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.
After 10 days, the rats were given a sweet, high-calorie chocolate pudding. Those exposed to the caloric and non-caloric sweet beverages ate more of the pudding.
The first experiment suggested that by breaking the connection between sweet taste and calories, artificial sweeteners interfere with the body’s natural ability to judge calorie content, Swithers says. The second, that the body is less able to recognize energy delivered in liquid form.
In a later set of studies the researchers fed rats yogurt sweetened with sugar or saccharine in addition to their regular chow and found that the rats that ate the no-calorie sweetener took in more calories overall and gained more weight.
The Research Part 2: The Observational Studies
Another study often cited in the news stories and blog postings followed people in San Antonio, Texas and showed that those who drank more diet sodas gained more weight over time.
Researchers analyzed data from the San Antonio Heart Study, which followed more than 5,000 adults for between seven and eight years.
Although people who drank both sugar-sweetened and diet sodas gained weight, diet soda drinkers were more likely to become obese. And the more diet sodas the participants drank the greater their weight gain.
The Framingham analysis included 9,000 middle-aged men and women followed for four years. Researchers found that compared to people who didn’t drink sodas at all, those who drank both sugar-sweetened and diet soda were more likely to develop metabolic syndrome -- a cluster of symptoms often linked to obesity that increase risk for heart disease and diabetes.
Because both of these studies were observational, it is impossible to say if the diet sodas played a direct role in the weight gain.
It may be that people switch to diet soda when they begin gaining weight without addressing other aspects of their diet that are causing the weight gain.
'Big Mac and Diet Coke' Mentality
It may also be that people with very poor diets disproportionately drink diet sodas.
Popkin calls this the “Big Mac and Diet Coke” mentality.
“Especially in America, we have a lot of people who eat high-fat, high-sugar diets, but also drink diet sodas,” he says.
Sharon Fowler, MPH, who led the San Antonio study, acknowledges this, but she also thinks something else is going on.
"I am not convinced these sweeteners are as safe as they should be, given their widespread use,” says Fowler, a faculty associate in the division of clinical epidemiology at the University of Texas Health Science Center in San Antonio. “I am concerned that we are in the middle of a giant experiment and we don’t know the outcome.”
Diet Soda Drinkers Who Diet Lose Weight
Popkin cites research, including his own, showing that people who drink artificially sweetened sodas as part of a calorie-restricted diet lose weight. So does Maureen Storey, PhD, who is senior vice president of science policy for the American Beverage Association.
“The current body of available science shows that low-calorie sweeteners -- such as those used in diet soft drinks -- can help reduce calories and aid in maintaining a healthy weight,” Storey tells WebMD.
She points out that the American Diabetes Association and the American Dietetic Association support the use of no-calorie sweeteners to restrict calories and sugar intake.
“Drinking diet beverages alone, however, is not enough to counter overeating -- the only way to maintain a healthy weight is by balancing calories consumed with calories burned.”
Nutrition researcher David L. Katz, MD, who directs the Yale Prevention Research Center, says the research as a whole suggests sugar substitutes and other non-nutritive food substitutes have little impact on weight one way or the other.
“For every study that shows there could be a benefit or harm, there’s another that shows no 'there' there,” Katz tells WebMD.
The Sweet Tooth Hypothesis
Katz agrees that the research linking diet sodas to weight gain is scant and inconclusive. But he's still concerned that artificial sweeteners condition people to want to eat more sweet foods.
“We refer to a ‘sweet tooth,’ not a ‘sugar tooth,’ “ Katz says. “I think that is absolutely right. Our taste buds don’t really differentiate between sweet in sugar and sweet from, say, aspartame. The evidence that this sweet taste is addictive is pretty clear.”
His theoretical concerns are bolstered by 20 years of real-world experience with his patients.
“What I have seen in my patients is that those who drink diet soda are more vulnerable to stealth sugars,” Katz says.
Katz says stealth sugars are those added to processed foods that don’t taste sweet, such as crackers, breads, and pasta sauce. They usually come in the form of high-fructose corn syrup.
Although some commercial pasta sauces contain no added sugars, others contain more than ice cream toppings, Katz says.
“The question is, who prefers marinara sauce with all that high-fructose corn syrup?” Katz says. “The answer is, a person with a sweet tooth.”
In their research review, Popkin and Mattes concede that use of no-calorie sweeteners probably does promote a preference for sweeter-tasting foods. But they conclude that it's not clear whether that affects weight gain -- and they say calorie-free sweeteners could help people control their weight, if used instead of higher-calorie sweeteners.
“But whether they will be used in this way is uncertain,” Popkin and Mattes write.