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Can Diet Sodas Actually Make You Gain Weight?

Evidence is mounting that diet sodas do the opposite of what they claim to do.

Woman holding glass of cola with Jeans too tight on belly fat and paunch, Diet concept, Weight loss concept.
Photo Credit: Niran Phonruang/Shutterstock

If you’re trying to lose weight, choosing diet soda over regular soda might seem like a good idea. You get a caffeine kick and a sweet, bubbly taste similar to the real deal, only without the sugar and calories. But while zero calories sounds a lot better than the 150 calories from a can of regular Coke, several studies have found that artificially sweetened sodas might actually lead to something you were trying to avoid: weight gain.

Although aspartame, sucralose, saccharin and other artificial sweeteners are FDA-approved, and the National Cancer Institute and other health agencies say they are generally safe in limited qualities, recent studies might make you think twice before drinking your next diet soda.

A study published in PLOS ONE in November 2016 tracked the body measurements and diets of 1454 participants (741 men, 713 women) in the U.S. over the course of 28 years, with a median followup of 10 years. The results showed that low-calorie sweetener users tend to have a higher average body mass index, a 2.6-centimeter larger waist circumference and a 53 percent higher incidence of abdominal obesity compared to participants who never reported using low-calorie sweetener.

Similarly, a 2008 study monitored the weights of 3,682 individuals for 7-8 years. After ruling out factors such as diet, exercise or diabetes status that might skew the data, the researchers determined that drinking artificially sweetened beverages (versus none) was associated with an almost doubled risk of being overweight or obese.

So why might diet soda cause weight gain? One reason is that even though artificial sweeteners are hundreds or even thousands of times sweeter than regular sugar, your brain is no fool.

"Some studies show that sugar and artificial sweeteners affect the brain in different ways," writes Harvard’s School of Public Health.

Case in point: In a 2008 UC San Diego study, researchers scanned the brains of volunteers who sipped water sweetened with sugar as well as water sweetened with sucralose, the zero-calorie artificial sweetener you’ll find in Splenda. The resulting MRI scans showed that the brain can distinguish between the calories from the non-caloric sweetener, although the conscious mind could not. As the authors of the study suggested, sucralose “may not fully satisfy a desire for natural caloric sweet ingestion.” Thus, artificial sweeteners might cause you to crave or consume more sugary foods, which tend to have more calories.

One of the most convincing arguments is the psychological factor. When I posed the question, “Do you think diet soda causes weight gain?" to my Diet Coke-loving friend, she admitted that the word “diet" might make diet soda drinkers feel like they can have a treat later.

"The same can be said for exercise. It’s not that exercise does not burn calories. People reward themselves for exercising by overeating,” she said, adding that “diet foods in general can be less satisfying, which may cause cravings for more food.”

A 2010 study published in the Yale Journal of Biology and Medicine concluded that “artificial sweeteners, precisely because they are sweet, encourage sugar craving and sugar dependence."

There are a number of other explanations for the diet soda/weight gain enigma, as Sharon Fowler, an adjunct assistant professor in the Department of Medicine at the University of Texas Health Science Center in San Antonio, explained to the Washington Post.

Besides thinking they have “extra permission [to eat]," soda drinkers might be altering our all-important gut bacteria, Flower noted. A 2014 study of mice found that sweeteners and/or the acid in diet soda may impact gut flora, which may lead to obesity and related ailments such as diabetes.

Of course, there is no definitive scientific proof that artificial sweeteners cause weight gain. I must note that aforementioned studies only show correlation, not causation. In fact, there are numerous diet soda studies that have conflicting results in which the beverage actually reduces the intake of calories and promotes weight loss or maintenance.

The American Beverage Association said in a statement to the Washington Post that "previous research, including human clinical trials, supports that diet beverages are an effective tool as part of an overall weight management plan. Numerous studies have repeatedly demonstrated the benefits of diet beverages—as well as low calorie sweeteners, which are in thousands of foods and beverages—in helping to reduce calorie intake."

While we should be wary of the source—the American Beverage Association is in the business of promoting soda-drinking—James Hamblin, a senior editor at The Atlantic, also acknowledged this point in an article where he argued that "artificial sweeteners probably don't cause weight gain, when used strategically."

"The conclusion lands in support of artificial sweeteners in the right context, specifically when they are substituted for sugar," Hamblin wrote, citing a 2014 American Journal of Clinical Nutrition meta-analysis. "People tend to see 'modest weight loss,' suggesting that low-calorie sweeteners (LCSs) indeed ‘may be a useful dietary tool to improve compliance with weight-loss or weight-maintenance plans.'"

Ultimately, the science is not settled. Nevertheless, sales of the most popular diet soda brands such as Diet Coke and Diet Pepsi have slumped year after year.

Consumers are increasingly wary of the purported health risks linked to the controversial sweetener aspartame as well as diet soda’s “unintended boomerang effect on appetite,” according to U.S. News. Consumers are switching to bottled water or healthier drinks such as tea and fruit or vegetable juices instead.

"There's plenty of scientific evidence suggesting that artificial sweeteners are linked to weight gain, not weight loss," said Gary Ruskin, co-director of U.S. Right to Know, a consumer group. "So how can Diet Coke and Diet Pepsi be advertised as ‘diet’ products?" 

In 2015, U.S. Right to Know asked the Federal Trade Commission (FTC) and Food and Drug Administration (FDA) to investigate Coca-Cola and PepsiCo for falsely advertising "Diet Coke" and "Diet Pepsi" as "diet" drinks. The FTC and FDA declined to act on the requests.

So what should a diet soda aficionado do in the face of all this bad press?

The best alternative is to take matters into your own hands and DIY. Get one of those home soda makers that are all the rage (there are several good models on the market) or keep a few bottles of sparkling water in the fridge. Then add a splash of grape juice, a few drops of vanilla, a sprig of mint, or use a more exotic recipe, and you'll have a glass of low-calorie bubbly goodness without any nasty chemicals, caramel coloring or artificial sweetener. 

Lorraine Chow is a freelance writer and reporter based in South Carolina.

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