In the observational study, those who drank at least one artificially-sweetened beverage a day were almost three times more likely to develop ischemic stroke (HR 2.96, 95% CI 1.26-6.97) and 2.9 times more likely to develop Alzheimer's disease (95% CI 1.18-7.07) over 10 years than those who abstained, Matthew Pase, PhD, of Boston University School of Medicine, and colleagues reported in the American Heart Association's journal Stroke.
For many people, such as people with diabetes or obesity, he said, diet sodas can be part of the gradual switch from sugary drinks.
Their study of nearly 4,400 adults also suggests diet drinks are more likely to cause strokes and dementia than those full of sugar.
Interestingly, the researchers didn't find any link between stroke or dementia and sugary beverages or regular sodas. Now just 38% of all soft drinks consumed are fully sugared, it said.
Of the 2,888 participants the study followed, there were only 97 cases of stroke and 81 cases of dementia.
They had filled in detailed questionnaires on their food and drink intake in the 1990s and were then tracked for ten years. The former indicates the manufacturer has not added any sugar to the product.
Other recent studies have found health risks that appear to be linked to diet fizzy drinks, such as a link between diet drinks and the risk of metabolic syndrome and type 2 diabetes. In fact, about 75 percent of dietary sodium actually comes from eating packaged and restaurant food. But after that, what we think those labels mean gets a bit fuzzy, according to a 2013 study.
Dr Mary Hannon-Fletcher, head of health sciences at Ulster University, said: "These data are sound as far as they go". In fact, studies have shown that no- and low-calorie sweetened beverages help reduce energy intake and body weight.
And so, in the late 1990s, low-fat diets swept the nation.
The researchers point out that preexisting conditions such as cardiovascular disease, diabetes and high blood pressure did not completely explain their findings.
Higher consumption of artificially sweetened soft drinks was associated with an increased risk of both stroke and dementia in an analysis of more than 4,000 participants in the Framingham Heart Study Offspring cohort, researchers found.
Matthew Pase, senior fellow in the department of neurology at Boston University School of Medicine, said: " Our study shows a need to put more research into this area given how often people drink artificially sweetened beverages.
"So, it was not surprising to see that diet soda intake was associated with stroke and dementia".
Pase, who studies how people can change behavior or diet to prevent dementia, said people need to be skeptical when deciding whether to select something with artificial sweeteners or real sugar.
"Even if someone is three times as likely to develop stroke or dementia", Pase said, "it is by no means a certain fate".
'In our study, 3 percent of the people had a new stroke and 5 percent developed dementia, so we're still talking about a small number of people developing either stroke or dementia'.
As a result of the burgeoning war on sugar, more people are turning to artificially sweetened foods and drinks as "healthful" alternatives. But they found other troubling signs.
Previous studies have shown they tend to be consumed by adults who are already overweight or obese. However both sugar and artificially-sweetened beverage consumption has been linked to cardiometabolic risk factors, which increases the risk of cerebrovascular disease and dementia.
"When the authors controlled for hypertension and diabetes and obesity the effects diminish, which implies that some of the effects of artificially sweetened beverages could still be going through a vascular pathway", he said about the new study.
But Gavin Partington, director-general of the British Soft Drinks Association, the industry body said: 'Despite their claims, the authors of this observational study admit they found no cause and effect and provide no science-based evidence whatsoever to support their theories. "NIH does not mention zero calorie sweeteners as a risk factor", the statement said. "So, I believe the mechanisms may be through vascular disease, though we can't prove it".