Drinking two or more diet drinks a day may increase the risk of heart disease, including heart attack and stroke, in otherwise healthy postmenopausal women, according to a new University of Iowa study. The findings were presented March 30 at the American College of Cardiology’s 63rd Annual Scientific Session in Washington, D.C.

The study, which analyzed diet drink intake and cardiovascular health in almost 60,000 women participating in the Women’s Health Initiative Observational Study, found that compared to women who never or only rarely consume diet drinks, those who consume two or more a day are 30 percent more likely to have a cardiovascular event and 50 percent more likely to die from related disease.

“This is one of the largest studies on this topic, and our findings are consistent with some previous data, especially those linking diet drinks to the metabolic syndrome,” says Dr.Ankur Vyas, a fellow in cardiovascular disease at UI Hospitals and Clinics, and the lead investigator of the study.

About one in five people in the U.S. consume diet drinks on a given day, according to data from the National Health and Nutrition Examination Survey (2009-10). But, according to Vyas, there is a relative lack of data about the cardiovascular health consequences of diet drinks.

Based on self-reported consumption of diet drinks over a three-month period, the researchers divided the 59,614 study participants into four consumption groups: two or more diet drinks a day, five to seven diet drinks per week, one to four diet drinks per week, and zero to three diet drinks per month. Each drink was defined as the equivalent of a 12-ounce beverage and included both diet sodas and diet fruit drinks.

After an average follow-up of 8.7 years, the primary outcome—defined as a composite of incident coronary heart disease, congestive heart failure, heart attack, coronary revascularization procedure, ischemic stroke, peripheral arterial disease, and cardiovascular death—occurred in 8.5 percent of the women consuming two or more diet drinks a day compared to 6.9 percent in the five-to-seven diet drinks per week group; 6.8 percent in the one-to-four drinks per week group; and 7.2 percent in the zero-to-three per month group.

The association persisted even after researchers adjusted the data to account for demographic characteristics and other cardiovascular risk factors, including body mass index, smoking, hormone therapy use, physical activity, energy intake, salt intake, diabetes, hypertension, high cholesterol, and sugar-sweetened beverage intake. On average, women who consumed two or more diet drinks a day were younger, more likely to be smokers, and had a higher prevalence of diabetes, high blood pressure, and higher body mass index.

But Vyas says the association between diet drinks and cardiovascular problems raises more questions than it answers, and should stimulate further research.

“We only found an association, so we can’t say that diet drinks cause these problems,” Vyas says, adding that there may be other factors about people who drink more diet drinks that could explain the connection.

“It’s too soon to tell people to change their behavior based on this study; however, based on these and other findings we have a responsibility to do more research to see what is going on and further define the relationship, if one truly exists,” he adds. “This could have major public health implications.”

Future research could include clinical studies, animal models, and even molecular and pharmacologic analyses to begin to explain what, if any, direct role diet drinks play in heart health.

In addition to Vyas, the study team included Linda Rubenstein, Jennifer Robinson, Linda Snetselaar, and Robert Wallace from the UI College of Public Health, and Rebecca Seguin at Cornell University; Mara Z. Vitolins at Wake Forest Medical School; Rasa Kazlauskaite at Rush University Medical Center; James Shikany at University of Alabama Birmingham School of Medicine; and Karen C. Johnson at University of Tennessee Health Science Center.

Note to Editors: This release is adapted from a news release prepared by the American College of Cardiology.