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The WHO declared aspartame “possibly” causes cancer. Here’s what that means.

Get ready for an earful about the health risks of Diet Coke, Trident gum, Equal, and other sugar-free items.

Stacks of boxed Coca-Cola and Coke Zero cans.
Coca-Cola and Coca-Cola Zero can boxes stacked in a Canadian grocery store in March 2015.
Roberto Machado Noa/LightRocket via Getty Images
Keren Landman, MD is a senior reporter covering public health, emerging infectious diseases, the health workforce, and health justice at Vox. Keren is trained as a physician, researcher, and epidemiologist and has served as a disease detective at the US Centers for Disease Control and Prevention.

On July 14, two World Health Organization (WHO) groups released the results of a fresh review of safety data on aspartame, along with consumer guidance on what to do with the results of that review.

The statement of the WHO’s International Agency for Research on Cancer (IARC) indicated they were classifying aspartame as “possibly carcinogenic to humans.” Meanwhile, a separate panel (JECFA, the Joint WHO and Food and Agriculture Organization’s Expert Committee on Food Additives) determined there was no need to change their recommended “acceptable daily intake” of aspartame — the amount people can safely ingest on a daily basis. The two committees’ statements were released jointly.

The news has lots of consumers spitting out their diet sodas — and it also has a lot of people confused about what to do with what feels like yet another wave of confusing and conflicting guidance about a food product. (Adding to the confusion: The US Food and Drug Administration — FDA — disputes the WHO’s findings.)

As we try to figure out what to add to and ban from our shopping carts, it’s worth understanding aspartame’s path from the lab to our refrigerators. It’s also worth examining what might set an IARC statement about a product’s safety apart from the assessments made by other agencies consumers trust with these decisions.

It’s also important to understand what this new classification really means, and what it does not — and moreover, what people can and should do with all of this new information.

Let’s get into it.

A history of scary, conflicting, and hard-to-trust guidance on aspartame

It can be hard for attentive consumers to know who to trust to weigh in on aspartame’s safety. That’s because since its earliest days, the product’s manufacturers — and the industry that now reaps enormous profits from its sale — have sought to influence its approval for consumer use.

When American pharmaceutical company G.D. Searle first tried to get aspartame approved by the FDA in 1973, it was denied: Independent scientists alleged the product could cause a range of neurologic disorders, and some alleged the company hadn’t been entirely above-board in its safety testing.

In January 1981, Ronald Reagan became the US president. His transition team included then-CEO of G.D. Searle, one Donald Rumsfeld. Although an independent FDA board warned that May that the drug might induce brain tumors, the agency’s newly installed director overruled them, and the FDA approved the product for consumer use that July.

In the 1990s, scientists began to raise concerns about aspartame’s safety. A neurologist published a study suggesting the product might play a role in causing brain cancer in humans, and the FDA’s own toxicologist raised concerns about the product’s cancer-causing potential.

To better characterize the drug’s risk, the Ramazzini Institute, an Italian nonprofit research laboratory, began studying its effects on rats in 1997.

This is a common first step in determining the safety of synthetic products produced for human consumption. But this study was particularly high-quality in that it included thousands of rats and gave them a range of aspartame doses. The purpose was to determine whether more aspartame led to more outcomes, a finding that would be strongly suggestive that aspartame was a cause of those outcomes and not just a coincidental exposure.

What the Italian scientists found did not look good: The rats that consumed aspartame had higher levels of malignant tumors in multiple organs, including kidneys, breasts, and the nervous system. These findings were noted even at low doses of aspartame — exposures similar to what the US and European authorities consider the acceptable daily intake of the product.

The study generated reams of controversy that have played out in the scientific literature ever since, with multiple groups of scientists across the globe independently re-evaluating the tissue samples from the Italian studies and coming up with their own conclusions about what they showed.

Interestingly, many of the scientists who disputed the Ramazzini Institute’s findings were funded by industry interests that profit from aspartame’s consumption: the American Beverage Association; Ajinomoto, an aspartame supplier; and the Calorie Control Council, to name just a few.

The upshot here is not that you need to launch all of your diet soda directly into the sun — at least, not yet. The guidance to come may yet indicate that there’s some amount of the product that’s safe to consume. Plus, to live on this planet is to routinely balance our desire for delight with the annoying reality that many fun things have some kind of health risk attached.

But consumers are entitled to have all the safety information companies do when making decisions about which health risks are worth taking. And the industry that produces and profits from aspartame has made complete information hard for consumers to get.

We deserve more transparency — and there’s a good chance the IARC’s guidance will at least achieve that.

Why the IARC’s guidance is especially authoritative

If aspartame’s safety has been under discussion for such a long time, why is IARC only making a statement about it now? Although that’s not entirely clear, experts say that the way IARC conducts its evaluations gives its assessments a particularly high level of integrity. That means it’s worth paying attention to the agency’s take on product safety.

In June, a group of authors from the Ramazzini Institute published a new paper describing a reanalysis of their original results using some new WHO tumor classifications. The study results “confirm and reinforce” their previous findings, write the authors.

It’s still not clear whether this study, or some other new science, is the reason for the IARC’s announcement. But whatever the reason for IARC’s timing, its assessment is not subject to the kinds of industry conflicts of interest that pose challenges to the FDA, or the analogous European Food Safety Authority (EFSA), said Erik Millstone, a longtime scholar of food safety policy and professor emeritus of science policy at the University of Sussex in England.

