No study has shown that aspartame causes memory loss in people. A 2025 study of 12,772 adults over 8 years tied heavy sweetener use to faster decline in thinking, but only in people under 60 (Study). Short trials that gave people aspartame on purpose mostly found no change in memory (Trial, Trial).
The question went viral through a YouTube Short of the author Michael Malice on Joe Rogan’s podcast. The clip, from the channel OptimalHealth, has about 1.8 million views. By his account, Dr Pepper Zero was his “main method of hydration.” He says words and names began to slip. Then, on a trip to New York, he drank full-sugar Dr Pepper and felt his thinking get quicker. The clip goes on to present “impaired memory, spatial learning deficits, and faster cognitive decline in adults under 60” as settled fact.
Each of those phrases traces back to a real study. None of those studies shows what the clip implies: that aspartame harms memory in people.
What Aspartame Is
Aspartame is a low-calorie sweetener about 200 times sweeter than sugar. Your gut breaks it into two amino acids, phenylalanine and aspartic acid, plus a little methanol. Amino acids are the building blocks of protein. Worries about the brain center on these two. A 2018 review proposed that aspartame may raise them in the brain and lower brain messengers such as dopamine and serotonin (Review). The same authors add that the findings are mixed and that more research is needed.
A 2007 review came down on the other side. It looked at doses up to 50 mg per kg, which is the US limit. It found these amino acids rise no higher than they do after an ordinary meal (Review). Keep in mind that Ajinomoto, a company that makes aspartame, paid for that review (News report). The authors say their expert panel did not know who was paying (Review).
Regulators set a safety limit called the acceptable daily intake, or ADI. The WHO and the FAO share an expert food committee called JECFA. In 2023, it kept the limit at 40 mg per kg of body weight a day (News release). The US FDA uses a higher limit of 50 mg per kg (Guidance). In WHO’s example, a 70 kg adult would need more than 9 to 14 cans of diet soda a day to go over the 40 mg limit. That assumes each can holds 200 to 300 mg and no aspartame comes from other foods.
So how much is in Malice’s drink? Dr Pepper’s Canadian product pages list the amounts. A 355 mL (12 oz) can of Diet Dr Pepper listed 184 mg of aspartame (Product page). A can of Dr Pepper Zero Sugar lists about 119 mg. It adds about 36 mg of a second sweetener, acesulfame potassium, and 41 mg of caffeine (Product page). The US version, launched in 2021, also uses a blend of aspartame and acesulfame potassium (Article). Its exact amounts may differ from the Canadian ones.
For a 70 kg adult, the 40 mg limit works out to about 15 cans of Diet Dr Pepper a day. For Zero Sugar, it is about 23 to 24 cans.
One group must avoid aspartame entirely. People with phenylketonuria (PKU), a rare inherited condition, cannot break down phenylalanine. The safety limit does not apply to them (Press release). That is why the label warns that the drink contains phenylalanine.
In 2023, the WHO’s cancer agency, IARC, also labeled aspartame “possibly carcinogenic” to humans, which means it might cause cancer. It based that on limited evidence about one type of liver cancer (News release). That label is about cancer, not memory.
The Brazil Study Behind the Clip
The “under 60” line comes from ELSA-Brasil, a long-running study of Brazilian civil servants. In the 2025 analysis, researchers followed 12,772 of them for about 8 years (Study). Their average age at the start was 52. They took thinking and memory tests three times: at the start, in the middle and at the end.
At the start, each person filled in one food questionnaire about the past year. From it, the team estimated how much of seven sweeteners each person had, aspartame included. The average was about 92 mg a day, all seven combined (Study). The top group averaged 191 mg a day. The American Academy of Neurology says that, for aspartame, this equals about one can of diet soda. The sweetener people ate most was sorbitol, not aspartame (Press release).
Split by age, the link showed up only in people under 60. In that group, the top sweetener users declined faster on overall thinking. They also declined faster on word fluency, which is how quickly words come to mind. In people 60 and older, the study found no link (Study). The academy adds that the link was stronger in people with diabetes (Press release).
The academy also gives a headline number. Across all ages, the top group’s thinking scores fell 63% faster than the bottom group’s. That is about 1.6 years of extra aging (Press release). The release was updated after a July 2026 correction, one of two the paper has had (Study). Harvard Health’s earlier brief reports 62% and ties that figure to people under 60 (Article). The release gives no percentage for that group alone, so don’t lean hard on any one number.
