In a nine-year study of more than 2,500 people, those who drank diet soda daily were 48% more likely to have a heart attack or stroke or die from those events, compared with those who rarely or never drank soda.
There was no increased risk of cardiovascular disorders among daily drinkers of regular soda, says study researcher Hannah Gardener, ScD, an epidemiologist at the University of Miami Miller School of Medicine.
The analysis, presented at the American Stroke Association International Stroke Conference (ISC), took into account a host of cardiovascular risk factors including age, sex, smoking, physical activity, alcohol and calorie consumption, metabolic syndrome, and pre-existing heart disease.
Still, the study doesn't prove cause and effect. And even though the researchers tried to account for risk factors that that could skew the results, they couldn't tease out everything, doctors caution.
"You try to control for everything, but you can't," says Steven Greenberg, MD, PhD, vice chair of the ISC meeting committee and professor of neurology at Harvard Medical School.
People who drink a lot of diet soda may share some characteristic that explains the association, he explains.
Maureen Storey, PhD, senior vice president of science policy for the American Beverage Association, says that the researchers failed to control for two important variables -- family history of stroke and weight gain -- that affect stroke risk.
Even Gardener says it's too early to tell people to skip soda based on this study alone. "But if confirmed, diet drinks may not be an optimal substitute for sugar-sweetened beverages."
The new study involved 2,564 people in the Northern Manhattan Study, with an average age of 69. About two-thirds of the participants were women, 21% were white, 24% African-American, and 53% Hispanic.
At the start of the study, people were asked to fill out a questionnaire that asked what foods and beverages they consumed and how often they consumed them.
They were also asked about their exercise routine, as well as whether they smoked or drank alcohol. The participants also had physical checkups.
Of the total, 901 said they never drank soda or drank it less than once a month, 282 said they drank at least one regular soda daily, and 116 reported they drank at least one diet soda daily.
Over the next nine years, 212 of them had strokes, 149 had a heart attack, and 338 died from vascular disease.
One drawback of the study is that participants were only asked about their soda habits at one time point; they could have changed over the study period, Gardener says. Also, there was no information on the types of soft drinks drunk, she says, pointing out that variations among brands, coloring, and sweeteners could have affected the results.
Philip B. Gorelick, MD, MPH, head of neurology and stoke research at the University of Illinois in Chicago, says the food questionnaire used in the study isn't a good way to gauge people's overall dietary patterns.
"You have to look at what people eat in totality," he tells WebMD. "People who are reducing calories by drinking diet soda may have an unhealthy dietary pattern, consuming a lot of fat and salt, for example. And that won't be picked up using a questionnaire like the one used here."
Storey says there is no diet soda-heart and stroke connection. "There is no scientific evidence to support the idea that diet soda uniquely causes increased risk of vascular events or stroke,” she says in a written statement. “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."
As for why diet soda might be bad for our hearts and brains, Gardener says that's still a big question mark.
Previous research linking regular and diet soda to diabetes and metabolic syndrome, both of which are risk factors for stroke and heart attacks, may offer a clue, she says.
These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.