice-based diet to other calorie-restriction plans, which may have produced similar results. She and her colleagues also did not measure blood sugar or insulin changes among the study participants, which may have shown the harms Lustig and Popkin outline above.
Finally, and most importantly, this was a three-day intervention. Spend any time reading the weight-loss literature, and you find nearly every diet works minor miracles in the short term, especially if it involves cutting calories. (You could probably eat nothing but Big Macs for three days, and if you kept your intake to 1,300 calories, you’d lose some weight.)
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One final black mark on the anti-juicing side of the ledger: There is no evidence that the human body is a cesspool of “toxins” that juices and smoothies can “cleanse” away. “Those claims are bull—they’re really misleading,” Popkin says. The human body does contain unhealthy chemicals and metabolites, but your liver and other self-defense systems do a great job of removing them. The belief that a juice cleanse will detoxify you is “silly celebrity stuff,” Popkin says.
That’s not to say fruits and vegetables aren’t sources of healthy nutrients. Many of them are loaded with antioxidants, which are molecules that prevent cell damage caused by harmful free radicals. But something most people don’t know: Antioxidants can turn into oxidants, or damage-causing molecules, if you get too much of them. “The idea says John Richie, a professor of public health sciences at Penn State University.
He mentions two infamous clinical trials conducted in the 1990s by researchers who—based on preliminary studies—believed the antioxidant beta carotene could help block the development of lung cancer in heavy smokers. “They found beta carotene supplementation not only didn’t prevent lung cancer in heavy smokers, it promoted it,” Richie says. “So just throwing more antioxidants at a problem isn’t necessarily going to solve anything.”
All of these are significant knocks against the whole juice/smoothie phenomenon. But there’s a big but: If your juices and smoothies include only vegetables—not fruits—and if drinking them isn’t adding a significant number of calories to your diet, then they can be an effective way to get more veggie-based nutrients into your system.
“Because vegetables have very little sugar to begin with, it doesn’t make much difference if you consume them whole or in a smoothie,” Lustig says. Popkin mostly agrees. “Apart from beets, which are very high in sugar, vegetable juices are healthy,” he says. He adds the disclaimer that juicing or blending vegetables is likely to strip out some of their vitamins—mostly due to “oxidation” caused by air exposure. “I wouldn’t say these are as healthy as eating a whole vegetable,” he adds. “But it’s not bad for you.”
To sum all this up, most of the “miracle” claims associated with juicing and cleanses are unsupported by science. And when it comes to juiced or blended fruit, there’s strong evidence of harm. But if you struggle to eat your vegetables, and you enjoy them juiced or blended? “Have at it,” Lustig says.
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