Diet Pepsi, Aspartame, and Other Sweeteners: What the 2023 WHO Findings and Recent Studies Mean

In 2015, PepsiCo announced that Diet Pepsi in the United States would no longer contain aspartame, responding to declining sales and consumer worries about the sweetener. The replacement blend of sucralose and acesulfame potassium drew its own criticism, with headlines warning that the company had swapped one harmful sweetener for another. Sales did not recover, and within a few years the aspartame-sweetened version returned, first as a separate “classic” product and later as the main formula.

The episode illustrates how consumer concern about artificial sweeteners shapes products, even as the science remains debated. In 2023, the World Health Organization’s cancer research agency classified aspartame as possibly carcinogenic to humans, while a separate WHO and FAO expert committee reaffirmed that it is safe within an acceptable daily intake. This article explains what each sweetener is, what research shows about aspartame, sucralose, acesulfame potassium, and newer sweeteners, and how to make sense of conflicting headlines.

Is aspartame in diet soda bad for you? For most people at normal intake, no clear harm is established. In 2023 the WHO’s cancer agency labeled aspartame “possibly carcinogenic,” a low-certainty classification, while its food-safety committee kept the long-standing acceptable daily intake, meaning typical diet-soda consumption stays within safe limits. Water is still the better default, but moderate diet soda is not proven dangerous.

What Sweeteners Are in Diet Sodas?

Diet sodas are sweetened with low- or no-calorie sweeteners such as aspartame, acesulfame potassium, sucralose, and sometimes stevia, instead of the sugar or high-fructose corn syrup in regular soda. Aspartame, discovered in 1965 and approved in the United States in the 1980s, is about 200 times sweeter than sugar. It is made from two amino acids, aspartic acid and phenylalanine, and breaks down in the body into these amino acids and a small amount of methanol, amounts comparable to those from some foods such as fruit juices. Because it contains phenylalanine, products with aspartame must carry a warning for people with phenylketonuria, a rare inherited disorder.

Sucralose, about 600 times sweeter than sugar, is made by chemically modifying sucrose with chlorine atoms. Most passes through the body unabsorbed.

Acesulfame potassium, about 200 times sweeter than sugar, is often blended with other sweeteners to improve taste. It is excreted largely unchanged.

Stevia-derived sweeteners, such as rebaudioside A and M, come from the stevia plant and are increasingly used in “natural” diet drinks.

Sugar alcohols, such as erythritol, are used in many low-sugar products and some beverages.

What Did the WHO Say About Aspartame in 2023?

In 2023 the WHO’s cancer agency (IARC) classified aspartame as “possibly carcinogenic,” a low-certainty category, while its food-safety committee reaffirmed the existing acceptable daily intake, so typical consumption remains within safe limits. In July 2023, two WHO-affiliated bodies released assessments of aspartame on the same day. According to WHO, the International Agency for Research on Cancer, citing limited evidence for carcinogenicity in humans, classified aspartame as possibly carcinogenic to humans (Group 2B), while the Joint Expert Committee on Food Additives reaffirmed the acceptable daily intake of 40 milligrams per kilogram of body weight. WHO noted that with a can of diet soft drink containing 200 to 300 milligrams of aspartame, a 70-kilogram adult would need to drink more than 9 to 14 cans per day to exceed the acceptable daily intake.

A summary of the IARC evaluation in The Lancet Oncology by Elio Riboli and colleagues explained that the classification was based largely on limited evidence linking aspartame to liver cancer in a few observational studies, with limited evidence in experimental animals and limited mechanistic evidence. It is important to understand what these two assessments mean. IARC evaluates hazard, whether something could cause cancer under some circumstances, without considering how much people consume. Group 2B also includes substances such as aloe vera extract and pickled vegetables in traditional Asian preparation. JECFA evaluates risk at actual exposure levels and concluded there was no convincing evidence of harm at intakes below the acceptable daily intake.

Do Artificial Sweeteners Cause Cancer or Other Harm?

