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Diet Sodas Cause Weight Gain? Not so Fast

What the research says about whether diet drinks are culprits, bystanders, or sweet tooth enablers.
By
WebMD Feature
Reviewed by Laura J. Martin, MD

Type “diet soda” and “weight” into your favorite search engine and you may be surprised by what you find.

“Drink More Diet Soda, Gain More Weight?” asks one headline. “Diet Soda: Doorway to Weight Gain” shouts another.

In a recent search of a popular Web browser, 49 of the first 50 hits were for stories warning diet soda drinkers that the beverages might make them pack on the pounds.

The sole exception was the Wikipedia entry for "diet soda," which also cited the weight gain concerns.

If you believe what you read on the Internet, it’s clear that drinking diet sodas causes weight gain, right?

Maybe, but probably not, obesity researcher Barry Popkin, PhD, tells WebMD. What is clear is that the science is far from conclusive.

Diet Soda, Weight Gain Evidence Scant

Turns out all the news stories and blog postings cite the same few studies: research in rats conducted by two investigators at Purdue University and two studies that followed soda drinkers over time.

Popkin, who heads the division of nutrition epidemiology at the University of North Carolina, Chapel Hill, says none of the studies makes a convincing case that no-calorie sodas contribute to weight gain.

No friend of the soft drink industry, Popkin’s own research links sugar-sweetened carbonated drinks to obesity and he has led a global effort to get the vending machines that sell them out of schools.

“The bloggers of the world have latched on to the notion that diet sodas cause obesity, but the science just isn’t there to back it up,” Popkin says.

In an analysis published last year, Popkin and co-author Richard D. Mattes, PhD, MPH, RD, who is a nutrition professor at Purdue University but was not involved in the rat studies, reviewed the research examining the impact of artificial sweeteners on weight.

They found little support for the notion that no-calorie sweeteners stimulate appetite or contribute to obesity in some other way, but they say more research is needed to know for sure.

The Research Part 1: The Rat Studies

When Purdue researchers Susan Swithers, PhD, and Terry Davidson, PhD, published their first studies in rats designed to test their theory that artificial sweeteners alter the body’s ability to regulate calorie intake, they were not prepared for the press attention their research received.

“Frankly, we were stunned,” Swithers tells WebMD. “It really was a small study.”

In the first study, two groups of rats were fed sweet, flavored, cola-like liquids. For one group, the liquid was always sweetened with sugar so there was a consistent relationship between the sweet taste and calories. In the second group, the sugar-sweetened liquids were alternated with liquids sweetened with the artificial sweetener saccharin, so that the relationship between the sweet taste and calories was inconsistent.

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