Look, I get it. Diet Coke tastes sweet because it has aspartame in it. Aspartame is a weird synthetic molecule that’s 200 times sweeter than sucrose. Half of the world’s aspartame is made by Ajinomoto of Tokyo—the same company that first brought us MSG back in 1909.
If you look on Wikipedia, you’ll see that aspartame is a methyl ester of the aspartic acid phenylalanine dipeptide, which isn’t, like, comforting.
It’s normal to have a prior that aspartame might be bad for you. Certainly, that was my prior. Without looking at any evidence, any reasonable person would think like this:
| aspartame is… | odds |
|---|---|
| …good for you | very unlikely |
| …harmless | plausible |
| …bad for you | plausible |
This makes the decision theory pretty simple: Consuming aspartame has little upside, but substantial downside.
The thing is, we do have evidence. We have a lot of evidence. The FDA calls aspartame “one of the most exhaustively studied substances in the human food supply”.
The other thing is, the alternative to aspartame often isn’t no aspartame but rather sugar or corn syrup or even perhaps even alcohol.
I don’t want to convince anyone to consume aspartame. But if we’re choosing between aspartame and other risky things, we should evaluate the relative risks.
What happens to aspartame after it goes into your body
Let’s forget about safety for a second, and just look at the causal chain. Say you drink a Diet Coke. What happens next?
Fact 1: Aspartame is quickly broken down in the gut.
After you drink a Diet Coke, the aspartame goes to your guts. After that, it’s very quickly broken down into 50% phenylalanine, 40% aspartic acid, and 10% methanol. For example, a can of Diet Coke contains 184 mg of aspartame. This becomes:
- 92 mg of phenylalanine
- 73.6 mg aspartic acid
- 18.4 mg methanol
This happens quickly and completely. No aspartame ever enters your bloodstream. The rest of your body only ever sees these three other chemicals. (click here or on any paragraph with a triangle for more details)
The European Food Safety Authority Report (EFSA) report: Scientific Opinion on the re-evaluation of aspartame as a food additive gives this figure (slightly modified):
The same report gives this discussion:
Fact 2: Phenylalanine is a standard amino acid you consume all the time.
We recently talked about phenylalanine. It is an essential amino acid. If you didn’t consume any of it then when your body tried to make certain proteins those proteins would get truncated, and then they wouldn’t do what they are supposed to do, and then you would die.
Fortunately, that’s almost impossible. From 2% to 5% of all protein in food is phenylalanine. The recommended dietary allowance for a 70 kg (154 lb) person is at least 2130 mg. Meat-eating men in the UK average 3500 mg per day, while vegetarians and vegans get slightly less.
Here are the amounts of phenylalanine in a few foods:
| potato | 170 mg |
| large egg | 340 mg |
| 8 oz (235 ml) glass of milk | 430 mg |
| 400g box of tofu | 3300 mg |
The 92 mg of phenylalanine you get from a Diet Coke is much less than what virtually everyone already gets from other sources.
RDA guidelines are here. For adults, the recommendation is at least 33 m/kg of phenylalanine (or tyrosine, a metabolite of phenylalanine). For a 70 kg (154 lb) person, that would be 2130 mg.
Around 1 in 12,000 babies is born with phenylketonuria, a serious genetic disorder that results in low levels of phenylalanine hydroxylase, making it difficult to metabolize phenylalanine. People with phenylketonuria need to carefully monitor their consumption of phenylalanine (from all sources). This is why there’s this scary ALL-BOLD WARNING.
If you had phenylketonuria you would know it already.
Fact 3: Aspartic acid is a standard amino acid you consume all the time.
Here’s a chart from Wikimedia with our friends circled:
Aspartic acid is not essential in humans, meaning that if you don’t eat it, your body can make it (usually from oxaloacetic acid). But that’s not likely, since almost everything with protein has aspartic acid including meat, grains, dairy, vegetables, and eggs. Men in the UK average 6600 mg of aspartic acid per day.
The 74 mg of aspartic acid you get from a Diet Coke is two orders of magnitude less than what most people get already.
Here’s Schmidt et al. (2015) again:
Fact 4: Methanol is a simple alcohol you consume all the time.
Methanol (CH₃OH) is the simplest alcohol molecule. It’s in lots of food. Here are some foods with larger average amounts.
| food | mg/kg methanol | typical serving | methanol |
|---|---|---|---|
| wine | 115.0 | 150 ml glass | 17 mg |
| tomatoes | 281.4 | medium 125g tomato | 35 mg |
| citrus fruit | 106.5 | medium 140g orange | 15 mg |
This vastly underestimates how much methanol you get. In land plants, the primary component of cells walls is pectin. Once in the body, pectin degrades into methanol. Here are some estimates of the indirect increase in methanol various fruits and vegetables cause in this way.
| food | mg/kg methanol | typical serving | methanol |
|---|---|---|---|
| root vegetables | 774 | medium 200 g potato | 155 mg |
| apples | 508.5 | medium 170 g apple | 132 mg |
| oranges | 531 | medium 140g orange | 74 mg |
| bananas | 657 | 120g without skin | 79 mg |
| avocados | 486 | 100g (flesh only) | 59 mg |
You get the idea. We eat things that contain methanol or metabolize into methanol all the time. It’s estimated that most people get between 130 and 1030 mg of methanol from food per day, much more than the 18 mg in a Diet Coke.
Now isn’t methanol toxic? Sure, if you consume enough of it. The LD₅₀ in rats is around 5600 mg/kg, as compared to 7300 mg/kg for good-old ethanol.
One “conspiracy theory” you hear about aspartame is that it becomes formaldehyde once it’s in the body. This is absolutely true: When metabolizing methanol, formaldehyde is created. But small amounts of formaldehyde are completely normal. The half-life of formaldehyde in human blood is around 1 minute, meaning it disappears almost immediately. You get more formaldehyde (via methanol) by eating an apple than by drinking a Diet Coke. Formaldehyde itself is also present in lots of foods, like meat, seafood, fruits, vegetables, and coffee.
The EFSA report:
Also the ESFA report:
Also Dhareshwar and Stella (2008):
The EFSA report again:
Fact 5: This doesn’t prove aspartame is safe.
To summarize the above:
- Aspartame is quickly broken down in the gut into phenylalanine, aspartic acid, and methanol. Aspartame itself never enters your bloodstream or touches any other part of your body.
- Phenylalanine is normal.
- Aspartic acid is normal.
- Methanol is normal.
(Incidentally, this same logic does not apply to other artificial sweeteners which mostly aren’t broken down at all.)
While informative, this does not prove aspartame is safe. Biology is crazy. But it should inform our priors. Speaking for myself, my previous model was that consuming aspartame would result in a crazy unknown synthetic chemical circulating around my body and doing god-knows-what. My updated model is that consuming aspartame results in slightly larger amounts of some totally normal chemicals.
This is reassuring. But even if they’re normal, could these chemicals still cause harm? Sure. Fortunately for us, aspartame was invented a long time ago, so we have lots of evidence.
The scientific consensus
How to think about this situation
Aspartame was first made in 1965 and was approved by the FDA in 1981. In the decades since, there have been hundreds of studies.
Given so many studies, focusing on individual papers is a mistake. With enough monkeys pounding away at enough typewriters scientists pounding away at enough science, lots of weirdness is expected.
The right strategy is to look at the entire pool of evidence. Some tiny number of people have the time and expertise to comb through the entire literature and synthe
