Tops of cans of artificially sweetened drinks.

Swapping out regular soda for diet versions might seem like a healthy move. After all, it keeps you from consuming about 150 extra calories and 39 grams of sugars for every 12-ounce can you drink. But evidence has been mounting in recent years that artificially sweetened beverages could have their own adverse health effects—including an increased risk of type 2 diabetes, weight gain, and heart disease.

Now, a new study published in Stroke, a journal of the American Heart Association, contributes to this growing body of evidence. The researchers analyzed data on diet-drink consumption in 81,714 women age 50 and older who participated in the ongoing Women’s Health Initiative study. (The type of artificial sweeteners in the drinks the women consumed isn’t known.)

“We didn’t find that stroke risk increased significantly for women consuming few diet drinks,” says Yasmin Mossavar-Rahmani, Ph.D., associate professor of clinical epidemiology and population health at the Albert Einstein College of Medicine, and lead author of the study. “But for those who regularly drank two or more 12-ounce cans a day, there was a tremendous increase in their risk of stroke.”

What the Study Found

After adjusting for known stroke risk factors, such as obesity, age, and high blood pressure, the researchers found that women who drank 24 ounces or more of diet beverages per day were 23 percent more likely to have a stroke than those who drank less than 12 ounces per week. They were also 29 percent more likely to develop heart disease and 16 percent more likely to die from any cause.

More on Diet Drinks

Other studies have shown a connection between diet drinks and stroke. For example, in April 2017, researchers at Boston University School of Medicine found that people age 45 and older who drank one or more diet sodas every day were three times more likely to have a stroke than those who didn’t drink them.

But this new study is one of the first to zero in on the link between diet beverages and specific subtypes of strokes. The researchers found that big consumers of diet drinks had an 81 percent greater risk of small artery occlusion, a stroke that’s caused by a blockage of small vessels in the brain, than women who consumed less than one per day. “These strokes are not generally as devastating as those caused by a large vessel blockage,” says Mossavar-Rahmani. “But if you have them repeatedly, they can lead to dementia.”

Greater Risk for Some Women

The study found that heavy consumption of diet drinks put certain women at far greater risk. Those who were obese (defined as having a body mass index of 30 or higher) were about twice as likely to have an ischemic stroke as obese women who rarely or never drank them.

For African-American women, those who drank the most diet beverages were about four times more likely to have an ischemic stroke. “We know that in general stroke rates are higher in African-Americans and in those who are obese,” says Mossavar-Rahmani. “But our data can’t explain why higher diet drink consumption increased risk so significantly for these groups and not for others.”

And big consumers of diet drinks who did not have a history of heart disease or diabetes were 2½ times more likely to have small artery occlusion compared with such women who rarely or ever drink them. 

What the Study Can't Tell Us

The study authors caution that their findings don't actually say that diet drinks cause stroke.

“Observational studies like this one attempt to associate behaviors with outcomes but cannot prove cause and effect,” says Robert Rankin, president of the Calorie Control Council, a group that represents the low-calorie and reduced-calorie food and beverage industry. “It is likely study subjects were already at greater health risk and chose these beverages to manage their calorie and sugar intake.”

And though this very large group was tracked for almost 12 years, the information on diet-drink consumption came from a questionnaire given three years into the study asking how often the women drank artificially sweetened drinks in the previous three months. “The results are derived from a snapshot in time,” says Hannah Gardener, Sc.D., associate scientist in the department of neurology at the University of Miami, who co-authored an editorial accompanying the study. “For some people, that may not reflect their long-term consumption pattern.”

It's also not known whether the women who drank large quantities of diet beverages had spent their previous years drinking sugar-sweetened sodas. “It’s possible that they were already on the road to heart disease when they switched to the artificially sweetened beverages,” says Ralph L. Sacco, M.D., professor of neurology at the University of Miami Miller School of Medicine. And because this study looked only at postmenopausal women, the results don’t necessarily apply to younger women or men.

What You Should Do

Despite many studies showing associations between diet soda intake and adverse health effects, researchers caution that they still don’t know exactly why diet sodas might be bad for you. It’s possible that ingredients in the beverages—including the artificial sweeteners—could damage blood vessels, affect metabolic function, or cause inflammation.

The best advice, given what we know so far, is to err on the side of caution. “This study adds to the growing consistency of research showing an association between diet soda consumption and heart disease risk,” says Sacco. “And since research has not shown that switching to artificially sweetened beverages is that helpful for weight loss, there’s very little reason to recommend drinking them.”

“We have enough data now to recommend people be cautious about diet soda consumption,” says Gardener. “The research shows that they are not harmless and that they may actually increase your risk of cardiovascular events, such as stroke.” She advises that water is the best beverage, and says if people use diet drinks to help wean themselves off sugar-sweetened ones, they should do so for a limited period of time as they transition to water and other healthier beverages.  

Correction: A previously published version of this story said that the researchers found that big consumers of diet drinks had an 18 percent greater risk of small artery occlusion stroke. The correct increase in risk is 81 percent. In addition, the Boston University Study was conducted April 2017, not April 2018.