The study, which followed almost 2,600 older adults for a decade, found that those who drank diet soda every day were 44 percent more likely than non-drinkers to suffer a heart attack or stroke.
The findings, reported in the Journal of General Internal Medicine, don't prove that the sugar-free drinks are actually to blame.
There may be other things about diet-soda lovers that explain the connection, researchers say.
"What we saw was an association," said lead researcher Hannah Gardener, of the University of Miami Miller School of Medicine. "These people may tend to have more unhealthy habits."
She and her colleagues tried to account for that, Gardener told Reuters Health.
Daily diet-soda drinkers did tend to be heavier and more often have heart risk factors like high blood pressure, diabetes and unhealthy cholesterol levels.
That all suggests that people who were trying to shed pounds or manage existing health problems often opted for a diet soda over the sugar-laden variety.
But even after the researchers factored in those differences -- along with people's reported diet and exercise habits -- they found that daily diet soda was linked to a 44-percent higher chance of heart attack or stroke.
Nevertheless, Gardener said, it's impossible for a study to capture all the variables that could be at work.
The findings do build on a few recent studies that also found diet-soda drinkers are more likely to have certain cardiovascular risk factors, like high blood pressure or high blood sugar.
This is the first study, Gardener said, to look at actual "vascular events" -- that is, heart attacks, strokes and deaths from cardiovascular causes.
The findings are based on 2,564 New York City adults who were 69 years old, on average, at the outset. Over the next decade, 591 men and women had a heart attack, stroke or died of cardiovascular causes.
That included 31 percent of the 163 people who were daily diet-soda drinkers at the study's start. In contrast, 22 percent of people who rarely or never drank diet soda went on to have a heart attack or stroke.
There was no increased risk linked to less-than-daily consumption. Nor was regular soda tied to heart attacks and strokes.
If diet soda, itself, somehow contributes to health risks, it's not clear how, Gardener said.
There's research in rats suggesting that artificial sweeteners can end up boosting food intake and weight. But whether results in rodents translate to humans is unknown.
"I don't think people should change their behavior based on this study," Gardener said. "And I wouldn't advocate drinking regular soda instead."
Regular soda is high in calories, and for people who need to shed pounds, experts often suggest swapping regular soda for the diet version.
A study out this month found that the advice may be sound. Obese people who were randomly assigned to drink water or diet drinks in place of sugary ones lost about five pounds over six months.
Gardener said that further studies such as hers are still needed to confirm a connection between diet soda and cardiovascular trouble.
Ultimately, she noted, clinical trials are considered the "gold standard" for proving cause-and-effect. That would mean randomly assigning people to drink diet soda or not, and then following them over time to see if there were differences in their rates of heart problems or stroke.
A study like that, Gardener said, would be "difficult and costly" -- since it would have to follow large groups of people over many years, and rely on people to stick with their assigned beverages.
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