Large studies have not established that approved sweeteners cause cancer at normal intake; some observational studies link heavy diet-drink consumption to health risks, but these cannot prove the sweeteners are the cause. A 2022 analysis of the French NutriNet-Santé cohort in PLOS Medicine by Charlotte Debras and colleagues followed 102,865 adults for a median of 7.8 years, using detailed dietary records that included brand names. Higher consumers of artificial sweeteners, especially aspartame and acesulfame potassium, had a modestly higher risk of cancer overall compared with non-consumers. The authors called for re-evaluation of sweeteners by health agencies. As an observational study, it cannot rule out that other differences between high and low consumers explain the association, such as underlying weight or health conditions that lead people to choose diet products.

Concerns have extended to other sweeteners. A 2023 study in Nature Medicine by Marco Witkowski and colleagues found that higher blood levels of erythritol, a sugar alcohol used in many low-sugar products, were associated with higher risk of major cardiovascular events in several patient groups, and laboratory experiments suggested erythritol may enhance blood clotting. Because the body also produces erythritol, and the study populations were at high cardiovascular risk, further research is needed to clarify these findings.

Sucralose has been studied for effects on gut bacteria and glucose metabolism, with mixed results, and some research has raised questions about a compound formed when sucralose is heated or metabolized. Regulators continue to consider sucralose safe within acceptable daily intakes.

WHO’s Guidance on Weight Control

In May 2023, WHO issued a guideline recommending that non-sugar sweeteners not be used as a means of achieving weight control or reducing the risk of noncommunicable diseases. The recommendation was based on evidence that non-sugar sweeteners do not provide long-term benefits for reducing body fat in adults or children and on observational findings of possible associations with type 2 diabetes, cardiovascular disease, and mortality in long-term use. WHO described the recommendation as conditional and noted that it does not apply to people with existing diabetes. The guidance encourages reducing sweetness in the diet overall, for example by choosing water, unsweetened drinks, and whole fruit.

A Brief History of Aspartame Approval

Aspartame was discovered accidentally in 1965 by a chemist working on an ulcer drug. The FDA first approved it in 1974, but the approval was stayed after objections about the quality of safety studies, and a public board of inquiry reviewed the evidence. The FDA approved aspartame for dry foods in 1981 and for carbonated beverages in 1983. Since then, it has been reviewed by regulators around the world. In 2013, the European Food Safety Authority completed a full re-evaluation of aspartame, including newer studies, and concluded it was safe at the acceptable daily intake of 40 milligrams per kilogram of body weight. The FDA’s acceptable daily intake is set slightly higher at 50 milligrams per kilogram. The controversial history of its approval has fueled persistent public distrust.

Do Sweeteners Affect the Gut Microbiome?

Some studies suggest certain artificial sweeteners can alter gut bacteria and glucose responses in some people, but the findings are mixed and their long-term health significance is not yet clear. A growing area of research examines whether non-sugar sweeteners affect gut bacteria and, through them, metabolism. A 2014 study in Nature by Jotham Suez and colleagues reported that commonly used sweetener formulations induced glucose intolerance in mice by altering gut microbiota, with similar effects observed in some healthy human volunteers. A 2022 randomized trial in Cell by the same research group gave healthy adults saccharin, sucralose, aspartame, or stevia in amounts below the acceptable daily intake for two weeks. Each sweetener altered the gut and oral microbiome and blood metabolites, and saccharin and sucralose significantly impaired glucose responses as groups, with effects varying between individuals depending on their microbiome. These findings suggest sweeteners are not metabolically inert, though the long-term health significance remains uncertain.

The Animal Studies Behind the Controversy

Much of the concern about aspartame and cancer stems from rodent studies conducted by the Ramazzini Institute in Italy in the 2000s, which reported increased lymphomas and leukemias in rats fed aspartame over their lifetimes, including at doses relevant to human exposure. Regulators in Europe and the United States reviewed these studies and identified methodological concerns, including the possibility that respiratory infections common in the rat colony affected results, and did not change their conclusions. IARC considered these and other animal studies in 2023 and rated the animal evidence as limited. Supporters of stronger regulation argue that the Ramazzini findings deserve more weight, and the debate illustrates how scientists can interpret the same data differently.

