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Why you can ignore the WHO claims that Diet Coke sweetener is a cancer risk

“The dose makes the poison.” That’s always what you have to bear in mind when you hear any claim about whether some substance or other is “toxic”, or even “causes cancer”. Many “poisons” only do you harm at very high concentrations, and even water can be toxic if you drink enough of it.

This week, we’ve seen a new claim that should set our alarm bells ringing. The World Health Organisation’s cancer research arm, the International Agency for Research on Cancer (IARC), is about to declare that aspartame, a sweetener used most famously in diet drinks but also in all sorts of other foods, is “possibly carcinogenic”.

This seems surprising, because aspartame has repeatedly been declared safe by multiple health authorities over the years after dozens of studies and reviews have been conducted.

Here’s what the IARC did. They have four categories, into which they put different “agents”. Group 1 is things they think definitely cause cancer. Group 2A is things that are “probably carcinogenic”. Group 2B is “possibly carcinogenic”. And Group 3 is “not classifiable”.

Aspartame isn’t currently anywhere on the list, but the proposal, after an updated review of the science, is to add it to Group 2B.

Just for context, here are some of the other things in Group 2B: Aloe vera. Working as a carpenter or joiner. Working in a dry cleaner. Pickled vegetables.

And here are some of the things from Group 2A, which are apparently even more serious cancer risks: Eating red meat. Working night shifts. Drinking hot beverages at above 65°C.

Hopefully you see the problem here, which makes the whole exercise somewhat academic: again, the dose makes the poison. At a certain level, all of these things might be hazardous to our health – but what is that level? We don’t get any useful information on that from the mere fact that something’s on the list.

And since we only know about the impending aspartame classification from a leaked news report, we don’t know any of the reasoning behind IARC’s decision. We don’t know what research they’ve looked at that’s made them change their mind on aspartame’s effects.

That research would have to be quite impressive if it was to overturn the very solid scientific consensus on aspartame and cancer. One problem is that a lot of the research finding links between aspartame and cancer is observational (that is, it’s not based on experiments or randomised trials). With observational research, it’s hard for scientists to tease apart whether being unhealthy causes you to drink more aspartame (maybe someone who’s obese tries drinking more diet drinks in an effort to lose weight), or whether the cause is the other way around.

If the dose makes the poison, what “dose” of aspartame might you need to increase your risk of cancer? How many cans of a particular diet drink? That’s the question a lot of people are interested in, but what does it even mean to “increase your risk”? Compared to what? To even begin to answer that question, you’d need to set an (arbitrary) criteria for what counts as a risk . We can only hope that the IARC will do so very clearly, otherwise their list will end up confusing more than it helps.

Aspartame has been a source of spurious health anxieties for decades, with a whole host of dodgy claims and dodgier “studies” claiming effects on cancer and other health outcomes besides. But these theories were never particularly plausible, given what we know about what happens to aspartame in the body, which is that it’s immediately broken down into other chemicals and doesn’t enter your circulation or “hang around” in any way that might pose a risk.

Are those other chemicals dangerous? No. For example, they include phenylalanine – and you get a great deal more of that from eating a potato or an egg than you do from drinking a can of an aspartame-sweetened beverage.

So not only is the epidemiological evidence on aspartame pretty clear, there’s no particular chemical reason to worry about it either. We’ll await the WHO panel’s reasoning to see if there’s something we’ve missed. But for now, there’s no reason to be concerned: feel free to crack open that can of Coke Zero.

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