Sweeteners, Diet Soda and Cognitive Decline: Evidence or Hype?

Originally published 2025 · Updated April 2026 with the 2025 Neurology eight-year prospective study on sweeteners and cognitive decline and subsequent expert analysis

In September 2025, a study published in Neurology — the medical journal of the American Academy of Neurology — made headlines around the world. It followed more than 12,000 adults for eight years and found that people who consumed the highest amounts of artificial sweeteners showed 62% faster global cognitive decline than those who consumed the lowest amounts — equivalent to approximately 1.6 years of accelerated brain ageing.

The coverage was predictably alarming. Diet soda was declared dangerous. Sweeteners were framed as cognitive poison. The nuance got buried under the headline.

Here is what the study actually found, what its limitations are, and what the broader evidence says about whether your sweetener habit is harming your brain.

TL;DR

• A 2025 study in Neurology followed 12,772 adults over an average of eight years and found that people under 60 who consumed the highest total amounts of artificial sweeteners showed 62% faster declines in thinking and memory skills compared to those who consumed the lowest amounts — equivalent to about 1.6 years of extra brain ageing.
• The sweeteners examined were aspartame, saccharin, acesulfame-K, erythritol, xylitol, sorbitol, and tagatose. Tagatose was the only one not associated with cognitive decline.
• The study was observational — it cannot establish causality. The authors themselves stated: "I cannot say to you that artificial sweeteners cause cognitive decline. We do know, however, that these sweeteners are associated with worse cognitive trajectories."
• No link between artificial sweetener consumption and cognitive decline was found in people over 60.
• The evidence for sweeteners causing harm is associational, not causal. But the evidence that they are entirely safe is also weakening.
• The most evidence-backed position: moderate use is likely fine; daily high-dose consumption warrants caution, particularly in middle age.

What the 2025 Neurology Study Actually Did

The study followed 12,772 adults from Brazil with an average age of 52, of whom 55% were women. Participants completed food frequency questionnaires at the start of the study, detailing what they ate and drank over the past year. Researchers divided them into three groups based on total artificial sweetener consumption — the lowest group consumed an average of 20mg per day, the highest consumed an average of 191mg per day. For aspartame, 191mg is roughly equivalent to one can of diet soda. Sorbitol had the highest average consumption at 64mg per day. Cognitive tests assessing verbal fluency, working memory, word recall, and processing speed were administered at the start, middle, and end of the study.

People under 60 who consumed the largest total amounts of sweeteners showed 62% faster declines in thinking and memory skills compared to those who consumed the lowest amounts. Participants in the middle tier showed a 35% faster rate of global cognitive decline — about 1.3 years of extra ageing — compared to the lowest consumers. When researchers broke results down by age, the link was only found in people under 60. They did not find links in people over 60.

The link to faster cognitive decline was stronger in participants with diabetes than in those without. "People with diabetes are more likely to use artificial sweeteners as sugar substitutes," the authors noted — meaning their overall exposure tends to be higher.

What the Study Cannot Tell Us

This is where careful reading matters more than the headlines.

The study is observational — it tracked what people ate and how their cognition changed, but it cannot establish that the sweeteners caused the cognitive decline. Several important alternative explanations exist:

Reverse causation — people who are beginning to experience cognitive changes may alter their diets in ways that increase sweetener consumption. This is a legitimate concern in observational dietary research.

Confounding from ultra-processed food consumption — the sweeteners studied are found primarily in ultra-processed foods, including diet sodas, flavoured waters, energy drinks, yoghurt, and low-calorie desserts. People who consume high quantities of artificial sweeteners are also consuming high quantities of ultra-processed foods more broadly, which have their own documented associations with cognitive decline. Separating the sweetener effect from the ultra-processed food effect is methodologically difficult.

Dietary assessment limitations — the study used a single food frequency questionnaire at baseline, completed once, to estimate eight years of dietary intake. People's diets change considerably over eight years, and self-reported dietary data is inherently imprecise.

Expert analysis published shortly after the study noted that the reported sweetener intakes — for example, xylitol at 3mg and erythritol at approximately 0.1mg per day — are far below real-world exposures and unlikely to have any biological effect at those concentrations. Emphasis, critics argued, should remain on reducing conventional sugar intake, given its well-established risks for brain and metabolic health.

A detailed critical analysis published in April 2026 concluded that sweeteners — at least based on the evidence presented in this study and many others — remain safe, and that the study cannot establish causation despite establishing an association.