Reading Sweeteners on Labels

In the United States, sweeteners appear by name in ingredient lists: aspartame, sucralose, acesulfame potassium (sometimes “acesulfame K”), saccharin, stevia leaf extract or rebaudioside, monk fruit extract, and sugar alcohols such as erythritol, xylitol, and sorbitol. In the European Union and many other countries, they may appear as E numbers: E951 for aspartame, E955 for sucralose, E950 for acesulfame potassium, E954 for saccharin, E960 for steviol glycosides, and E968 for erythritol. Aspartame-containing products must carry a phenylalanine warning. Many products use blends of two or more sweeteners to improve taste.

Sweeteners appear in more products than many people realize, including flavored yogurts, protein bars, chewing gum, “light” juices, some breads and sauces, children’s medicines, and toothpaste. People who want to limit sweeteners can check ingredient lists across these categories, not just diet sodas.

Are Artificial Sweeteners Safe for Children?

Approved sweeteners are considered safe within intake limits, but health authorities generally recommend water and milk over diet sodas for children, since sweet drinks of any kind are not needed in their diets. Children are increasingly exposed to non-sugar sweeteners through reduced-sugar products. The American Academy of Pediatrics has noted that evidence on long-term effects in children is limited and has encouraged clear labeling of sweetener content. Because children have lower body weights, the same amount of sweetener represents a higher dose per kilogram. Most experts advise that water and milk be children’s main beverages, limiting both sugary and artificially sweetened drinks.

Diet Drinks and Long-Term Health

Large cohort studies have examined whether diet drinks are associated with long-term health outcomes. A 2019 analysis in Circulation by Vasanti Malik and colleagues followed more than 118,000 U.S. health professionals for decades. Sugar-sweetened beverage consumption was associated with higher total and cardiovascular mortality in a dose-dependent way. Artificially sweetened beverages were associated with higher total and cardiovascular mortality only at high intakes, four or more servings per day, and only among women, which the authors said required further confirmation. Replacing sugar-sweetened beverages with artificially sweetened ones was associated with modestly lower mortality risk, though replacing them with water was preferable.

Such studies are prone to reverse causation: people who are already overweight or have diabetes are more likely to choose diet drinks, which can make diet drinks appear harmful. Researchers try to adjust for this, but residual confounding remains a concern. Overall, the evidence suggests that switching from sugary drinks to diet drinks is likely better than continuing sugary drinks, and that water or unsweetened beverages are better still.

Sugar Versus Sweeteners

Concerns about sweeteners should not obscure the established harms of excess sugar. High intake of added sugars, especially from sugary drinks, is linked to weight gain, type 2 diabetes, heart disease, and tooth decay. WHO recommends limiting free sugars to less than 10 percent of daily calories, ideally less than 5 percent, and U.S. dietary guidelines recommend less than 10 percent. For many people, the most effective change is reducing sweetened drinks of any kind and becoming accustomed to less sweet tastes, which research suggests is possible over a few weeks.

How Acceptable Daily Intakes Are Set

Regulators set acceptable daily intakes using animal and human studies. They identify the highest dose that caused no observed adverse effect in long-term animal studies, then divide it by a safety factor, typically 100, to account for differences between animals and humans and among people. The resulting ADI represents an amount that can be consumed daily over a lifetime without appreciable risk. Exceeding the ADI occasionally does not mean harm will occur, because of the built-in margin. Critics argue that ADIs may not capture subtle effects, such as changes in gut bacteria or effects of breakdown products, that were not measured in older studies, which is why regulators periodically re-evaluate sweeteners as new research appears.

New Questions About Sucralose

Research on sucralose has raised questions about compounds related to it. A 2023 laboratory study in the Journal of Toxicology and Environmental Health, Part B by Susan Schiffman and colleagues reported that sucralose-6-acetate, an impurity in sucralose manufacturing that may also form in the gut, was genotoxic in cell-based screening assays and affected gene expression in human intestinal tissue models, and estimated that a single sucralose-sweetened drink might exceed a threshold of toxicological concern for genotoxicity. These were laboratory findings, and regulators have not changed sucralose’s approved status, but the study illustrates why ongoing research and re-evaluation are important. Studies have also found that heating sucralose to high temperatures, as in baking, may generate potentially harmful chlorinated compounds, leading some experts to advise against using it in high-heat cooking.

