Can Diet Sodas Cause Cancer? Making Sense of the WHO's Aspartame Classification

To drink or not to drink? That is the question. Diet sodas have long been in the crosshairs of self-appointed health aficionados -- you don't have to scroll too far through TikTok to see someone telling you to give up sodas altogether, go to the gym and take control of your life. For me, a diet soda lover, it's easy to ignore. Scroll on. 

The latest advice from the World Health Organization isn't quite as easy to scroll past.

Aspartame, a widely used artificial sweetener often found in sugar-free soda and thousands of other food products, has been classified as "possibly carcinogenic to humans" by the WHO's International Agency for Research on Cancer. However, a second group, the Joint WHO and Food and Agriculture Organization's Expert Committee on Food Additives, didn't alter its acceptable daily intake limit.

The updated classification on aspartame, handed down in a hotly anticipated report published by international experts on July 14, was first revealed ahead of time in a Reuters report on June 30. This led to a buffet of worried and alarming headlines, like "Bombshell discovery about Diet Coke" and "Why you must avoid sweeteners like Aspartame." Leapfrogging off the announcement, an opinion piece in Bloomberg suggested "Cancer Risk or No, Diet Soda Is Bad For You."

Considering the ubiquity of diet soda across the world, the new grouping does seem troubling at first. But it's important to note the IARC's classification system doesn't examine risk -- it examines and classifies the types of agents, exposures or other things that may cause cancer, from chemicals to occupational or environmental sources, and lumps them into four different categories. 

For instance, in 2015, the IARC declared processed meats would be classified as group 1 carcinogens, placing foods like bacon in the same group as alcohol, tobacco and asbestos. Aspartame has been placed in category 2B, "possibly carcinogenic to humans," dropping it alongside agents like whole aloe vera extract, carpentry and joinery, gasoline, certain strains of human papillomavirus and working in the textiles manufacturing industry.

These comparisons are slightly confusing. When communicating these findings in the past, it's been difficult to understand just what the new classifications mean: The processed meat ruling in 2015 led to similarly alarming -- and incorrect -- headlines that eating processed meats is somehow as risky as smoking. 

The JECFA evaluation should put diet soda drinkers at ease: Despite IARC's new classification, JECFA saw no reason to alter the acceptable daily intake of aspartame, which is currently set at 0-40 mg/kg body weight. 

However, the WHO is advocating for more research.

"The assessments of aspartame have indicated that, while safety is not a major concern at the doses which are commonly used, potential effects have been described that need to be investigated by more and better studies," said Francesco Branca, director of the WHO department of nutrition and food safety.

So, what's the concern, what does the research show and what does this new categorization mean? How much diet soda can you drink? Or should you change your dietary habits and cut it out? Here's what the evidence says.

What is aspartame?

Aspartame was discovered by accident. While working on an anti-ulcer drug, the chemist James Schlatter mixed the amino acids aspartic acid and phenylalanine, and then -- well, it's not exactly clear. Some sources suggest he accidentally spilled his chemical mixture onto his hand and noticed a distinctly sweet taste when he licked his finger to shuffle some papers. Other accounts suggest Schlatter simply decided to taste his mixture and noted that it "tasted good." Whatever the case, on that day in 1965, aspartame was born. 

It's officially known to chemical scientists as N-L-α-aspartyl-L-phenylalanine-1-methyl ester, but you've likely come across aspartame under its brand names Equal or NutraSweet. You may have also found it listed under various names in ingredient lists, including "sweetener (951)." Per gram, aspartame has a similar amount of calories to sugar, but because it tastes far sweeter, manufacturers can use less in their products to get a similar taste. 

When you ingest products that contain aspartame, it's broken into three pieces: the two amino acids and methanol. This is a process that occurs rapidly when aspartame reaches the gut and does not linger in the blood or organs. The European Food Safety Authority notes (PDF), "Any effect reported to occur in the body following ingestion of aspartame will be caused by one or more of the three constituents." 

The new IARC classification

The IARC classification system is not designed to assess the risk of developing cancer from ingesting aspartame. The process of classifying an agent into one of its four buckets involves studying the available scientific evidence -- from research and experiments to clinical trials -- and evaluating if there is enough of a foundation to say "this could cause cancer."

Take processed meats, which the IARC dubbed a group 1 carcinogen in 2015. The classification system states this agent is "carcinogenic to humans." It may seem like dropping a particular thing into this category means it should be avoided at all costs. 

