Why the aspartame in Diet Coke and Coke Zero probably isn’t worth worrying about

This essay was originally published on Gideon Meyerowitz-Katz’s Medium blog.

Aspartame is an artificial sweetener that is used in a lot of products, including Diet Coke and Coke Zero. Like most artificial sweeteners, we have a wonderful love/hate relationship with it. It’s sweet but doesn’t have any calories, so it’s great for people who want to lose a bit of weight. Also, a bunch of people are convinced that anything artificial is basically poison, and that diet soft drinks are probably killing us all.

This has hit the headlines recently, because apparently the International Agency for Research on Cancer (IARC), a World Health Organization body, is preparing to declare aspartame a class 2B carcinogen. This has caused a huge uproar, because aspartame is one of the most commonly used artificial sweeteners in the world, and also because cancer is very scary.

Fortunately for people like me who really like our diet drinks, the evidence really isn’t that compelling. Aspartame probably isn’t giving you cancer.

IARC categories

The first point to consider in this discussion about aspartame is the way that the IARC classifies things that could potentially cause cancer in human beings. They have four categories:

1. Causes cancer

2A. Probably causes cancer

2B. Possibly causes cancer

3. Unclassifiable as a cancer risk

There are a few interesting points to make here off the bat. Firstly, the IARC doesn’t ever consider the magnitude of risk. There are class 1 carcinogens that cause cancer in every person exposed to them, and other class 1s that almost never cause cancer even in massive, lifelong doses. For example, both processed meat and plutonium are considered class 1 carcinogens, even though the risk from bacon is decidedly lower than that posed by nuclear explosions.

Now, as the headlines state, the IARC is moving aspartame up to a class 2B carcinogen, which means it “possibly” causes cancer. For context, I downloaded the IARC database of human carcinogens, and the class 2B also includes:

1. Coconut oil soaps
2. Aloe vera
3. Pickled vegetables
4. Talcum powder
5. Working in the textiles industry
6. Nickel

And a whole host of other things as well. Class 2B does not mean that something definitely or even probably causes cancer — it means that there is some suggestion that the thing could plausibly cause cancer, and perhaps a small amount of evidence indicating that it does.

The evidence

So we know that class 2B isn’t necessarily a problem — in practice, it’s mostly a way for the IARC to call for more research into a question. But what has the evidence shown thus far?

Well, there have been quite a few studies looking at aspartame and other artificial sweeteners over the years. Generally, they have overwhelmingly been quite reassuring about the potential for cancer risk.

In fact, there are quite a few large epidemiological studies looking at hundreds of thousands of people that have failed to find a link between aspartame or other sweeteners and different forms of cancer.

There’s this one from 2012, which grouped together two large cohorts called the Nurses’ Health Study and the Health Professionals Follow-Up Study, which together include more than 100,000 people, and found no association between non-Hodgkin’s lymphoma, myeloma, or leukemia, and diet soda. There’s this study, which looked at the same two groups of study participants and found no increased risk of pancreatic cancer except in one small subgroup. Another paper from 2014 looked at another large cohort of people and found no increased risk of a range of cancers even in people who drank diet drinks daily. One very recent case-control study looked at a range of other cancers and specifically aspartame consumption, and found that in general there was no increased risk observed for people who had more aspartame.

A recent systematic review of epidemiological studies, which also included a review of the toxicological literature, summarized this evidence — over more than a dozen large studies, there is very little evidence that aspartame and other sweeteners cause an increased risk of cancer. In fact, I could only find a single paper, which was published in 2022, that found a reasonably consistent correlation between aspartame intake and cancer, and even then it was not a strong connection.

Do I drink the Diet Coke?

Now, this literature isn’t perfect. All of these studies have issues, and it’s entirely possible that there is some very vague link that we’ve missed despite decades of research into this question. It is relatively simple to identify when something probably does cause cancer, but it takes an enormous amount of effort to show that it probably doesn’t.

However, as an epidemiologist who looks into these issues regularly, I found the studies very reassuring.

Part of this is the actual risks that we’re talking about here. Remember what I said right at the start — the IARC does not ever define how risky something is, just what the evidence says about whether it can cause cancer at all.

Take the study I mentioned just before, which found an association between aspartame and cancer. In this paper, researchers looked at the Nutri-Net cohort of people, which includes more than 100,000 individuals followed up between 2009-2021, and checked to see whether those who reported having more aspartame were more likely to get cancer than those who had none.

They found that, on average, people who ingested no aspartame got cancer at a rate of about 31 in 1,000 during this period. For people who had a “higher” intake of the chemical, the risk of cancer was instead 33 in 1,000. In other words, going from having no aspartame at all, to drinking it regularly for a decade, increased the risk of cancer by 0.2%.

That’s a tiny risk by any measure. Ignoring all of the potential confounders here and why it’s unlikely that this is a causal relationship, it’s still a bit of a meaningless risk for the average individual. It might be meaningful to population health workers but even then possibly not.

It’s also always useful to remember the alternative to artificial sweeteners: sugar. While there’s some very vague evidence that aspartame might be bad for your health, there’s strong and consistent evidence that excess adiposity (too much body fat) is worse. Drinking water is probably the best thing to do for you in many ways — if nothing else, it’s free — but if the choice is between sugar-sweetened and aspartame-sweetened, it’s a bit more of a complex question.

And that’s why I’m personally not worried about my Diet Coke/Coke Zero intake. Yes, there’s some vague evidence that aspartame may be associated with an increased risk of cancer. There’s also some evidence it isn’t! But even the evidence suggesting that it might be shows a risk so low that it’s unlikely to mean much to my life.

Gideon Meyerowitz-Katz is an epidemiologist from the University of Wollongong and a science writer and communicator.