Do they cause weight gain, cravings, or diabetes? Here’s what the latest evidence shows.
“Diet drinks are associated with weight gain, new research suggests,” read the headline in the Washington Post this July.
That wasn’t the first time people who drink diet sodas were told that they may be setting themselves up to fail.
“Some experts believe that artificial sweeteners trigger sweetness receptors in the brain, which cause the body to prepare itself for an influx of calories,” said an article on time.com in July 2016. “Even though those calories don’t arrive, the body still craves them, and that may cause people to eat more calories overall, putting them at a risk for weight gain.”
Diet soda can make you gain weight? Not so fast.
“Some observational studies show that people who drink diet beverages are at a greater risk of being obese or gaining weight,” says Vasanti Malik, a research scientist at the Harvard T. H. Chan School of Public Health.
But those kinds of studies—which ask people about their diet and then track their health for years or decades—can’t prove that diet sodas cause weight gain.
“People who are overweight or whose weight is creeping up may opt for diet drinks to control their weight,” says Malik.
In other words, people may consume diet drinks because they’re gaining weight.
What’s more, notes Malik, “In our work, diet drinks are not associated with weight gain.”1
That could be because her study, which followed 125,000 adults for about 20 years, took more factors into account—like diet, sleep, exercise, and time watching TV—than others.
“Most people gain weight over time,” explains Malik. “Our study showed that if you drink sugar-sweetened beverages, you gain more weight. But our results also suggest that if you switch from sugar-sweetened drinks to diet drinks, water, lowfat milk, coffee, or tea, you gain less weight over time.”
And that’s backed by strong evidence from a large double-blind, randomized clinical trial.
When Dutch researchers randomly assigned 641 children to drink a cup a day of a drink sweetened with either sugar or a low-calorie sweetener (sucralose plus acesulfame potassium) for 1½ years, the kids who got the sugar-sweetened drink gained more weight and body fat.2
So diet drinks cause less weight gain than sugary drinks. But are dieters worse off with diet drinks than water?
In one study, researchers enrolled 318 overweight or obese adults who reported getting at least 280 calories a day from beverages.3
“That could include soda, sports drinks, alcohol—anything but plain milk,” says co-author Deborah Tate, a professor of health behavior and nutrition at the Gillings School of Global Public Health at the University of North Carolina.
The diet-drink group was told to replace at least two servings a day of caloric beverages with diet drinks, the water group was told to replace at least two servings with water, and the control group got general advice about healthy eating.
After six months, “all three groups had lost [an average of] 2 percent of their weight—about four or five pounds,” says Tate. However, people in the diet-drink group were twice as likely to lose at least 5 percent of their weight as those in the control group. The water group did almost as well.
“The diet-drink group may have had an easier time replacing caloric beverages than the water group,” says Tate.
So, over the short term, diet drinks may help some people kick the sugary-drink habit more than water does.
But Tate is cautious. “Over time, people could start compensating by consuming more calories,” she says.
Malik also hesitates to endorse diet drinks over water. “For people who drink soda often, diet drinks are probably better,” she says. “But the goal isn’t to switch from regular soda to diet soda. It’s to switch to healthier drinks like water, seltzer, or unsweetened coffee or tea.”
Bottom Line: If you want to lose—or not gain—weight, switch from sugary drinks to diet drinks or water.
And what about the claim that low-calorie sweeteners cause people to crave calories?
Some studies find that the brain responds differently when people taste artificial sweeteners than when they taste sugar.4,5 But there’s little evidence that those differences actually change what people eat.
Strike one: If low-calorie sweeteners caused people to crave sugar, why would the Dutch trial—and smaller, shorter studies in adults—find that people gain less weight on diet drinks than on sugary beverages?
Strike two: Short-term studies don’t find much evidence for cravings. For example, 31 adults ate no more calories at lunch and dinner on days when their pre-meal snack was crackers and cream cheese sweetened with stevia or aspartame than on days when the cream cheese was sweetened with sugar.6
In fact, on the aspartame and stevia days, they ended up eating about 300 fewer calories than on the sugar day.
Strike three: In Tate’s study, people who drank diet beverages ate fewer desserts than people who drank water.7
“We don’t know why,” says Tate. “It could be that diet drinks provide sweetness, caffeine, carbonation, and overall enjoyment without any calories. So maybe they satisfy some of those sweet cravings. Either way, our study didn’t show that replacing caloric drinks with diet drinks causes people to seek out more sweet foods.”
Bottom Line: There’s no good evidence that low-calorie sweeteners increase cravings for sweet foods.
“Diet sodas may alter our gut microbes in a way that increases the risk of metabolic diseases such as type 2 diabetes—at least in some of us,” reported NPR in 2014.
The source: an Israeli study in which mice fed low-calorie sweeteners (saccharin, sucralose, or aspartame) for 11 weeks had worse glucose tolerance—that is, their blood sugar spiked higher after they drank a sugar-laden liquid—than mice fed ordinary sugars or water.8
“Those findings really caught people’s attention,” says George Kyriazis, an assistant professor at Sanford Burnham Prebys Medical Discovery Institute in Orlando, Florida.
What’s more, low-calorie sweeteners didn’t affect glucose tolerance if the mice were also given antibiotics, suggesting that the sweeteners impair glucose tolerance by disturbing the balance of gut bacteria.
However, all the sweeteners weren’t the same. Aspartame appeared to have little or no impact on glucose tolerance, while saccharin and sucralose affected only some “responders.”
“The results were mind-boggling because artificial sweeteners have different chemical structures and probably won’t be processed in the same way by gut bacteria,” says Kyriazis.
For the rest of the study, the researchers used only saccharin because it caused the highest spike in blood sugar in the mice. They gave seven people who didn’t drink much diet soda about 10 packets’ worth of saccharin each day.
“Four of them developed glucose intolerance after a week,” notes Kyriazis.
And when their stool samples were transferred to mice that had been raised in a sterile environment—so they had no gut bacteria—the mice also became glucose intolerant.
But since 2014, no researchers have done a similar study. (Kyriazis has a pilot study under way.)
“It’s too early to conclude that artificial sweeteners can be harmful or beneficial or neutral for glucose tolerance,” says Kyriazis.
What about long-term studies that ask people what they drink and look at who gets type 2 diabetes? Some report a higher risk in diet-beverage drinkers, but the best studies do not.9,10
“At first, we also found an increased risk for diabetes with diet-beverage intake,” says Harvard’s Vasanti Malik, who co-authored one of those studies, which followed 40,000 men for 20 years.10
“But we were able to control for dieting behavior, recent weight gain or loss, family history of diabetes, and other factors that influence risk that other groups hadn’t taken into account.”
Once they did, “the link between diet-soda intake and diabetes risk vanished.”
Bottom Line: If you’re worried that diet soda may increase your risk of diabetes, ask your doctor to test your hemoglobin A1c (a long-term measure of blood sugar).
1 Int. J. Obes. 37: 1378, 2013.
2 N. Eng. J. Med. 367: 1397, 2012.
3 Am. J. Clin. Nutr. 95: 555, 2012.
4 Am. J. Clin. Nutr. 82: 1011, 2005.
5 NeuroImage 39: 1559, 2008.
6 Appetite 55: 37, 2010.
7 Am. J. Clin. Nutr. 97: 604, 2013.
8 Nature 514: 181, 2014.
9 Diabetes Care 32: 688, 2009.
10 Am. J. Clin. Nutr. 93: 1321, 2011.