A new study links diet soft drinks to an increased risk for stroke and dementia.

Researchers studied more than 4,000 people over 45 who had filled out food-frequency questionnaires and had periodic health examinations between 1991 and 2001. The scientists tracked their health over the next 10 years and found 97 cases of stroke and 81 cases of dementia.

The study, in the journal Stroke, found that compared with those who did not drink diet soda, people who drank one to six artificially sweetened drinks a week had twice the risk of stroke. There were similar, although weaker, associations for dementia risk. The reasons for the link remain unknown.

The study adjusted for age, sex, education, physical activity, diabetes, smoking and many other characteristics that might affect the risks. But the senior author, Dr. Sudha Seshadri, a professor of neurology at Boston University School of Medicine, said that there were additional variables the study could not address. For example, she said, people might have switched to diet soda because they already had cardiovascular problems.

Still, she added, there are health benefits associated with some drinks, like tea or coffee, “but not with soda of any kind, either diet or not.”