Andrew Budson teaches neurology at Harvard Medical School. In a Harvard Health brief, he called it “curious” that the study found no link in people over 60. In his view, small effects are hard to spot in older adults, whose memory changes vary widely. He does not read the result as proof that sweeteners are harmless after 60 (Article).
Six of the seven sweeteners, aspartame among them, were each tied to faster decline in thinking. The one exception was tagatose, a rare sugar (Study). Erythritol was one of the six. It has its own debate over blood vessels, covered in our look at erythritol and stroke risk.
The study has limits. People reported their diet only once, on a questionnaire about 8 years before the last test (Article). People who dropped out of the study may differ from those who stayed. Other habits that go along with sweetener use could also explain part of the link (Study).
Or the link could run the other way. People in poorer health, such as those with diabetes, may be more likely to choose diet drinks (Study). If those conditions also affect the brain, the diet drink would be a sign of poor health, not the cause.
The researchers themselves are careful. They say their results “suggest the possibility of long-term harm” (Study). The academy notes the study found links and “did not prove that they were a cause” (Press release). Harvard’s headline says sweeteners “may” speed decline. The Short states it as fact.
Testing Aspartame and Memory Loss
The clip’s “spatial learning deficits” come from mice. In a 2023 study, male mice drank water with aspartame for up to 16 weeks (Study). The doses were 7% to 15% of the FDA limit, scaled to body size. The authors say that matches 2 to 4 small, 8 oz diet sodas a day for a person.
The mice learned a maze more slowly than mice on plain water. They also did worse on a test of spatial working memory, which is memory for where things are. Their sons and daughters never drank aspartame, yet they showed similar problems. The grandchildren, bred from one dose group, mostly did not (Study).
The study was small: each group had only 8 male mice. Once the mice had learned the maze, aspartame did not hurt how well they remembered it. The mothers also drank aspartame water for 1 to 5 days during mating. So the problems in the offspring may not come only from the fathers (Study). Above all, these were mice. Mouse studies are good at raising questions, but they can’t tell us what happens in people.
What happens when people get aspartame on purpose? A few small trials have tested it. In a 1998 trial, 48 healthy young adults spent 20 days each on aspartame, on sugar and on a dummy (Trial). A coin flip decided the order. Neither the volunteers nor the researchers knew which was which. Some took 15 mg per kg a day. Others took 45 mg per kg, about the aspartame in 17 cans of Diet Dr Pepper for a 70 kg adult.
Phenylalanine in their blood went up. Yet no test of memory, thinking, brain waves or behavior changed (Trial). An even smaller 1990 trial gave 10 adults one dose of aspartame and, on a separate day, a dummy. Their recall of a word list 2 and 24 hours later did not differ between the two days. Neither did their scores on attention and arithmetic tests (Trial).
A 2014 trial found something different. Twenty-eight healthy university students each ate two diets for 8 days, one high in aspartame and one low. They had a 2-week break between the two (Trial). The high diet gave 25 mg per kg a day, against 10 mg on the low one. Even the high dose sat well below the safety limit. Both diets contained aspartame, so the trial compared a high dose with a low one, not aspartame with none.
On the high diet, people were more irritable and more depressed. They also did worse on a test of spatial orientation, the sense of where things are around you. Their working memory did not change (Trial).
So the trials partly disagree. One found mood and orientation changes below the safety limit. The others found no change, even at higher doses. All of them were small, and nobody stayed on aspartame for more than 20 days. These trials can’t prove that aspartame is safe for memory. They can’t prove that it harms memory either.
The big reviews split the same way. In 2013, Europe’s food safety agency, EFSA, concluded that aspartame does not harm the brain, behavior or thinking of children or adults. It judged the 40 mg limit safe for most people, though not for people with PKU (Press release). The 2018 review called the data mixed and urged caution (Review).
Is Full-Sugar Soda Better?
Malice’s switch felt like proof, and his sharper thinking may well have been real. But a lot changed at once. He swapped a diet soda for a sugary one, so he took in sugar and calories he had not before. He was also on a trip, which changes routine and setting. He knew what he was drinking, and there was nobody to compare him with. That is why trials hide what people drink and use a comparison group.
Larger studies point in two directions. The Framingham Heart Study followed adults for up to 10 years (Study). People who drank a diet soft drink every day had about three times the risk of ischemic stroke as people who drank none. That is the kind of stroke caused by a blocked blood vessel. Daily drinkers also had a higher risk of Alzheimer’s dementia.
