The aspartame limit doesn't mean what you think
Read time: 4 minutes
I'm currently in Zagreb, and I had a travel day from hell on Saturday!
Woke up at 3 am. Bus to Dublin Airport at 3:30 am. Arrived at the airport at 4:45 am. Flight scheduled for 6:50 am. Flight delayed to 9 am. Got to the lounge for a few hours. Walked towards my gate. Flight delayed by a further 30 minutes. Then a further 30 minutes again. Oh, and the flight I'm catching is to Paris, where I had a connecting flight to Zagreb at 1 pm. My flight then gets delayed until 3 pm! I waited two hours in the queue for the customer service desk. Got rebooked onto a flight to London Stansted for 1 pm. I then had to pay for a new flight from London to Zagreb. My flight to London was delayed by over an hour because Trump was taking off from Dublin Airport, so all flights were grounded. Finally got to Zagreb and into my Airbnb at 10 pm, where my mate was waiting for me. All on four hours of sleep!
Man, I'm still not recovered today 😂
Anyway, enough about me; let's get into this week's newsletter, and I promise you it's a good one...
Everyone has an opinion on diet drinks.
Ask 10 people whether Diet Coke is worse for you than the full-sugar version, and you'll get 10 answers, every one of them delivered with total confidence.
❌ It causes cancer.
✅ It's completely fine.
❌ It wrecks your gut.
✅ It's better than sugar.
❌ It's worse than sugar because it tricks your body into something or other.
Almost nobody in that conversation has read a word about it. Least of all whoever's being loudest.
So most people land somewhere vaguely uneasy. They drink the stuff, they half-expect it's doing something to them, and they don't know enough to settle the argument either way.
Which is exactly the gap a good scary number walks straight into.
The one going round at the moment says aspartame becomes a problem once you pass 40 mg per kilo of bodyweight a day. It gets shared with a huge study attached.
A million people. Finally proven. That number is real enough.
The claim bolted onto it falls apart the second you look at where the number came from, and I think it's worth 5 minutes of your time to see how.
Mostly because the same trick gets played on you constantly, and it's easy to spot once you know the shape of it.
What that number actually is
40 mg per kg of bodyweight is the Acceptable Daily Intake for aspartame.
The World Health Organisation's food additives panel reviewed it in July 2023 and confirmed it should stay where it is (1).
Now, the way these limits get set is the bit nobody explains. Researchers start with the highest dose in the animal studies that produced no observable effect whatsoever.
Then they divide it. Usually by 100. That gap is deliberate padding, built in to cover the fact that people aren't rats, that some of us are more sensitive than others, and that nobody wants the legal limit sitting anywhere near the interesting bit.
So 40 mg/kg sits roughly one hundredth of the way below a dose where nothing was happening at all ⤵
Calling it the point where aspartame becomes a problem has it precisely backwards.
It's the point regulators chose because it's so far away from any problem that they could be confident about it.
What you'd actually have to drink
For a 70 kg adult, going past that limit means more than 9 to 14 cans of diet drink a day, depending on the brand (1).
EVERY day. Not just one random Saturday where you went a bit crazy.
And the average person who consumes aspartame at all gets somewhere around 4.9 mg/kg daily, about a tenth of the limit.
So unless you're putting away double figures of Diet Coke daily, this number was never about you 🥤
If I stopped there, you'd be entitled to think I'd been paid by Coca-Cola, but the honest position is that the evidence is genuinely mixed, and almost everyone shouting about it is ignoring half of what's there.
In 2023, the WHO's cancer agency classified aspartame as possibly carcinogenic to humans, on the basis of limited evidence relating to liver cancer (1).
"Limited evidence" is doing a lot of work in that sentence. It's the category for "we saw something, we can't rule out chance or other explanations, somebody should look harder."
Then there's the newest one, published in February this year. Researchers followed 100,004 American adults and found that people who consumed aspartame had around a 13% higher risk of cancer overall than people who consumed none (2).
Sounds fairly damning if you ask me.
Then you read the next line of the same paper ⤵
No clear dose-response relationship was found.
In plain terms: the people knocking back loads of it were no worse off than the people having the occasional one.
That matters enormously, and I'd want you to understand why.
The bit that makes you harder to fool
If something genuinely causes harm, more of it should generally cause more harm.
That's the pattern researchers look for. It's most of what separates "this causes that" from "these two things happen to turn up together."
When the dose-response is missing, the usual explanation is that something else is going on, and in this case, the obvious candidate is that people who drink a lot of diet fizzy drinks are different from people who drink none, in about a hundred ways that have nothing to do with sweeteners.
They're more likely to be heavier. More likely to be dieting. More likely to have been told by a doctor to cut sugar.
Untangling that from a questionnaire is genuinely hard, and the researchers themselves say so.
Aspartame is not the hill I'd choose to die on, and it's nowhere near the top of the list of things costing you results.
If diet drinks are replacing sugar, that's a win. Someone swapping 4 cans of full-sugar cola for the diet version has cut a serious amount of sugar out of their week. That's a real, measurable change. Worrying about the sweetener while ignoring that would be daft.
If you're drinking 8 a day, have a think about why. Not because of the aspartame. Because that's usually a caffeine habit, a boredom habit, or an "I need something that isn't water" habit, and those are worth looking at on their own terms.
Water is still the base. Diet drinks aren't health food. Nobody ever improved their health by adding Diet Coke.
The real lesson here
That 40 mg figure spread because it sounded precise. Precision is enormously persuasive. A number with a decimal point in it feels like someone did the work, and most of us don't stop to ask which work, or what the number was measuring in the first place.
It's the same reason "66 days to build a habit" gets repeated endlessly by people who've never read the study it came from.
So next time a number like that lands in your feed, ask two things.
What was actually measured?
And more of it, more harm?
You'll be right more often than most of the internet.
And remember, if you've ever got any questions or want some advice, all you've got to do is hit that reply button.
Right, I'm off to find a gym in this city,
Catch you next time.
Mark ✌️
