Much Ado About Nothing: Diet Coke is Fine
If you are reading this, you probably said “I’m trying to cut back on diet soda” in my presence. And I probably flipped my lid.
In a past life, I would have sent you seven links to various health organizations and just left it at that. I had all of them in Evernote, just waiting to be copy-pasted. But that was never very effective, so let’s try something different.
First off: relax. Take a deep breath. Diet Coke is fine.
I mean, mostly. It’s bad for your teeth, just like any acidic drink.1 And it has caffeine – 46mg/can to be exact, about the same as half a cup of black tea. And if you’re in the 0.004% of people with phenylketonuria, stay away – Diet Coke contains phenylalanine, and that shit will cause brain damage.
But cancer? Diabetes? Something imprecise-yet-vaguely-sinister about your gut biome? None of these claims would pass muster in any other context. So let’s get into it.
The Evidence
When people attack Diet Coke, what they’re really attacking is aspartame.2
Aspartame is one of the most studied chemicals in the human food supply (fda.gov). At the time of writing, PubMed's archive contained over 700 studies with aspartame in the title.
That’s a lot. More than you or I can read. Fortunately we don’t need to read them – just about every major government pays a small army of people to read these studies, pull out the common findings, and interpret those findings in the context of what is safe for citizens of their country to do.
So let’s look at some of those agencies. Here is the US Food and Drug Administration. The European Food Safety Authority.3 Health Canada. Food Standards Australia and New Zealand. The UK Food Standards Agency. The United Nations and World Health Organization (JECFA).4 Every one of them has concluded aspartame is safe for human consumption,5 at between 40-50mg/kg/day – enough that a 130-pound adult could drink one 12-pack of Diet Coke every afternoon and be fine.
Of course, governments don’t have a monopoly on health recommendations. We also have the American Heart Association. The American Diabetes Association. The Academy of Nutrition and Dietetics. Every one of them finds aspartame to be safe.
If you want to get a bit more into the details, the American Academy of Pediatrics published a great policy statement that not only finds aspartame to be (surprise surprise) safe, it walks through many of the shortcomings in the studies that found aspartame to cause tumors.
How safe is “safe enough”
Some of you might be thinking that I left out an organization. Earlier this year (2023, in case this post is being read past that), The International Agency for Research on Cancer (IARC) classified aspartame as “possibly carcinogenic to humans” (who.int). It raised quite a stir. Such a stir that a bunch of news agencies published ominous headlines warning that the WHO found aspartame to be possibly carcinogenic.
Here’s the thing: the IARC is a research agency. It runs studies, conducts meta-analyses, things like that.
What it doesn’t do is recommend actual policy to actual citizens – they leave that to the WHO’s food safety agency, JECFA. The IARC and JECFA work hand-in-hand, which is why the WHO did not publish the IARC’s findings alone. The announcement that states the IARC re-classified aspartame as a possible carcinogen contains a second paragraph, that states JECFA maintained its position that aspartame is safe.6
But maybe that’s not satisfying – let’s talk about why the IARC’s classifications don’t directly translate to policy recommendations.
The IARC uses four classifications (who.int): “Carcinogenic to humans”, “probably carcinogenic to humans”, “possibly carcinogenic to humans”, and “not classifiable as to its carcinogenicity to humans”. Aspartame falls into the third category.
These classifications have nothing to do with exposure level, severity, or likelihood. The “carcinogenic to humans” list includes drinking alcohol, smoking cigarettes, and being exposed to plutonium. I think we can all agree that these are dangerous. I think we can also all agree that they are dangerous to different degrees.
When the IARC announced that they are classifying aspartame as possibly carcinogenic to humans, they meant that they found limited evidence supporting: some amount of aspartame carries some amount of risk of causing some amount of cancer (who.int).
In their own words, the “possibly carcinogenic” means (emphasis mine):
“A positive association has been observed between exposure to the agent…and cancer for which a causal interpretation is considered by the Working Group to be credible, but chance, bias or confounding could not be ruled out with reasonable confidence.”
The IARC put aspartame in the same category that it put magnetic fields created by power lines, magnetic fields created by cell phones, birth control pills,7 and pickled vegetables. I would say “if you’re thinking twice about drinking Diet Coke, consider thinking twice about living anywhere with cell reception or power”. Except that, you know, the IARC’s designations mention nothing about risk levels, so I really can’t say.
This is why we should take cues from organizations that are focused on the practical questions. Not “is it possible that some amount of this is unsafe”, but “what amount is unsafe, and how risky is it?”. JECFA’s recommendations should carry the day here, along with the recommendations of literally every other food safety agency I could get my hands on.