The authors of the study themselves are clear on this: the finding is associational, not causal. It identifies a relationship worth investigating further — not a proven harm.

The Broader Sweetener Evidence: What We Know

The 2025 Neurology study does not exist in isolation. The sweetener evidence base has been developing rapidly, and the picture is more nuanced than either the "completely safe" or "cognitive poison" framings suggest.

Gut Microbiome Effects

This is the most mechanistically plausible pathway through which sweeteners might produce cognitive effects — and the one with the most consistent supporting evidence. Several commonly used sweeteners, including saccharin, sucralose, and acesulfame-K, alter gut bacterial composition in ways that reduce microbial diversity and disrupt the gut-brain axis.

The gut-brain connection is real and well-established — approximately 95% of the body's serotonin is produced in the gut, and gut bacterial metabolites influence neuroinflammation, mood, and cognitive function. A sweetener that disrupts gut microbiome composition could theoretically influence brain function through these indirect pathways. The evidence is preliminary but biologically coherent.

Cardiovascular Concerns

Earlier in 2025, research linked erythritol — a sugar alcohol widely used in keto and low-sugar products — to elevated cardiovascular risk. A Cleveland Clinic study found higher erythritol blood levels were associated with increased risk of heart attack and stroke. The mechanism proposed involves erythritol's effects on platelet aggregation. This finding has been somewhat contested and requires replication, but it was significant enough to generate clinical attention.

Blood Sugar Regulation

The intuition behind sweeteners — that replacing sugar with something sweet but non-caloric would reduce sugar intake and its metabolic consequences — has been questioned by research showing that sweeteners may still trigger insulin responses and potentially dysregulate appetite-regulating hormones. The evidence here is mixed and depends considerably on the specific sweetener and the study population.

What Is Clear

Some things are well established. Sweeteners do not cause tooth decay — a genuine and significant benefit over sugar. They are lower in calories — relevant for weight management. The FDA and EFSA have established acceptable daily intakes for all approved sweeteners based on the available safety data, and the doses used in most dietary contexts remain well within these limits.

What is less clear is the long-term effect of chronic daily exposure, particularly in people who consume sweeteners heavily — multiple diet drinks per day, plus sweetener-containing yoghurt, protein bars, chewing gum, and other products — over years and decades.

The Specific Sweeteners: A Quick Guide

• Aspartame — the most studied. WHO declared it "possibly carcinogenic" in 2023, a classification the FDA disputed. The 2025 Neurology study associated it with cognitive decline. Not recommended in pregnancy.
• Saccharin — one of the oldest artificial sweeteners. Associated with cognitive decline in the 2025 study. Has known effects on gut microbiome composition.
• Acesulfame-K — widely used in combination with aspartame. Associated with cognitive decline in the 2025 study. Less studied than aspartame or saccharin.
• Sucralose — not included in the 2025 Neurology study but has documented effects on gut microbiome. Generally considered safe at normal dietary doses.
• Erythritol — a sugar alcohol found naturally in some fruits. Used widely in keto and low-sugar products. The 2025 cardiovascular research raised concerns that require further investigation.
• Xylitol and sorbitol — sugar alcohols with established dental health benefits. Associated with cognitive decline in the 2025 study at high doses. Well tolerated at moderate levels; high doses cause gastrointestinal effects.
• Stevia and monk fruit — plant-derived sweeteners not included in the 2025 Neurology study. Have fewer documented concerns in the current evidence base and are often recommended as preferable alternatives to synthetic sweeteners.
• Tagatose — the only sweetener in the 2025 study not associated with cognitive decline. A rare sugar with a smaller evidence base.

What to Actually Do With This Information

The honest answer is that the evidence does not yet support eliminating all sweeteners from your diet — but it does support not treating them as unconditionally safe.

The most evidence-aligned position, as stated by the lead author of the 2025 study: "It is wise to limit consumption as much as possible, ideally avoiding daily use."

Practically, this means:

• Diet soda daily is worth reconsidering. Not because one can causes measurable harm, but because it represents a consistent daily exposure to multiple sweeteners — often acesulfame-K and aspartame in combination — over years.
• Occasional use is unlikely to be meaningfully harmful based on current evidence. A diet drink at a social occasion, sweetener in occasional coffee, or a piece of sugar-free gum is not what the concerning research is describing.
• Stevia and monk fruit are preferable alternatives to synthetic sweeteners for people who want to reduce sugar intake without switching entirely to water and unsweetened drinks. The evidence base for harm with these plant-derived options is currently smaller.
• Sparkling water with fruit is the most effective swap for daily diet soda consumers who want to maintain the carbonation ritual without the sweetener exposure.

