The Strange Reason Diet Soda Makes You Fat

Want one reason for your beer belly? How about 100 quintillion? That’s about how many bacteria live in your gut. And scientists now believe these bacteria can have a significant impact on your weight.

Consuming high amounts of fructose (a type of sugar), artificial sweeteners, and sugar alcohols (another type of low-calorie sweetener) cause your gut bacteria to adapt in a way that interferes with your satiety signals and metabolism, according to a new paper in Obesity Reviews.

“An evolution of the gut flora to this new sweetener-rich environment has a potential to negatively impact our health,” says Amanda Payne, Ph.D., lead author of the review.

How does that happen? As bacteria in the gut process food, they give off byproducts called short-chain fatty acids. These can be beneficial and serve as energy in the body. But as the sweetener-adapted bacteria thrive and become more efficient at processing large amounts of high-fructose corn syrup, artificial sweeteners, and sugar alcohols, they also produce more and more short-chain fatty acids.

In those high amounts, Payne says, short-chain fatty acids decrease satiety signals. “This signaling may cause disruptions in our feeling full and hence prevent us from stopping to eat when we should,” Payne says.

As if overeating isn’t enough, the short-chain fatty acids also promote inflammation in the lining of the gut. Just how? Scientists aren’t yet sure. But they do know that inflammation damages gut tissue and results in leaky gut syndrome. Pleasant as it sounds, it means bacteria leak through that damaged gut tissue into the blood stream and cause further inflammation there. That’s a serious problem that can lead to insulin resistance and an increased risk for coronary artery disease, stroke, and type 2 diabetes.

This could partly explain the link researchers have found between drinking diet soda and being overweight. In one study, people who drank two or more diet sodas a day had five times the increase in waist circumference over a 10-year period compared to people who didn’t drink any diet soda. There are a few explanations for the findings. Maybe people drink more diet soda because they’re trying to lose weight. Calorie-free sweetness may also confuse the brain into craving more sugar.

As for your gut, at this point it’s not clear if one diet soda a day is less damaging to the gut flora than ten. “I will say from a personal perspective that I don't drink sodas—diet or regular—and I rarely eat processed foods, especially if they have high-fructose corn syrup listed on their label,” Payne says. Your best bet is to consume products containing these sweeteners in moderation, and drink mostly water. For gut flora to thrive, eat a wide variety of fruits, vegetables, and whole grains.

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