This next bit might only be persuasive to Californians, but I think it bears mentioning. There is a law in this state called Proposition 65. If you don’t live here, a summary: any business that exposes people to something that might be carcinogenic needs to put up a sign saying so.
This very reasonable-sounding law has led to literally every coffee shop, bar, and restaurant in California putting up warning sings about carcinogens. Parking garages have signs stating that the very air is carcinogenic. Here’s the kicker: even Proposition 65 does not require a warning about aspartame. This is the law that requires public warnings for (among many other things): alcohol, french fries, prune juice, green beans, roasted asparagus, white boards, magic markers, Christmas lights, dental offices, anything with a gasoline engine, fish, chips, and toast.8 California’s “carcinogen” bar might seem impossibly low, but the IARC’s “possibly carcinogenic” designation is still below it. Even California only counts the substances the IARC actually lists as “human or animal carcinogens” (ca.gov).9
But isn’t it icky?
I think most people read this and think, “yeah, I can have a can of Diet Coke now and then”. I am taking a more extreme position: I drink between four and six cans a day, and I am behaving perfectly safely when I do so.10
Drinking six cans of soda just sounds bad. I think there’s a puritanical, moralizing impulse buried in all of us, that just can’t stand to watch somebody enjoy themselves without consequence. Diet Coke has to be bad for you. It’s a mass-produced, artificially-synthesized, very-sweet brown drink that comes in a can. Come on.
And like, I’m not saying it definitely isn’t. I mentioned the carbonation thing, I mentioned the caffeine thing, and the phenylalanine warning is right on the can.
But the rest of the hand-wringing is baseless.11 We have evaluated aspartame as best we know how, and that evaluation says it is fine. Does that mean aspartame is conclusively safe? That the latest study that’s making its rounds on Twitter X is definitely overblown? No, I suppose not. There’s always potential for some new body of work to show that aspartame really is dangerous, and we just didn’t detect it before. But the same could be said of anything – we might as well be talking about Russell’s teapot.
We live in a time of tremendous human progress. A time where we have engineered a soda that tastes delicious, has no calories, and has survived forty years of intense scrutiny by health agencies world-wide. Kick back, crack open a can, and enjoy it.
Diet Coke has a pH around 3.10, making it better than cranberry juice and worse than club soda (nih.gov). My dentist recommends drinking a glass of water after every soda and who knows, maybe that helps.
Diet Coke consists of carbonated water, caramel color, aspartame (200mg), phosphorus acid, potassium benzoate, natural flavors, citric acid, and caffeine. I don’t have time to defend each of these, but I have never seen an attack on any of them except aspartame and caffeine. In the mid-2000s it was fashionable to question artificial food dyes, but caramel color isn’t artificial – it’s a natural coloring produced by heating sugar (or some other carbohydrate).
In 2021, the EFSA did open a re-evaluation into two other sweeteners related to aspartame, although it is ongoing. As of today, these are all authorized for human consumption.
I abbreviated this; the fully-spelled-out name is “The Joint Food and Agriculture Organization of the United Nations / World Health Organization Expert Committee on Food Additives”. However, writing such a thing would be barbarous.
While the Commonwealth countries’ articles on aspartame tend to reference the EFSA’s, the FDA’s, and JECFA’s assessments, they also state that their own agency conducts a review and safety assessment.
The National Cancer Institute has a similar relationship with the FDA. While the NCI’s fact sheet on artificial sweeteners and cancer lists lots of individual (conflicting) studies that show possible correlation between aspartame and cancer, they also state quite plainly that “the FDA reviewed numerous safety studies that were conducted on each sweetener to identify possible health harms. The results of these studies showed no evidence that these sweeteners cause cancer or other harms in people.”
There are two kinds of birth control pills, those with estrogen and progestogen combined, and progestogen only. The former is classified as carcinogenic, the latter only possibly so. The combined pill is more common.
The full list of chemicals can be found here, and you can find the household items that might contain any of chemicals in the various fact sheets
Prop 65 doesn’t only look at the IARC’s list, it also looks at lists from the EPA, the FDA, and OSHA. Unsurprisingly, none of these list aspartame.
Somehow, my teeth are fine, but I’ll chalk that one up to luck and genetics
If you are not convinced, and have actual evidence suggesting I shouldn’t be either, leave it in the comments! I will look into the evidence incorporate it. I hope this post stands as an ever-strengthening testament to the safety of Diet Coke. And if I’m actually persuaded, I will recant the whole thing and post your evidence right at the top.