A sprinkle-covered donut and a mug of coffee

A new study from Yale University has found that consuming sucralose (used in diet drinks and foods like low-calorie ice cream and yogurt) together with carbs may increase your risk of weight gain and type 2 diabetes.

“There have been lots of studies about artificial sweeteners, but little or no attention to whether they're given in isolation or eaten with other foods,” says the study's lead author, Dana Small, Ph.D., a professor of psychiatry, director of the Modern Diet and Physiology Research Center, and divisional director of nutritional psychiatry at Yale University School of Medicine. The study suggests that consuming sugar substitutes with carbs may be particularly problematic.  

What the Study Found

When you eat carbohydrates, blood sugar (glucose) and insulin levels rise. The insulin shuttles the glucose into the body’s cells, where it’s used for energy, and glucose levels fall back into the normal range. But in some people, the cells don’t respond—they become insulin-resistant—and the body keeps making insulin to bring the glucose level down. Over time, this reaction can lead to type 2 diabetes and weight gain. 

More on Sugar Substitutes

In the study, published in the journal Cell Metabolism, the researchers looked at the effect on the body and brain of 45 healthy participants who drank beverages that contained sucralose alone, sugar alone, or a combination of sucralose and maltodextrin (a type of carbohydrate that has calories but is flavorless). 

Neither the sucralose- or sugar-only drinks produced an abnormal glucose/insulin response, but the sucralose/maltodextrin drink did. And in the brain, the combination caused changes that over time could alter how well the body handles sugar. 

To rule out the possibility that maltodextrin itself was responsible, the researchers added a drink that contained only maltodextrin. The subjects' responses to that were also normal. 

A spokesperson for the the Calorie Control Council, which represents the low-calorie food and beverage industry, said the organization was "still reviewing the small study’s findings and methodology but stands by the overall safety and benefits of sucralose and other commonly used low-calorie sweeteners."

Why are the changes triggered by the blend of sucralose and carbs so worrisome? “There’s no simple answer to that,” Small says. But when your body isn’t responding properly to insulin, you’re liable to overproduce it, which can overtax your pancreas. You’re likely to have high blood sugar levels, too. And the extra glucose in your body has to go somewhere, she points out. “It may be stored in the liver, which can lead to fatty liver. Or it can potentially harm other organs, and lead to prediabetes, cardiovascular disease, and even cognitive problems.” 

Drinking a diet beverage with a carb-containing meal or eating foods that contain both carbohydrates and sucralose occasionally may not be a problem. But Small notes that the total amount of sucralose consumed by the participants during the two-week study was the equivalent of having 14 packets of Splenda—well within the range of what the average person might actually consume. “Presumably, with regular consumption in a drink, diet yogurt, diet cola with fries, you could easily accumulate enough to be a factor in diabetes,” Small says.

More research on sucralose, as well as on aspartame and other sugar substitutes, is needed to confirm these results. 

The Takeaways

So what’s a consumer with a sweet tooth to do? “I don’t want the message to be that you should drink sugar-sweetened beverages, because the evidence of their negative effects is incontrovertible,”  Small says. And she recognizes that for some people—those with diabetes or prediabetes or those watching their weight—diet drinks may be helpful. While water is always preferable, an occasional diet soda can still be an option as long as you don’t drink it with other foods, she advises. What's not known is how much time there should be between eating carbs and having a diet beverage.

You should also check ingredients lists on packaged food. “There are many products on the market that contain both sucralose and sugars or carbohydrates—even in some that aren't labeled 'diet' or 'reduced sugar',” says Amy Keating, R.D., a CR nutritionist. 

But until there’s more evidence of their benefits and drawbacks, the wisest course may be to avoid sugar substitutes or at least minimize their consumption. “I’m convinced that when it comes to artificial sweeteners, we don’t know enough about them, so I avoid them,” Small says.