Comparing Common Sweeteners

Sweetener	Sweetness vs sugar	Notable points
Aspartame	~200×	IARC Group 2B (2023); ADI 40 mg/kg (JECFA); PKU warning required
Sucralose	~600×	Mostly unabsorbed; ongoing research on gut and metabolic effects
Acesulfame potassium	~200×	Often blended; linked with aspartame in French cohort cancer association
Stevia extracts	~200–400×	Plant-derived; highly purified extracts considered safe
Erythritol	~0.6–0.8×	Sugar alcohol; 2023 study linked blood levels with cardiovascular events

What the Diet Pepsi Story Shows

PepsiCo’s experience reflects a broader pattern. Companies respond to consumer concerns by reformulating, but replacement ingredients can raise their own questions, and taste changes can alienate loyal customers. After Diet Pepsi removed aspartame in 2015, some consumers complained about the taste and sales did not improve, prompting the company to bring back an aspartame version in 2016 and later return to aspartame as the main sweetener. Other beverage companies have shifted toward stevia blends or zero-calorie options marketed as “natural.” For consumers, the lesson is that “free of” claims do not necessarily mean a product is healthier; the replacement ingredients matter too.

Practical Advice

Moderation is reasonable. Occasional diet soda is unlikely to pose meaningful risk for most people, and staying well below the acceptable daily intake is easy for typical consumers.

Water and unsweetened drinks are best. Plain or sparkling water, unsweetened tea, and coffee provide hydration without sweeteners or sugar.

Diet drinks may help some people cut sugar. For people who drink sugary sodas, switching to diet versions reduces sugar and calories, though WHO advises against relying on sweeteners for long-term weight control.

People with PKU must avoid aspartame. Check labels for the phenylalanine warning.

Children and pregnancy. Many experts advise limiting sweetened beverages of all kinds for children; pregnant people can discuss sweetener use with their clinician.

Common Myths About Sweeteners

“Aspartame turns into formaldehyde and poisons you.” Aspartame breaks down partly into methanol, which the body converts to formaldehyde and then formic acid, but amounts from typical intake are small and comparable to those from foods such as fruit juices and tomatoes.

“IARC said aspartame causes cancer.” IARC classified it as possibly carcinogenic based on limited evidence; this reflects hazard, not a demonstrated risk at typical intake.

“Natural sweeteners are always safe.” Stevia and monk fruit extracts are considered safe, but erythritol, often marketed as natural, has raised new questions, and any sweetener can be overconsumed.

“Diet soda is as bad as regular soda.” Evidence suggests sugary drinks carry clearer risks; diet drinks are not ideal, but they avoid added sugar.

“Sweeteners cause cravings for sweets.” Research on whether sweeteners increase sugar cravings is mixed; individual experiences vary.

Key Terms

Acceptable daily intake (ADI): The amount of a substance that can be consumed daily over a lifetime without appreciable health risk.

Non-sugar sweetener: A sweetener providing little or no calories, such as aspartame, sucralose, or stevia.

IARC Group 2B: “Possibly carcinogenic to humans,” based on limited evidence.

Phenylketonuria (PKU): An inherited disorder in which the body cannot process phenylalanine, requiring avoidance of aspartame.

Sugar alcohol: A carbohydrate such as erythritol or xylitol used as a reduced-calorie sweetener.

The Bottom Line

Diet Pepsi’s brief switch from aspartame to sucralose and acesulfame potassium, and its later return to aspartame, reflects persistent consumer worries about sweeteners. In 2023, IARC classified aspartame as possibly carcinogenic based on limited evidence, while WHO and FAO experts reaffirmed an acceptable daily intake that typical consumers rarely approach. Observational studies have linked high sweetener intake to modestly higher cancer and cardiovascular risks, and trials show sweeteners can alter gut bacteria and glucose responses, but causation is unproven. WHO advises against relying on sweeteners for weight control. For most people, occasional diet drinks are a better choice than sugary ones, and water or unsweetened drinks are best.

