A study published online in the journal Stroke in April 2017 got a lot of diet-beverage drinkers worried. Using data from thousands of people in the Framingham Heart Study, researchers linked daily consumption of diet beverages to increased risk of stroke and dementia over a 10-year period.

It wasn’t the first study to report such results. For instance, two studies in 2012— the Northern Manhattan Study and one using data from the Nurses’ Health Study and Health Professionals Follow-Up Study—also linked diet sodas to increased stroke risk. In contrast, several other studies have not found any such associations.

It’s possible that diet drinks may have detrimental effects on blood vessels in the brain through some unknown biological pathway. But because this study, like previous ones, was observational, it does not prove cause and effect.

Rather, it may be that people predisposed to such vascular conditions—such as those with diabetes— are more likely to drink diet beverages. As an accompanying editorial explained, “People at increased risk of vascular events because of preexisting vascular risk factors may switch from regular to diet soft drinks in an attempt to control weight and insulin resistance. It is entirely possible that the intake of ASB [artificially sweetened beverages] starts after the cardiovascular risk is increased and, therefore, is a marker of a high-risk profile rather than being a causal risk factor for stroke or dementia.” In fact, when the researchers controlled for vascular risk factors and diabetes, the association between diet drinks and dementia was no longer significant.

Another review of observational studies, in CMAJ in July 2017, linked high intake of sugar substitutes with increased risk of hypertension, metabolic syndrome, type 2 diabetes, and cardiovascular events. But these studies were not of high quality and, despite attempts to control for confounding variables, they are still subject to bias. Moreover, “it is not uncommon for hypotheses based on observational evidence to fail when tested in RCTs [randomized controlled trials], and these data should therefore be interpreted with caution,” the researchers warned.

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