The beverage findings should be "a wakeup call to pay attention to diet sodas," said Dr. Steven Greenberg. He is a Harvard Medical School neurologist and vice chairman of the International Stroke Conference in California, where the research was presented on Wednesday.
A simple solution, health experts say, is to drink water instead.
Doctors have no chemical or biological explanation for why diet soda may be risky. It could be that people who drink lots of it also fail to exercise, weigh more, drink more alcohol or have other risk factors like high blood pressure and smoking. However, the researchers took these and many other factors into account and didn't see a change in the trend.
"It's reasonable to have doubts, because we don't have a clear mechanism. This needs to be viewed as a preliminary study," said lead researcher Hannah Gardener of the University of Miami.
But for those trying to cut calories, "diet soft drinks may not be an optimal substitute for sugar-sweetened beverages," she said.
The numbers come from the Northern Manhattan study, which enrolled about 2,500 adults over 40 in the New York area from 1993 to 2001 through random phone calls. Half are Hispanic and one-fourth are black, making it one of the few studies to look at these risks in minorities, who have higher rates of stroke.
Participants filled out a standard survey about their diets at the start of the study, and their health was tracked for nearly 10 years. In that time there were 559 strokes or heart attacks, 338 of them fatal.
Daily diet soda drinkers (there were 116 in the study) had a 48 percent higher risk of stroke or heart attack than people who drank no soda of any kind (901 people, or 35 percent of total participants). That's after taking into account rates of smoking, diabetes, waistline size and other differences among the groups.
No significant differences in risk were seen among people who drank a mix of diet and regular soda.
Earlier studies have tied diet and regular soda consumption to greater risk of diabetes and a group of weight-related problems called the metabolic syndrome.
Some diet soda critics have suggested it can promote a sweet tooth, affecting behavior and how much of a person's diet comes from sugary sources rather than healthier fruits, vegetables and grains.
These sorts of studies just observe groups of people and are not strong enough evidence to prove risk.
"It's too preliminary to suggest any dietary advice," but other big studies should look at this question, Gardener said.
Greenberg, of the stroke association, called it "a real-world" look at possible risk.
Dr. Maureen Storey, senior vice president of science policy for the American Beverage Association, said in a statement that there is no evidence "that diet soda uniquely causes increased risk of vascular events or stroke."
"The body of scientific evidence does show that diet soft drinks can be a useful weight management tool, a position supported by the American Dietetic Association. Thus, to suggest that they are harmful with no credible evidence does a disservice to those trying to lose weight or maintain a healthy weight."
The beverage group's statement also noted researchers didn't adjust their results for family history of stroke. Gardener, the researcher, said that's not "a substantial weakness."
The same federally funded study also looked at a more conventional health risk - salt. It found higher risks for people eating more than 1,500 milligrams a day. That's the limit the American Heart Association recommends, but last week's new dietary guidelines from the government say it's OK to have a little more.
Researchers found that stroke risk rose 16 percent for every 500 milligrams of salt consumed each day. Those who took in 4,000 or more milligrams of salt had more than 2.5 times greater risk of stroke compared to those who limited themselves to 1,500 milligrams.
A teaspoon of salt contains about 2,300 milligrams of sodium. About three-fourths of the salt we eat, though, comes from processed foods, especially tomato sauce, soups, condiments, and canned foods.
Stroke conference: www.strokeconference.org
Heart Association on salt: http://bit.ly/GHA0W