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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.

The Research Part 1: The Rat Studies continued...

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.

In another study, rats were fed high-calorie chocolate pudding or chocolate milk with their regular food. At the end of the month, the chocolate milk group had gained significantly more weight.

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.”

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