The dementia result rests on just 81 people who developed dementia, 63 of them with Alzheimer’s. It also faded once the team accounted for diabetes, blood pressure and other heart risks. The stroke link held (Study). One more check left out people with diabetes. That check took only basics such as age and sex into account, and the dementia link was still there.
The authors say they cannot tell whether diet drinks raised the risk or people with diabetes simply chose diet drinks. The study also lumped all diet drinks together, so it cannot single out aspartame.
In the same analysis, sugary drinks were not linked to stroke or dementia (Study). Does that make sugar the safer pick? A second Framingham paper says no (Study). It took a one-time snapshot of about 4,000 adults, using memory tests and brain scans.
People who had more than 2 sugary drinks a day had smaller brains and poorer memory than those who had less than 1. The authors put the brain-size gap at about 2 years of extra brain aging. Sugary drinks here meant soda, fruit drinks and fruit juice. The survey could not tell juice with added sugar from juice without. Even one glass of fruit juice a day went with a smaller brain and weaker memory (Study). When the team also accounted for diet and exercise, the brain-size link weakened, but the memory link held.
Diet soda showed up in that paper too. It was linked to a smaller brain, but not to worse memory (Study). The authors warn that some findings may be due to chance. A snapshot also can’t show which came first, so none of this proves cause. Neither soda has been shown to be good for your brain.
Frequently Asked Questions
Is Aspartame Safe?
Within the official limit, regulators say yes. WHO’s expert committee kept that limit at 40 mg per kg a day in 2023 (News release). EFSA found no harm to the brain or thinking at the amounts people now eat (Press release). The evidence on memory in people is still unsettled. People with PKU must avoid aspartame.
How Much Aspartame Is Too Much?
The official limit is 40 mg per kg of body weight a day, or 50 mg in the US (Guidance). For a 70 kg adult, WHO puts that at more than 9 to 14 cans of diet soda a day. One small trial found more irritability and low mood at 25 mg per kg, compared with a lower dose (Trial). For the same adult, that is roughly 10 cans of Diet Dr Pepper or 15 of Zero Sugar.
Is Aspartame Worse Than Sugar?
For the brain, studies in people don’t give a clear answer. In Framingham, daily diet soda was tied to more dementia over 10 years, but the link faded after accounting for diabetes (Study). In a separate snapshot, sugary drinks went with smaller brains and poorer memory (Study). Both studies watched people rather than testing a drink, so neither proves cause.
Can Diet Soda Affect Your Memory?
It might, but nobody has shown that it does. In Brazil, people under 60 with the highest sweetener intake declined faster on thinking tests (Study). Short trials that gave people aspartame mostly found no memory change (Trial). Only longer trials can settle it.
Key Takeaways
- The Short states a maybe as fact. Harvard’s report says sweeteners may be linked to faster decline in thinking and memory. It does not say they cause it (Article).
- The under-60 signal has soft spots. People reported their diet once, at the start (Article). People with health problems may simply choose diet drinks more often (Study).
- The maze problems were in mice. Mice on aspartame learned a maze more slowly. By the authors’ math, the doses matched 2 to 4 small diet sodas a day (Study).
- Short human trials mostly found nothing. Twenty days of high doses changed no test in 48 young adults (Trial). One 8-day trial found more irritability and worse spatial orientation, but no memory change (Trial).
- Sugar is not the fix. More than 2 sugary drinks a day went with smaller brains and poorer memory (Study).
- Not proven harmful, not proven harmless. Regulators judge intakes within the limit safe for most people (Press release). The trials that tested memory in people lasted weeks, not years.
Make Water Your Main Drink Again
Malice’s phrase, “main method of hydration,” is the most useful part of the clip. If any soda, diet or regular, is your main drink, there is a simple way out of this debate. Water, sparkling water, unsweetened tea and plain coffee raise none of these sweetener questions.
If you have noticed brain fog or mood changes, try a small test. Drop diet soda for two weeks and notice how you feel. It won’t prove anything, but it costs nothing. If the fog stays, see a doctor, since many things can cause it.
People with PKU must avoid aspartame. For most other adults, an occasional can sits far below the limits regulators set. This piece covers the brain only. Sweeteners and the gut are a separate question, covered in artificial sweeteners and gut health.
Settling the question would take longer trials. Those would give people aspartame or a dummy for months and test their memory before and after. Until such trials exist, the fair answer is that nobody knows for sure.
The evidence on aspartame and memory is still unsettled. Water is the one drink nobody is arguing about.
This article is for educational purposes and is not medical advice. Talk to a qualified clinician before changing your health regimen.
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