Regulatory Differences Around the World

Countries do not always agree on sweeteners. Cyclamate was banned in the United States in 1970 after rat studies raised cancer concerns, but it remains permitted in the European Union, Canada (as a tabletop sweetener), and many other countries, where later reviews found it safe within limits. Saccharin carried a U.S. warning label for years after rat bladder cancer studies, until research showed the mechanism did not apply to humans and the warning was removed in 2000. Approval of newer sweeteners, such as specific stevia components and monk fruit extracts, also varies by country. These differences show that sweetener regulation involves scientific judgment and changes as evidence develops.

Stevia and Monk Fruit

Highly purified extracts from the stevia plant, mainly steviol glycosides such as rebaudioside A and M, have GRAS status in the United States and are approved in many countries. Whole stevia leaves and crude extracts have not been approved as food additives in the U.S. because of limited safety data. Monk fruit (luo han guo) extracts, sweetened by compounds called mogrosides, also have GRAS status. These sweeteners are often marketed as natural alternatives, and research to date has not raised major safety concerns, though long-term studies are limited. Many products blend them with erythritol, so checking labels for all sweeteners is useful.

Key Facts at a Glance

IARC classified aspartame as possibly carcinogenic (Group 2B) in July 2023, citing limited evidence, while JECFA reaffirmed an acceptable daily intake of 40 mg per kilogram of body weight; a 70-kilogram adult would need more than 9 to 14 cans of diet soda daily to exceed it. A French cohort of 102,865 adults associated higher intake of aspartame and acesulfame potassium with modestly higher cancer risk. A 2022 trial found sucralose and saccharin impaired glucose responses in healthy adults. WHO advised in 2023 against using non-sugar sweeteners for weight control.

Sweeteners During Pregnancy and Breastfeeding

Approved sweeteners are considered acceptable in pregnancy within acceptable daily intakes, with the exception that people with phenylketonuria must avoid aspartame. Some observational studies have reported associations between sweetened beverage consumption during pregnancy and outcomes such as preterm birth or childhood weight, but these findings are not consistent and may reflect other factors. Many clinicians advise moderation and emphasize water, milk, and unsweetened drinks during pregnancy. People with specific concerns can discuss sweetener use with their obstetric provider.

Further Reading

WHO publishes its 2023 non-sugar sweetener guideline and its aspartame assessment summaries online. The FDA and the European Food Safety Authority provide information on approved sweeteners and acceptable daily intakes. Peer-reviewed studies cited in this article offer detail on cancer, cardiovascular, and metabolic research. Reading regulatory assessments alongside new studies helps put headlines into context.

What Is Still Unknown

Several important questions about sweeteners remain open. Researchers do not know whether the gut microbiome changes seen in short trials persist and whether they affect long-term health. Observational associations with cancer and cardiovascular disease need confirmation in studies designed to address reverse causation. The effects of lifelong exposure beginning in childhood, now common because of widespread use in foods, have not been studied directly. The health effects of sweetener blends and breakdown products are less studied than individual sweeteners. Regulators in Europe and elsewhere continue to re-evaluate approved sweeteners as new data emerge, and conclusions may change.

Practical Ways to Cut Back

People who want to reduce both sugar and sweeteners can try gradual steps: mixing diet or regular soda with sparkling water, flavoring water with citrus, cucumber, or mint, choosing unsweetened tea and coffee and adjusting slowly to less sweetness, and checking labels on yogurt, cereals, and protein bars for added sugars and sweeteners. Many people find that preferences for sweetness decline after a few weeks of lower intake.

Readers with specific health conditions, such as diabetes, kidney disease, or phenylketonuria, should discuss sweetener choices with their clinician or dietitian, since needs vary.

For most healthy adults, the evidence supports a measured view: diet drinks are not health foods, but occasional consumption within acceptable intakes is unlikely to pose meaningful risk, and they are a reasonable step down from sugary drinks for people trying to cut sugar.

