Researchers at Columbia University in New York and Miami's Miller School of Medicine say the Northern Manhattan Study began in 1993 with 3,298 participants age 40 and older -- 63 percent women, 21 percent white, 24 percent black and 53 percent Hispanic -- who are still tracked.
"If our results are confirmed with future studies, then it would suggest that diet soda may not be the optimal substitute for sugar-sweetened beverages for protection against vascular outcomes," lead author Hannah Gardener of the University of Miami said in a statement.
At the beginning of the study, the researchers asked subjects how much and what kind of soda they drank. After an average follow-up of 9.3 years, 559 vascular events occurred in the diet soda drinkers, the study found.
The findings, presented at the American Stroke Association's International Stroke Conference, said when the researchers also accounted for patients' metabolic syndrome, peripheral vascular disease and heart disease history, the increased risk persisted at a rate 48 percent higher.
The Calorie Control Council, a non-profit association representing the low-calorie food and beverage industry, said the study has several weaknesses, including:
-- The research is not a published study and has not been peer-reviewed for a scientific journal.
-- Soda intake is based on self-reported consumption.
-- The study is observational and does not show cause and effect and the authors do not appear to have controlled for family history of heart disease, stroke or weight gain.
-- The sample size of the diet soda drinkers was 4.5 percent of the total study sample.
-- The study population was not representative of the U.S. population.