Americans trying to stay healthy have abandoned sugary drinks for diet drinks in droves over the past few decades on the theory that the latter is better than the former. Now, more evidence has emerged to refute that rationale.

Indeed, a new study shows an association between diet soda and both stroke and dementia, with people drinking diet soda daily being almost three times as likely to develop stroke and dementia as those who consumed it weekly or less.

“This included a higher risk of ischemic stroke, where blood vessels in the brain become obstructed and Alzheimer’s disease dementia, the most common form of dementia,” said Matthew Pase, a Boston University School of Medicine neurologist and the lead author of the study published in the journal Stroke.

While emphasizing that the research did not show causation, only a correlation, Pase said in a video explaining the study that diet drinks “might not be a healthy alternative.”

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The study, described only as a hypothesis by its lead author was surrounded by caveats. While the risk was greater, the absolute numbers were low. “In our study,” the lead author said, ” three percent of the people had a new stroke and five percent developed dementia, so we’re still talking about a small number of people developing either stroke or dementia.”

The lead author also noted its many limitations in an accompanying commentary from the American Heart Association:

The participants were overwhelmingly white, and it is possible that ethnic preferences may influence how often people select sugary or artificially sweetened drinks ….People did not drink sugary sodas as often as diet sodas, which Pase said could be one reason the researchers did not see an association with regular soda since the participants may have been health conscious and just not consuming them as frequently. The main limitation, Pase said, is the important point that an observational study like this cannot prove that drinking artificially-sweetened drinks is linked to strokes or dementia, but it does identify an intriguing trend that will need to be explored in other studies.

Still, people should be “cautious” about their intake of diet sodas, Pase said, noting that more study is needed.

And they should most definitely not retreat to sugary drinks, he said. They have been associated not only with obesity and its consequences, such as diabetes, but with poorer memory and smaller overall brain volumes.

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The study kept track of 2,888 individuals age 45 and over for the development of a stroke and 1,484 participants age 60 and older for dementia over a 10 year period. All are participants in the famous Framingham Heart Study, several thousand men and women who have had blood tests done periodically since the 1970s.

The study “found that those who reported consuming at least one artificially sweetened drink a day, compared to less than one a week, were 2.96 times as likely to have an ischemic stroke, caused by blood vessel blockage, and 2.89 times as likely to be diagnosed with dementia due to Alzheimer’s disease,” said a summary from the AHA.

A parallel study of sugary drinks did not find an association with stroke or dementia.

The artificial sweeteners consumed by those in the study included saccharin, acesulfame-K, and aspartame. Other sweeteners, including sucralose, neotame and stevia have been approved by the FDA since, the study said.

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The results were adjusted for variables such as age, sex, caloric intake, diet quality, physical activity and smoking. (For those seeking more detail, the study is downloadable in its entirety.)

“So, the bottom line is, ‘Have more water and have less diet soda,” Christopher Gardner, director of Nutrition Studies at the Stanford Prevention Research Center, said in an AHA release. “And don’t switch to real soda.”

He added “Nobody ever said diet sodas were a health food.”

The AHA release quoted Rachel K. Johnson, professor of nutrition at the University of Vermont: “We need to be cautious in the interpretation of these results. It doesn’t prove cause and effect. When you see these kinds of associations, you want to always ask what is the biological plausibility, what is the mechanism that might be causing this?”

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“We have a robust body of literature on the adverse effects of sugary drinks. Absolutely the message is not to switch to sugary drinks,” she said.

The American Beverage Association was quick to defend diet drinks.

“Low-calorie sweeteners have been proven safe by worldwide government safety authorities as well as hundreds of scientific studies and there is nothing in this research that counters this well-established fact,” it said in a statement. It added:

While we respect the mission of these organizations to help prevent conditions like stroke and dementia, the authors of this study acknowledge that their conclusions do not — and cannot — prove cause and effect.

This post has been updated and corrected. An original version said stevia was an artificial sweetener. In fact, it comes from a plant.

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