The bigger picture: the research consistently shows that the best strategy for brain health over time is a diet rich in whole plant foods, healthy fats, and antioxidants — not simply replacing sugar with sweeteners. Certain nutrients, like healthy fats, antioxidants, carotenoids, vitamin E and choline, are particularly beneficial for reducing inflammation and promoting brain health over time.

FAQ

Do artificial sweeteners cause cognitive decline?

The honest answer is: we do not know yet. A major 2025 study in Neurology found a significant association between artificial sweetener consumption and faster cognitive decline in middle-aged adults — equivalent to 1.6 years of accelerated brain ageing at the highest consumption levels. However, the study was observational and cannot establish that sweeteners caused the decline. Confounding from ultra-processed food consumption and methodological limitations mean the finding requires replication and further mechanistic research before causal conclusions can be drawn.

Which sweeteners are most concerning based on current evidence?

The 2025 Neurology study associated six sweeteners with faster cognitive decline: aspartame, saccharin, acesulfame-K, erythritol, xylitol, and sorbitol. Tagatose was the only sweetener studied that was not associated with cognitive decline. Separately, erythritol has been associated with elevated cardiovascular risk in a 2025 Cleveland Clinic study. Plant-derived sweeteners including stevia and monk fruit have fewer documented concerns in the current evidence base.

Is diet soda bad for you?

Diet soda is not proven to be harmful at moderate consumption levels. It does not cause tooth decay and is significantly lower in calories than regular soda. However, it typically contains multiple sweeteners — often aspartame and acesulfame-K in combination — and daily consumption represents a consistent long-term exposure to compounds whose long-term effects are still being characterised. The lead author of the 2025 Neurology study recommends limiting sweetener consumption as much as possible, ideally avoiding daily use.

Are stevia and monk fruit safer than artificial sweeteners?

Based on current evidence, yes — plant-derived sweeteners including stevia and monk fruit have fewer documented safety concerns than synthetic sweeteners like aspartame, saccharin, and acesulfame-K. They were not included in the 2025 cognitive decline study, and the existing evidence base for harm is smaller. They are generally considered preferable alternatives for people who want to reduce sugar intake while limiting artificial sweetener exposure.

How much sweetener is too much?

Regulatory bodies including the FDA and EFSA have established acceptable daily intakes for approved sweeteners based on available safety data, and typical dietary consumption remains within these limits for most people. The 2025 Neurology study's highest consumption group averaged 191mg of total sweeteners daily — roughly equivalent to one can of diet soda. The concern from the study is not acute toxicity but chronic daily exposure over years. Occasional use at moderate levels is unlikely to be meaningfully harmful based on current evidence.

What is the best alternative to diet soda?

Sparkling water — plain or with natural fruit flavour — provides the carbonation and refreshment of diet soda without sweetener exposure. Kombucha is an alternative that provides some carbonation and gut microbiome benefits. Unsweetened green or herbal tea provides antioxidants and, in the case of green tea, mild caffeine without sweetener exposure. For those who prefer sweetness, drinks sweetened with stevia or monk fruit are preferable to those using synthetic sweeteners.

The Bottom Line

The 2025 Neurology study is significant and should be taken seriously — not as proof that sweeteners cause cognitive decline, but as a meaningful signal that warrants both scientific follow-up and individual caution. An eight-year prospective study of nearly 13,000 people finding a consistent association between sweetener consumption and accelerated cognitive decline cannot be dismissed simply because it is observational.

At the same time, the evidence does not justify eliminating all sweeteners from your diet or treating every diet drink as a health hazard. What it does support is treating daily, high-dose sweetener consumption with more caution than the "safe sugar alternative" marketing suggests — particularly in middle age, and particularly through the route of daily sweetened beverages.

The bigger lever for brain health remains what you eat consistently, not what you occasionally avoid. A diet rich in whole plants, quality fats, and diverse fibre produces documented cognitive benefits that dwarf whatever effect moderate sweetener avoidance might achieve.

For a structured approach to reducing sugar and sweetener dependence, the Sugar Reset and Aspartame Reset from the Reset Series cover the practical steps for reducing intake without white-knuckling the craving. Pair them with the Reset Companion for daily nudges and personalised support.

Related reading: Fibremaxxing: What It Is, Whether It Works, and How Much Is Too Much · The Keto Diet: Who It Actually Works For — and Who It Doesn't · Do Ultra-Processed Foods Increase Heart Disease Risk?

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