If you eat bacon, the evidence IARC evaluated suggests you may be at higher risk of certain types of gut cancers. But if you put bacon on your head and wear it like a hat, you're not more likely to get brain cancer. IARC does not evaluate the ways bacon can cause cancer, just the evidence it has the ability to. It may seem obvious a bacon hat won't cause cancer while ingesting it might, but it's an important distinction. The risk of cancer from bacon, then, is likely to be drastically different from other group 1 carcinogens like UV radiation or smoking. 

The IARC's 25 experts evaluated approximately 1,300 studies and placed aspartame into the second lowest category: 2B. (The lowest, category 3, is where "unclassified" agents are placed because of insufficient evidence showing cancer-causing effects.)

In short, the IARC found "limited evidence" aspartame could cause cancer in humans based on three major studies, featuring four major cohorts. Those studies demonstrated a positive association between consumption of artificially sweetened drinks and liver cancer. However, the IARC notes bias and confounding factors cannot be ruled out. 

Similarly, the IARC found "limited evidence" aspartame causes cancer in experiments involving animals and limited evidence that aspartame itself exhibits the characteristics of a cancer-causing agent. Basically, if aspartame doesn't linger in the body and is broken down into its three parts, how does it do damage? Some studies the IARC cites suggest it might cause some inflammation and alter cell proliferation and death. Again, it notes the studies evaluated had flaws.

Safe consumption limits

JECFA has not made any changes to the acceptable daily intake, or ADI, for aspartame and originally set it to 40mg/kg body weight. (The US Food and Drug Administration sets this limit at 50mg/kg.)

That means an 80kg (176-pound) person would need to ingest about 3,200mg of aspartame to reach the limit -- a staggering amount. Considering a standard can of Pepsi Max contains about 125mg, this Pepsi Max fiend would need to drink about 25 cans a day to exceed the JECFA ADI, and even more for the US FDA's limit. Diet Coke is similar, at about 192mg of aspartame per can, so around 19 cans a day.

On the other hand, a study published in the journal PLoS Medicine in March 2022 examined associations between artificial sweeteners aspartame, acesulfame-K and sucralose, and health risks. More than 102,000 adults participated in the study, and the scientists followed their intake over several years. The results suggested there was an increased risk of cancer for those who consumed aspartame, at levels below the current ADI, compared to those who didn't consume artificial sweeteners at all. 

JECFA also assessed evidence for noncancer effects. It considered some of the studies showing association between type 2 diabetes and conditions related to blood flow in the brain as "not convincing." 

Another factor to consider is how aspartame might affect the microbiome -- the billions of bacteria that reside in the body. JECFA found the results were inconsistent, citing one study where both the oral microbiome and the gut microbiome were altered due to aspartame consumption. 

Both the IARC and the WHO are encouraging further, well-designed studies to study how aspartame may affect different aspects of human health and implore research to continue into its association with type 2 diabetes, regulation of insulin and cancer. 

The big question: Should I drink diet soda?

I think a better question to ask, especially if you're a heavy drinker of diet sodas, is: Why do you drink diet soda?

A disclosure: I drink a small lake's worth of Pepsi Max (Australia's far superior version of Pepsi Zero Sugar) every year, sometimes exceeding 50 fluid ounces a day. I have a good chunk of skin in the game here and this updated classification should definitely affect my drinking habits -- or, at the very least, make me think about them. 

I really like the taste of Pepsi Max. I don't drink it because I think it's going to make me thinner or abstain from sugars, or because I think it's a healthier alternative to Pepsi. It's just that the 16 tablespoons of sugar in a bottle of full-strength soda makes my teeth feel furry. 

The WHO recently advised people not to use sugar-free sweeteners as a way to control their weight. Recent studies have shown that replacing a sugar-filled can of Coke with a Diet Coke doesn't carry any significant benefit if you're trying to lose weight. 

Ultimately, after rummaging through the data and talking with experts, I'm not particularly concerned about aspartame's new classification. The new classification feels like a gentle prod for scientists to continue to research aspartame's effects on the body and, perhaps, for consumers to evaluate their drinking habits. But the evidence for its cancer-causing effects is not very strong. 

Aspartame is also present in other products such as chewing gum, sugar-free Jell-O, yogurts and the tabletop sweeteners Equal and NutraSweet. If you're choosing to be aspartame-conscious, it's worth thinking about these products, too.

For me, the best advice comes from my dad -- himself a Pepsi Max drinker: "Everything in moderation, including moderation."