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When you grab a diet soda instead of the full-sugar version, you might think you’re making the healthy choice — and you are, at least when it comes to your weight. But according to a new study, people who drink diet soda habitually could be putting themselves at risk for stroke.

Researchers reported their findings at the American Stroke Association’s International Stroke Conference 2011. The study, which analyzed data on 2,564 participants of the ethnically diverse, longitudinal Northern Manhattan Study (NOMAS), divided people into several groups based on their soda consumption: those who drank no soda; those who drank regular soda, either moderately (one to six servings per week) or every day; and those who drank diet soda, either moderately or every day. There were two other groups who went back and forth between regular and diet soda, drinking either moderately or every day. (More on Time.com: Big Breakfasts Are Out. 5 Better-For-You Morning Meals)

The researchers found that people who drank diet soda every day were 48% more likely to suffer a stroke or other vascular event over a nine-year span, compared with people who didn’t drink any soda. That risk persisted after researchers accounted for factors like participants’ age, sex, smoking status, exercise, alcohol consumption and peripheral vascular disease or heart disease history.

About 35% of the respondents didn’t drink soda at all, and 24% said they drank diet soda; a total of 116 total participants drank diet soda every day. By the end of the nine-year follow-up, 559 participants had suffered some type of vascular event, including 221 strokes, 149 heart attacks and 338 total deaths. (More on Time.com: The Updated Egg: Less Cholesterol, But Is It Healthy?)

But the researchers said that more investigation would be needed before they’d feel comfortable recommending patients stay away from the fizzy stuff. The study’s findings were intriguing, but they don’t prove cause and effect; for one thing, people who drink soda every day, diet or otherwise, may also engage in other less-than-healthy behaviors, like eating a lot of high-fat, high-cholesterol foods. Researchers also acknowledge that relying on participants’ self-reported dietary behavior was a significant limitation of the study.

Further, participants were asked about their soda-drinking habits only once, at the start of the study; their behaviors may have changed significantly over the follow-up period. Researchers also did not collect data on family history of stroke nor did they measure long-term changes in participants’ weight. (More on Time.com: Bud in a Bottle: New Marijuana Soda to Launch in Feb.)

“If our results are confirmed with future studies, then it would suggest that diet soda may not be the optimal substitute for sugar-sweetened beverages for protection against vascular outcomes,” said Hannah Gardener, lead author and epidemiologist at the University of Miami Miller School of Medicine in Miami, Fla., in a statement.

But, then, we already knew that. The optimal substitute is water.

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