Dr. Ralph Sacco, chair of neurology at the University of Miami Leonard M. Miller School of Medicine and senior author on the report, said, "As a first step, we just looked at total calories. It's a work in progress."
Although the authors controlled for vascular disease and metabolic syndrome, Miami's Sacco theorized that drinking diet soda might increase stroke and heart attack risk by modulating vascular risk factors and metabolism.
"When we control for these, we still see an association but it's attenuated a bit," Sacco said, adding that people who drank daily diet soda had a higher mean body-mass index and were slightly more likely to have diabetes.
"I think we need confirmation in other, larger data sets," Sacco said. "We find this intriguing but it requires follow-up."
Critics also noted the failure to control for a family history of cardiovascular disease or hyperlipidemia, both of which are known risk factors for stroke and heart attack, according to Dr. Nehal Mehta, director of the Inflammatory Risk Clinic at the University of Pennsylvania.
And the study population may have been at a higher-than-normal risk for cardiovascular events, with an event rate of 22 percent compared with the expected rate of 8 to 10 percent, Mehta said.
Roughly three-quarters of the study participants were African-American or Hispanic; both considered to be at a higher risk for cardiovascular problems.
But some experts welcomed the study as a "wake-up call" for many to ditch diet soda in favor of water and other more natural beverages.
"Any way you slice it, soda drinking is not healthy and should be done sparingly," said Dr. Peter McCullough, consultant cardiologist and chief academic and scientific officer for St. John Providence Health System and the Providence Park Heart Institute in Southfield, Mich.
"The study highlights the increasingly negative information we are getting about the consumption of non-caloric sweetened beverages," said Dr. Jana Klauer, a New York City-based private practice physician specializing in weight control and nutrition. "People drink them to save calories and enjoy a sweet taste, but diet soda hasn't lead to weight loss and now appears to be causing more problems than it solves."
Dr. Cam Patterson, chief of cardiology and director of the University of North Carolina McAllister Heart Institute, said, "We can't ignore this association but we really don't know what it means right now. It is implausible to me that diet drinks actually cause heart disease, but it does make sense that people who drink diet sodas might make other lifestyle choices, like exercising less, that contribute to the results of this study.
"I'll continue to pack a diet soda with my lunch, but I'll look more carefully at what else is in my lunch box," Patterson said, "and I'll pay more attention to what I'm doing while I'm drinking my diet soda."