How Diet Pepsi Compares Today

Beverage formulations change frequently, and the same brand may use different sweeteners in different countries or product versions, such as caffeine-free, zero-sugar, or flavored varieties. Checking the ingredient list on the specific can or bottle is the only reliable way to know which sweeteners a product contains.

Shoppers who want to avoid particular sweeteners can also look for products labeled unsweetened, which contain neither sugar nor non-sugar sweeteners, though labels such as “no added sugar” may still include sweeteners.

Ingredient lists in the European Union and some other regions may show sweeteners as E numbers, so knowing the common codes helps when shopping abroad: E951 for aspartame, E955 for sucralose, and E950 for acesulfame potassium.

Frequently Asked Questions

Is aspartame bad for you?

For most people consuming normal amounts, there is no clearly established harm. In 2023 the WHO’s cancer agency classified aspartame as “possibly carcinogenic,” a low-certainty label, while its food-safety committee kept the long-standing acceptable daily intake. That intake is far above what typical diet-soda drinkers consume, so moderate use stays within limits regulators consider safe.

Does aspartame cause cancer?

The evidence is weak and uncertain. The IARC placed aspartame in its “possibly carcinogenic” category, the same group that includes aloe vera extract, based on limited evidence. Large human studies have not convincingly shown that aspartame causes cancer at normal intake levels, and major food-safety bodies continue to consider it acceptable within established daily limits.

Is diet soda healthier than regular soda?

Diet soda avoids the sugar and calories of regular soda, which can help with weight and blood sugar compared with sugary drinks. However, the WHO advises against relying on non-sugar sweeteners for long-term weight control, and some studies link heavy diet-drink use to health risks. Water remains the healthiest default, with diet soda a lower-sugar alternative.

How much aspartame is safe to consume?

Regulators set an acceptable daily intake that, for aspartame, corresponds to a large number of diet sodas per day for an average adult, far more than most people drink. People with the genetic condition phenylketonuria (PKU) must avoid aspartame because they cannot process one of its components, which is why products carry a PKU warning.

Do artificial sweeteners affect blood sugar or gut bacteria?

Possibly in some people. Although sweeteners like aspartame have few or no calories and little direct effect on blood sugar, some research suggests certain sweeteners may alter gut bacteria and glucose responses in some individuals. The findings are mixed and their long-term importance is unclear, so the overall health impact remains an active area of study.

References

1. World Health Organization. Aspartame hazard and risk assessment results released. July 14, 2023. who.int
2. Riboli E, Beland FA, Lachenmeier DW, et al. Carcinogenicity of aspartame, methyleugenol, and isoeugenol. Lancet Oncology. 2023;24(8):848–850. PMID 37454664
3. Debras C, Chazelas E, Srour B, et al. Artificial sweeteners and cancer risk: Results from the NutriNet-Santé population-based cohort study. PLOS Medicine. 2022;19(3):e1003950. PMID 35324894
4. Witkowski M, Nemet I, Alamri H, et al. The artificial sweetener erythritol and cardiovascular event risk. Nature Medicine. 2023;29(3):710–718. PMID 36849732
5. World Health Organization. Use of non-sugar sweeteners: WHO guideline. 2023. who.int
6. Suez J, Korem T, Zeevi D, et al. Artificial sweeteners induce glucose intolerance by altering the gut microbiota. Nature. 2014;514(7521):181–186. PMID 25231862
7. Suez J, Cohen Y, Valdés-Mas R, et al. Personalized microbiome-driven effects of non-nutritive sweeteners on human glucose tolerance. Cell. 2022;185(18):3307–3328.e19. PMID 35987213
8. Malik VS, Li Y, Pan A, et al. Long-Term Consumption of Sugar-Sweetened and Artificially Sweetened Beverages and Risk of Mortality in US Adults. Circulation. 2019;139(18):2113–2125. PMID 30882235
9. Schiffman SS, Scholl EH, Furey TS, Nagle HT. Toxicological and pharmacokinetic properties of sucralose-6-acetate and its parent sucralose: in vitro screening assays. Journal of Toxicology and Environmental Health, Part B. 2023;26(6):307–341. PMID 37246822

Last updated: October 6, 2026