People that drank two or more sugary beverages of any kind per day were more likely to have poorer memory, smaller overall brain volume, and a significantly smaller hippocampus. Researchers also found that higher intake of diet soda, at least one per day, was associated with smaller brain volume.
In a second study, researchers looked at whether participants had suffered a stroke or been diagnosed with dementia due to Alzheimer’s disease. Interestingly, there was no association between sugary beverage intake and stroke or dementia. But people who drank at least one diet soda per day were nearly 3 times as likely to develop stroke and dementia.
While no of this data proves causation, there is a growing body of research showing that excess refined sugar does increase inflammation which crosses the blood-brain barrier and acceleration brain aging. Regarding the diet soda, there have been studies linking artificial sweeteners to disruption of the gut microbiome which also causes inflammation which can lead to brain aging.