No connection was found between those health risks and other sugary beverages, such as sugar-sweetened sodas, fruit juice and fruit drinks.
The study found that people who reported drinking at least one artificially sweetened drink a day compared with less than one a week were 2.96 times as likely to have an ischemic stroke, caused by blood vessel blockage, and 2.89 times as likely to be diagnosed with dementia due to Alzheimers disease.
Scientists have put forth various hypotheses about how artificial sweeteners may cause harm, from transforming gut bacteria to altering the brain's perception of "sweet", but "we need more work to figure out the underlying mechanisms", said Boston University's Matthew Pase, who is lead author on the two studies. Artificially-sweetened beverages, however, also have health risks of their own.
Previous studies have looked at artificial sweeteners impact on stroke risk.
In response to the study being published, the American Beverage Association issued a press release, stating that extensive studies have shown that ASBs are "safe for consumption".
The new research suggests that the potential harms of artificial sweeteners may extend to the brain.
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Dr Mary Hannon-Fletcher, the Head of Health Sciences at Ulster University in the United Kingdom, said that although the research was good, the methods of getting the data had limitations.
Over seven years, researchers studied thousands of people over the age of 45 from the area of Framingham, Mass., on their drinking and eating habits. Ischaemic strokes occur when blood can not get to the brain because of a blockage, often one caused by a blood clot forming in either an artery leading to the brain or inside a vein in the brain itself.
At the end of 10 years of follow-up, 97 study subjects suffered a stroke. "When the researchers controlled for other risk factors, particularly cardiovascular risk factors, it explained most of the association between artificially sweetened beverage intake and the development of dementia".
The study was unable to determine a cause-and-effect relationship between diet soda drinks and the health risks, meaning more research is needed to find out whether they are actually a direct result of artificially sweetened drinks.
"This makes it more likely that there is a group of people who both use artificially sweetened drinks and are at higher risk of dementia, presumably because they have a risk factor, such as diabetes, for which a low sugar diet has been recommended". For the dementia part of the study, they followed almost 1,500 people over age 60 in the Framingham group. For lower consumption (between 1 and 6 drinks per week), the risk is increased by an ischemic stroke (2.6 times more risk), but not to develop dementia.
"In our study, three per cent of the people had a new stroke and five per cent developed dementia, so we're still talking about a small number of people developing either stroke or dementia".
Dr Rosa Sancho, Head of Research at Alzheimer's Research UK, said: 'As people are becoming more aware of the consequences of a high-sugar diet, many are turning to artificially-sweetened diet fizzy drinks as an alternative to those with lots of sugar.