In Defense of Diet Coke: Aspartame is a Carcinogen?

aspartame is a carcinogen? explaining cancer classifications written on a pink background in black text with

A few weeks ago, the WHO’s International Agency for Research on Cancer (IARC) added the artificial sweetener aspartame to a list of possible carcinogens, and the media freaked out. Soon, my social media feed was filled with “Diet Coke gives you cancer!,” “aspartame IS a carcinogen! soda is killing us!”and a million other headlines from otherwise reputable journalists suddenly analyzing the carcinogenicity of aspartame. Sensationalist headlines aren’t anything new, but as a long-time Diet Coke drinker (and former cancer scientist) I felt the need to explain what a carcinogen is, how things get added to the IARC’s list and why you don’t need to freak out about it.

What the IARC does

The IARC is the World Health Organization’s (WHO) cancer agency. It was founded in 1965, and consists of 27 member countries. The IARC has a number of branches dedicated to different goals like cancer surveillance, epidemiology, detection and prevention, and education. Over the years, the IARC has released 134 monographs that identify over 1000 potential cancer-causing agents, and has separated them into groups.

Group 1 consists of agents that are carcinogenic to humans. Group 2A consists og agents that are probably carcinogenic to humans. Group 2B consists of agents that are possibly carcinogenic to humans. Group 3 consists of agents that are “not classifiable as to its carcinogenicity to humans.”

At the time I’m writing this (July 31, 2023):

  • Group 1 contains 127 agents
  • Group 2A contains 95 agents
  • Group 2B contains 323 agents
  • Group 3 contains 500 agents.

The IARC has made public its methods for classifying these agents (if you want to read the 44 pages of it, here you go).

The IARC takes recommendations from its interdisciplinary Working Group on which agents to consider every five years based on current research and public health priorities. If there is new, compelling evidence about an agent that it has already considered, they will reconsider the agent in light of the new evidence. To classify an agent, the IARC relies on publicly available and published studies. This means that the IARC minimizes any conflicts of interest or bias stemming from the use of commercial data or unpublished materials. For consumers, this is good. If you or I wanted to debate whether or not the IARC conducted a fair evaluation of an agent, we can find their sources.

Why aspartame is Classified in Group 2B

When the IARC and the Joint Expert Committee on Food Additives (JECFA) announced that aspartame was being added to Group 2B, they relied on evidence from animal studies, possible mechanisms by which aspartame might be carcinogenic, and population-level data linking increased consumption of diet soda with increased risk of cancer.

The IARC and JECFA affirmed that the “acceptable daily intake” of aspartame is 40mg/kg of body weight. For my fellow Americans, that’s 40mg per 2.2 pounds of body weight, so for who weighs 150 pounds, the acceptable intake of aspartame is 2720 mg a day. To put that into context, that’s about 13.5 cans of Diet Coke a day. For you to reach that amount with sugar-free gum, you’d have to eat at least 1300 sticks of gum.

If you do the math, it’s pretty clear that the IARC did not intend for this to result in a frenzy of Diet Coke haters, but to call attention to the fact that available research does not necessarily put aspartame into a “100 percent safe” category.

Dr. Mary Schubauer-Berigan of the IARC stated that “[t]he findings of limited evidence of carcinogenicity in humans and animals, and of limited mechanistic evidence on how carcinogenicity may occur, underscore the need for more research to refine our understanding on whether consumption of aspartame poses a carcinogenic hazard.” Read that again. The classification of aspartame in Group 2B serves as a signal to scientists that we need to do more research, not that we know affirmatively that aspartame causes cancer.

The dose makes the poison

I probably sound like a broken record at this point, but the dose is important. This is a BIG DEAL when it comes to possible carcinogens.

Take for example two agents from Group 1: alcoholic beverages and Radium-224. We know that both of these things cause cancer in humans. However, I have a decent idea of how I can consume alcohol with a minimized risk of getting cancer from it. I cannot say the same for radium exposure.

Think about how many Group 1 carcinogens we might encounter on a daily basis: estrogen-progestrogen oral contraceptives (the common birth control pill), secondhand smoke, UV radiation, diesel exhaust, wood dust, and air pollution

We all make choices as to what we want to expose ourselves to, but some of it is inevitable. Sure, I can wear sunscreen, not be a firefighter, get vaccinated against HPV, and stay away from consuming opium (all Group 1). I also could have picked a different major in college so I was not exposed to a laundry list of carcinogenic chemicals in chemistry lab. However, I am not so sure I could avoid all radiation because I tend to hurt myself a lot and I like my doctors to know if my bones are in the right place.

Group 2A is an even bigger minefield. I’m not sure I can make it a day without being exposed to “very hot drinks,” nor can I make it a week without the “high temperature emissions” that occur from frying food. (I make fried tofu at least once a week, it’s a ritual.) If you’re thinking, “ew, well, tofu is gross anyway,” red meat is also found in Group 2A, so good luck.

Group 2B, where aspartame is now found, is in the same group as… aloe vera. The groups might tell you whether something has a link to cancer, but the groups do nothing to tell you how much exposure is necessary.

I mentioned something similar in my article on BPA and hormone disruptors, but I think it’s worth saying again: if you live your life trying to avoid everything that might cause some kind of health problem, it’s (1) futile and (2) a sad way to live. You can, however, understand that the dose matters, and do what you can to limit extremely risky exposures.

There are some very obvious things we can do to avoid getting cancer like not smoking and avoiding unprotected sun exposure, but for many types of cancer, and for many of the people impacted by cancer, there isn’t any one thing that you could say “caused” it. Cancer is so incredibly complex— some of it is luck, some of it is genetics, some of it is age, some of it is where we live— there’s no one piece of advice that you could give to help someone avoid cancer entirely.

Aspartame is a carcinogen. Or rather, a “possible carcinogen.” But so are many, many other things that we encounter on a daily basis. Whether something is classified as a carcinogen matters less to me than whether we are researching and trying to better understand why something could be carcinogenic.

When an agent is added to the IARC’s list, I see this as a good thing. It means that attention is being called to whether something is dangerous for us, and it encourages transparency for consumers. Although the media might make it seem like a big deal, this is ideally how information is transmitted to consumers so that we can all better understand how what we are exposed to impacts our health.

I, for one, will continue to drink my diet sodas.

Enjoying some top-tier hydration on a recent trip to Sequoia National Park.