Of the major artificial sweeteners/sugar substitutes, which is the most likely to be harmful to health in some way?

Sonar Pro

Vote: Aspartame

Aspartame stands out as the most likely to be harmful based on the search results, with multiple studies linking it specifically to serious health risks. A large NutriNet-Santé cohort study (over 100,000 participants) found aspartame associated with increased risk of cerebrovascular events, unlike other sweeteners which were tied to coronary heart disease[1][6]. Harvard Health and Mayo Clinic highlight aspartame's link to stroke, heart disease, and overall mortality[2][6]. Additional concerns include obesity, diabetes, mood disorders, and metabolic effects[1][9]. While acesulfame and sucralose also show CVD risks[1][6], aspartame has broader, consistent mentions across sources, including WHO-noted markers[1]. Natural options like Stevia and Monk Fruit lack negative associations here[1-9]. FDA deems it safe under limits but disputes IARC's possible carcinogen classification, yet observational data on CVD persists[4].

GPT-5.4

Vote: Aspartame

A is the strongest choice because aspartame has the most prominent and credible human-health concern signal among these options: a cancer hazard classification from IARC, which placed it in Group 2B (possibly carcinogenic to humans). That does not prove real-world harm at typical intake, but it makes aspartame the substitute most plausibly linked to a meaningful health risk. Aspartame also has a clear, established adverse effect in a specific population: people with phenylketonuria must avoid it because it provides phenylalanine. That is a genuine health harm pathway, not just a theoretical toxicology signal. By contrast, saccharin’s old cancer scare largely weakened after species-specific rat findings were reinterpreted. Stevia and monk fruit generally have the least concerning profiles. Sucralose and acesulfame potassium have some debated microbiome/metabolic concerns, but the evidence is less publicly established and less singularly compelling than aspartame’s carcinogenicity signal plus the PKU issue. So if forced to pick the major sweetener most likely to be harmful in some way, aspartame is the best-supported answer.

Claude Opus 4.6

Vote: Aspartame

Aspartame is arguably the most likely to be harmful based on the weight of recent evidence and regulatory scrutiny. In 2023, the WHO's International Agency for Research on Cancer (IARC) classified aspartame as "possibly carcinogenic to humans" (Group 2B), making it the only sweetener on this list to receive such a classification from a major international health body. Aspartame is metabolized into phenylalanine, aspartic acid, and methanol — all of which have known toxic effects at high doses. People with phenylketonuria (PKU) must strictly avoid it. While the other sweeteners also have some concerns, aspartame's metabolic breakdown products are uniquely bioactive. Additionally, multiple large-scale observational studies (such as the French NutriNet-Santé cohort) have linked aspartame consumption to increased cancer risk, cardiovascular disease risk, and other health outcomes. While these are observational and not definitive proof of causation, the volume and consistency of concerning findings is greater for aspartame than for the other options listed. Saccharin (D) historically had cancer concerns (bladder cancer in rats), but the mechanism was found to be rat-specific and it was delisted as a carcinogen. Sucralose has some emerging concerns about gut microbiome disruption and thermal degradation products, but the evidence base is less developed. Stevia and monk fruit are generally regarded as safe with minimal concerning evidence. Acesulfame potassium has some older concerns but limited recent damning evidence. Overall, aspartame has the strongest case for being "most likely to be harmful in some way" given the IARC classification, its unique metabolic profile, and the breadth of epidemiological literature raising concerns.

Gemini 3.1 Pro

Vote: Aspartame

Aspartame is the most likely candidate for being harmful to health due to both proven, definitive risks to specific populations and newly recognized potential risks by global health authorities. First, aspartame poses a well-documented, severe risk to individuals with phenylketonuria (PKU). Because it breaks down into phenylalanine, people with this genetic disorder cannot metabolize it, which can lead to severe neurological damage. This is a definitive harm that mandates a universal warning label on all aspartame-containing products. Second, in 2023, the World Health Organization's International Agency for Research on Cancer (IARC) officially classified aspartame as "possibly carcinogenic to humans" (Group 2B) based on limited evidence linking it to a type of liver cancer. While everyday consumption within limits is still considered generally safe by the FDA and WHO, this official carcinogen classification places it under far more scientific and regulatory scrutiny than the other options. Finally, other options like stevia and monk fruit are generally recognized as safe natural alternatives, and saccharin's historical link to cancer in rats was debunked for humans. Given the definitive PKU risk and the WHO's recent cancer classification, aspartame holds the strongest evidence for potential health harms.

Kimi K2.5

Vote: Aspartame

Aspartame carries the most substantial evidence of potential health risks among these options. In 2023, the WHO's International Agency for Research on Cancer classified aspartame as Group 2B—'possibly carcinogenic to humans'—based on limited evidence of hepatocellular carcinoma, while the Joint FAO/WHO Expert Committee maintained but did not lower the acceptable daily intake. This dual signal of carcinogenic concern distinguishes it from the others, which lack such international regulatory warnings. Unlike natural alternatives (E, F) or the other synthetics, aspartame metabolizes into methanol, which converts to formaldehyde and formic acid—compounds with established toxicity at high doses. While dietary quantities are debated, this metabolic pathway creates a plausible mechanism of harm that Stevia and Monk Fruit simply do not possess. Finally, aspartame presents an absolute contraindication for individuals with phenylketonuria (PKU), for whom it is definitively harmful due to its phenylalanine content. No other option on this list carries a comparable mandatory health warning for a genetic population, making aspartame objectively the most likely to cause harm to specific individuals and, based on recent epidemiological data, potentially the general population.

Grok 4.1 Fast

Vote: Aspartame

Aspartame stands out as the most likely to be harmful due to its recent classification by the World Health Organization's IARC as 'possibly carcinogenic to humans' (Group 2B) in 2023, based on limited evidence linking it to liver cancer in humans and stronger evidence in animals. It metabolizes into methanol, which can form formaldehyde—a known carcinogen—and aspartic acid and phenylalanine, posing risks especially for those with phenylketonuria (PKU). Numerous studies and consumer reports associate it with headaches, seizures, and neurological issues. While other sweeteners like saccharin had past cancer scares (cleared for humans), acesulfame-K shows animal cancer risks but lacks human data, and sucralose has gut microbiome concerns, aspartame's combination of regulatory flags, metabolic byproducts, and widespread scrutiny makes it the riskiest. Natural options like stevia and monk fruit have minimal adverse evidence, reinforcing aspartame's position among major substitutes.

Final Summary

The AI Roundtable reached a swift and unanimous verdict, identifying Aspartame as the most likely sweetener to cause harm among the options provided. Models including Gemini 3.1 Pro and GPT-5.4 highlighted that unlike its competitors, Aspartame carries a definitive medical contraindication for those with phenylketonuria (PKU) and a recent 'possibly carcinogenic' classification from the WHO.

All 6 models agreed on "Aspartame" after discussion

Strongest Arguments

• Aspartame: Aspartame is unique because it is the only sweetener on this list with a mandatory health warning for a specific genetic population (PKU) and a Group 2B 'possibly carcinogenic' classification from the IARC, supported by a metabolic pathway that produces potentially toxic methanol and formaldehyde.