According to a study of more than 2,500 people presented today as a poster at the American Stroke Association International Stroke Conference in Los Angeles, people who drank diet soda daily had a 61 percent increased risk of cardiovascular events compared to those who drank no soda, even when accounting for smoking, physical activity, alcohol consumption and calories consumed per day.
"This study suggests that diet soda is not an optimal substitute for sugar-sweetened beverages, and may be associated with a greater risk of stroke," Hannah Gardener of the University of Miami and her colleagues reported at the conference.
But the questionnaire-based study garnered criticism by experts in diet, nutrition and vascular disease.
"This study has major flaws and should not change anyone's diet soda consumption," said ABC News Chief Health and Medical Editor Dr. Richard Besser.
The researchers used data obtained though the multi-ethnic, population-based Northern Manhattan Study to examine risk factors for stroke, heart attack and other vascular events such as blood clots in the limbs. While 901 participants reported drinking no soda at the start of the study, 163 said they drank one or more diet sodas per day.
"One of the many flaws here is that participants were asked about soda intake at only one point in time, when they entered the study," Besser said. "It is difficult to imagine that people's intake of soda is constant during that period."
Association, Not Causation
Connie Diekman, a registered dietician and director of University Nutrition at Washington University in St Louis, said, "Population-based studies provide some 'food for thought' but shouldn't be used as the basis of nutrition guides for individuals. This study would be another one that indicates more controlled studies are needed."
Drinking regular or diet soda has previously been linked to diabetes and metabolic syndrome, a precursor to diabetes. Surprisingly, Gardener and colleagues failed to detect an increased cardiovascular risk among daily drinkers of regular soda.
"Unfortunately, it may be that individuals with poor dietary habits do resort in some kind of calorie balancing and continue to eat high-calorie sweet foods but reduce their guilt by drinking diet soda," said Dr. Howard Weintraub, clinical director of the New York University Center for the Prevention of Cardiovascular Disease, explaining the propensity to wash down a high-fat meal with low-cal soda.
Although the authors went on to control for metabolic syndrome (a component of which is obesity), peripheral vascular disease and cardiac disease history later in their analysis -- lowering the magnitude of increased risk to 48 percent -- they did not account for variations in eating habits.
"In my 20 years of clinical practice, patients who consume diet soda tend to have more of a sweet tooth; to get more sweet cravings; to eat more foods with added sugar; and to like and eat more processed food than patients who avoid both regular and diet soda," said Dr. David Katz, director of Yale University's Prevention Research Center.
'Work in Progress'
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."