A can of diet soda contains roughly the same amount of artificial sweetener that researchers just linked to memory and thinking skills deteriorating 62 percent faster than in people who barely touch the stuff.
The finding comes from a large cohort study tracking thousands of middle-aged and older adults over roughly eight years. It is one of the largest studies to date connecting everyday sweetener intake, not sugar, to measurable cognitive decline.
What the researchers actually measured
The team followed more than 12,000 adults with an average age in the mid-50s, tracking their diets through detailed food questionnaires and giving them repeated memory and cognitive tests at multiple points over the study period. Participants were split into groups based on how much artificial sweetener they consumed daily, from nearly none to the highest quartile.
Those in the highest-consuming group saw their global cognitive scores decline 62 percent faster than the lowest-consuming group. Verbal fluency, the ability to quickly generate words and categories and a common early marker of cognitive aging, declined even faster in that same group, particularly among participants over 60.
The amount separating high from low consumption in the study was smaller than most people would assume. Researchers noted that the threshold for the highest exposure group was comparable to what a person would get from one to two servings of diet soda a day, not some extreme end of sweetener use reserved for people chugging six cans daily.
Which sweeteners were involved
The study tracked seven low- and no-calorie sweeteners commonly used in diet drinks and packaged foods: aspartame, saccharin, acesulfame-potassium, sucralose, sorbitol, xylitol, and tagatose. Aspartame and saccharin, the two most common in diet colas, showed some of the clearest associations with faster decline.
This distinction matters because it separates the sweeteners themselves from sugar, which the same researchers also tracked in parallel. Sugar intake did not show the same relationship with cognitive decline in this cohort, which argues against a simpler explanation where people with generally worse diets just performed worse on testing.
Why a sugar substitute might affect the brain at all
Artificial sweeteners were designed to pass through the body largely unmetabolized, avoiding the blood sugar spikes tied to regular sugar. But a growing body of research suggests they still interact with biology in ways that were not fully anticipated when many of these compounds were approved decades ago.
Several mechanisms are under active investigation. Sweeteners can alter the gut microbiome, the community of bacteria that produces compounds known to influence brain inflammation and communicate with the brain through the vagus nerve.
Other research has pointed to changes in insulin and glucose regulation triggered by non-caloric sweetness itself, independent of actual sugar intake, a mismatch some researchers believe confuses metabolic signaling over years of repeated exposure. There is also early evidence that some sweeteners can cross the blood-brain barrier in measurable amounts, though what that means functionally for long-term cognitive health is still being worked out.
This is not the first time diet soda specifically has drawn scrutiny. A widely cited study out of Boston University published in the journal Stroke in 2017 found that people who drank at least one artificially sweetened beverage a day had a meaningfully higher risk of stroke and dementia over the following decade compared to those who drank them less than once a week. That study could not isolate which ingredient was responsible, but the newer research narrows the suspect list considerably.
The diabetes connection made it worse
The association between artificial sweeteners and faster cognitive decline was notably stronger among participants who had type 2 diabetes at the start of the study. That subgroup showed a more pronounced decline than the overall cohort, a signal the study authors flagged as worth deeper investigation given how often diet beverages are recommended specifically to people managing blood sugar.
Diabetes is already an independent risk factor for dementia and cognitive impairment, largely through its effects on blood vessels in the brain. If artificial sweeteners compound that risk rather than sidestepping it, that complicates advice that has told diabetics for years to swap sugar for substitutes with little concern about the substitute itself.
What this study does not show
Observational cohort studies like this one can establish a strong association but cannot prove that sweeteners directly cause cognitive decline. People who drink a lot of diet soda may differ from low consumers in other ways the study could not fully capture, including overall diet quality, physical activity, and existing metabolic health.
The researchers did adjust for standard variables including age, education, physical activity, and cardiovascular risk factors, and the association held after those adjustments. But causation in nutritional epidemiology is notoriously difficult to pin down from a single study, however large the sample.
It is also worth noting that moderate consumers, people having an occasional diet drink rather than a daily one, did not show the same steep decline as the highest-consumption group. The signal was concentrated at the upper end of intake, not spread evenly across anyone who has ever touched a diet soda.
What to do with this information
This is not a case for panic over an occasional diet soda, but it is a reasonable prompt to look at how often diet and zero sugar labeled products actually show up in a daily routine. Someone drinking a diet soda with every meal sits in a meaningfully different exposure category than someone having one every few days.
Water, unsweetened coffee or tea, and naturally flavored sparkling water remain the most well-studied ways to cut back on added sweeteners without introducing this open question. For anyone managing diabetes and leaning on diet beverages specifically for blood sugar control, this is worth raising with a physician rather than adjusting course alone based on a single headline.
The bigger takeaway may be less about any single ingredient and more about how much is still unknown about substances introduced into the food supply decades before the tools existed to track their long-term effects on the brain.