When these other agencies approved aspartame, they did it “on the basis of corporate data, most of which wasn’t in the public domain,” said Millstone. In a letter to the EFSA, he quoted from the agency’s own statements indicating it considered unpublished data in its assessment.

On the other hand, he said, the IARC is more selective in its use of unpublished, confidential commercial data, and it takes greater care to exclude people with conflicts of interest from contributing to its evaluations.

A few years ago, Millstone and a co-author looked closely at how the European Food Safety Authority had weighed the 154 studies on aspartame safety when it looked to assess the product in 2013. About half of the studies favored aspartame’s safety and about half indicated it might do harm.

The agency had judged all of the harm-suggesting studies — but only a quarter of the safety-affirming studies — to be “unreliable,” wrote the authors. And the agency had applied looser quality standards to the studies suggesting safety than it had to the studies suggesting harm. Agency reviewers pushed back against Millstone’s assessment. And in any case, aspartame has remained on the European market.

Millstone is not alone in thinking that IARC has higher standards for assessing product safety than other consumer product agencies. A senior scientist at the Center for Science in the Public Interest independently evaluated the European Food Safety Agency’s aspartame approval process and judged the agency’s conclusions were “not sound and not supported by the science; they contradict established criteria and principles by IARC.”

“In the early 1970s, US standards were noticeably higher than European standards,” said Millstone. “But over time, the FDA has been increasingly subordinated to commercial interests.”

The FDA has rules about who can serve on its advisory committees that are aimed at preventing conflicts of interest. However, a recent investigation by ProPublica found that consultants employed by McKinsey worked for the FDA on drug safety monitoring projects while simultaneously working for pharmaceutical companies directly affected by those projects. A recently passed congressional bill aims to reduce such conflicts.

In a statement Thursday, the FDA disagreed with IARC’s conclusion, saying, “FDA scientists reviewed the scientific information included in IARC’s review in 2021 when it was first made available and identified significant shortcomings in the studies on which IARC relied.”

Industry influence in consumer safety agencies means some government agencies have bent over backward to discount unwelcome results, said Millstone. But there’s something else going on that’s even harder to combat, he said: “Institutional inertia.”

“These supposedly authoritative institutions are loath to admit that they have ever made a mistake in the past,” he said, “especially if the evidence showing that they’re making a mistake had been available earlier — and they’d ignored it.”

What to do with this new information

Shady behavior and conflicts of interest along aspartame’s path to approval notwithstanding, the IARC’s statement indicating a possible link between aspartame and some cancers raises a lot of questions: What does this designation really mean? How big of an aspartame-associated cancer risk does it imply? And what should consumers do — and not do — with this information?

The most important thing to know about IARC’s carcinogen ranking system is that it’s intended to help people understand the quality of the science linking cancers with any given exposure — whether that exposure is something you ingest, like aspartame, or a behavior, like working nights.

But that ranking says absolutely nothing about how much of a risk any of those effects portend.

Let’s say, for example, that a bunch of high-quality studies reliably suggest eating mashed potatoes every day increases a person’s lifetime risk of cancer from 1 percent to 1.2 percent — but that eating french fries increases a person’s cancer risk from 1 percent to 20 percent. Although the risk associated with french fries in this scenario is much higher than that of mashed potatoes, IARC would classify both of these foods as “carcinogenic” — not “probably” or “possibly,” but definitely carcinogenic — because of the strong causal relationship for both.

To be clear, this is a completely fake scenario: So far no potato preparations have made it onto IARC’s list. However, as Vox has written in the past, processed meats and cigarettes — substances with wildly different cancer risks — are both IARC-classified as carcinogenic because the science underlying both is so strong.

IARC’s classification of aspartame indicates the strength of the science linking it to cancer is a good deal lower than the one in this example (again, the classification calls it “possibly carcinogenic,” not definitely). It’s true that some of the relative scientific weakness might be related to how hard it is to study health outcomes associated with foods. In that case, yes — there might be a small increase in aspartame-associated cancer risk that scientists haven’t yet identified.

However, that possibility shouldn’t prevent people from eating or drinking aspartame-containing products as an alternative to ones that contain lots of sugar, said Therese Bevers, who directs the Cancer Prevention Center at the University of Texas MD Anderson Cancer Center.

“My big concern is that I don’t want people saying, ‘Oh my gosh, I’ve got to stop diet sodas, I’m gonna get sugared sodas,’ and then they start drinking those and gain weight, which we know is one of the major cancer risks,” said Bevers. “And that has solid data.” A better outcome of the recommendation would be if people who drink a ton of diet soda replaced some of it with water.

Another potential positive outcome would be if the announcement provoked scientists to study aspartame more rigorously — in other words, to develop a higher quality of evidence about its link with cancer, said Bevers. After all, there are several as-yet unexplained emerging cancer trends, like rising rates of colon cancers in young adults, and increasing brain tumor rates in children and teens.

And while the existing animal data on aspartame is compelling, animal data isn’t sufficient evidence for a recommendation affecting humans, said Bevers. “It would be wonderful if everything that we found in mice studies translated to the same outcome in humans, but it doesn’t. It’s inconsistent.”

Perhaps one of the best outcomes of labeling aspartame as possibly carcinogenic is that it’s “going to stimulate investigators to want to better understand it, and understand mechanisms — and get stronger data to prove it is or it isn’t,” said Bevers.

Update, July 14, 10:40 am ET: This story was originally published on July 1 and has been updated to include details of the WHO groups’ statements on aspartame and an explanation of how to understand the new information.

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