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Zero-Calorie Sweeteners Are Making You Fat

In some cases, research shows they may be even worse for you than real sugar.

Markham Heid

Richard Levine/Getty Images

The conflicting headlines around low-calorie sweeteners could give you whiplash. While one study links them to belly fat, another finds "no evidence" tying them to higher body weights. The more you read about aspartame, stevia, sucralose, and all the other sugar substitutes that food manufacturers are pumping into the stuff we eat, the more muddled the health picture becomes.

But don't be fooled: Artificial sweeteners are not helping you lose weight or stay fit. In some cases, they may be even worse for you than real sugar. 

"Right now, the simple answer is this is science versus the food industry," says Robert Lustig, a neuroendocrinologist at the University of California. When asked about all the conflicting research, Lustig says those conflicts evaporate when you remove studies funded by makers of artificial sweeteners.

You don't have to take his word for it. One analysis from the Center for Behavioral Medicine at Northeastern Ohio University found that 100 percent of industry-funded aspartame studies concluded that the sugar substitute was safe, while 92 percent of independent studies came to just the opposite conclusion. Another meta-study, this one from Johns Hopkins, determined that industry funding and study authors' "financial conflicts" introduced bias into their flattering or benign findings on artificial sweeteners.  

"I'm not blaming artificial sweeteners for America's obesity epidemic, but I think they're a part of it," says Kristina Rother, chief of the National Institute of Health's section on pediatric diabetes and metabolism. 

Rother has published more than a dozen studies on artificial sweeteners. She says when she first started to raise concerns about their role in weight gain, some of her friends were incredulous. "A lot of people were like, 'Kristina, are you saying something with no calories can cause weight gain?'" She laughs. "It was like I was challenging the law of thermodynamics."

While it took a while for Rother and other researchers to get a grip on the specific health impacts of artificial sweeteners, the real-world evidence is hard to miss. 

Worldwide, roughly 600 million adults and 42 million children are obese, per the World Health Organization. Those numbers are astronomical compared to the rates of global obesity 40 years ago. During those same four decades, the use of artificial sweeteners has jumped dramatically. 

"At the very least—the very least—it's clear that artificial sweeteners are not a solution to combat obesity," Rother says. "We now have plenty of evidence to show that people who ingest artificial sweeteners on a frequent basis are just as obese and just as likely to develop diabetes and have all these other negative health effects as people who ingest sugar." 

The idea that "diet" or "light" or "low-cal" soft drinks and snack foods are your ticket to a slim, healthy body are out the window—calories be damned. 

But how is it that something with no calories can make you fat and sick? 

"There are lots of potential explanations, but one involves cognitive distortion," says Susan Swithers, a metabolism researcher at Purdue University. "People think artificial sweeteners are healthier than sugar, or that drinking diet soda is saving them calories, and that belief gives them license to overindulge." 

Another more insidious explanation: Artificial sweeteners seem to "break the link" between what normally happens in your mouth, your brain, and your digestive system when you eat something sweet, Swithers says. First, your taste buds and your brain tell your digestive system that it needs to prepare to break down and metabolize sugar. Your pancreas starts producing insulin, a hormone that allows your body to use or store sugar in the form of glucose. But if that sweet taste your mouth registered comes from artificial sweeteners, your digestive system never gets that expected hit of sugar. Swithers says this may increase your desire to go eat something that actually contains sugar—almost as though the artificial sweetener created an itch your digestive system demands to scratch.

"This could also create issues when you eat real sugar, because your body's ability to predict what happens when you taste something sweet has been pushed out of line," she adds. 

Some of the latest research also suggests artificial sweeteners are messing with your microbiome—the complex ecosystem of microorganisms that live in your gut and support your digestion. 

"Some [artificial sweeteners] may impact the composition and function of the gut microbiome in a way that drives an increased risk of developing impaired glucose tolerance, a condition predisposing a person to adult-onset diabetes," says Eran Elinav, author of an influential study linking sugar substitutes to unhealthy gut changes. 

Speaking in part about Elinav's research, Rother says these artificial sweetener-induced microbiome changes may make your gut more efficient when it comes to absorbing certain nutrients. While that may sound like a good thing, over-absorption could ultimately be the link that ties no-cal sugar substitutes to weight gain, she says. 

"Nothing is definitive," UCSF's Lustig adds. "But if you look at trends in the data—on stuff building on other stuff—you see everything pointing toward artificial sweeteners being bad for us."

Whether "bad for us" means "worse than regular sugar," however, the jury's still out. 

"Personally, I think regular sugar is probably worse," Lustig says. "If sugar is a ten and water is a zero, I put artificial sweeteners at about a six." But he's quick to add this may depend on your current health status, or how much sweet stuff you're packing into your diet.

"Sure, if you're drinking a liter a day, I think sugar will be worse than artificial sweetener," Swithers says. But what if you rarely eat sweets or drink soda, and your health is good? "I'd go for the real stuff," she says. "At least then your body's natural responses are appropriate."

Rother agrees. "If you only have [sugar or artificial sweeteners] once in a while, I think it's better to go for the real thing," she says.

But don't kid yourself. As Rother puts it, if you're deciding between diet soda and regular soda, "the only correct answer is, drink water."