The study tracked people, not lab rats, for eight years. Researchers followed 12,772 adults (average age 52, 55% women), estimating consumption of seven artificial sweeteners (aspartame, saccharin, acesulfame, erythritol, sorbitol, xylitol, and tagatose) from food-frequency questionnaires taken at the start of the study, then re-tested memory and thinking skills over time.
The dose that mattered was smaller than "heavy use." The lowest-intake group averaged 20 mg/day; the highest averaged 191 mg/day, which the researchers note is roughly what one can of diet soda delivers in aspartame alone. This was not an extreme-consumption study. The higher-intake group's habits look ordinary.
62% faster decline, but only under 60. Adults under 60 in the highest-intake group had 62% faster declines in memory and thinking than the lowest-intake group, equivalent to about 1.6 additional years of brain aging. Adults over 60 showed no measurable link at all. Most headlines led with the 62% number and left the age cutoff for paragraph six, if it appeared.
Diabetes widened the gap further. People with diabetes who were also high sweetener consumers showed the fastest declines in the study. Whether that reflects the sweeteners, the underlying metabolic condition, or both together is not something a single observational study can separate out.
This is an association, not a mechanism. The researchers were explicit that more research is needed before concluding sweeteners cause the decline. Food-frequency questionnaires are self-reported and imprecise, and people who drink a lot of diet soda differ from people who don't in ways beyond the soda itself. What the study does establish is a large, tracked cohort showing a consistent pattern in one specific age band, not a single fluke reading.
If you swap sweeteners in specifically to manage blood sugar, fasting insulin is the marker that tells you whether that trade-off is working. See what a fasting insulin test measures.