Is Diet Coke Bad For You?

If you spend any time on the internet at all you will no doubt at some point been told that you shouldn’t drink diet coke because it’s full of chemicals. This is because there are many who would make the claim that the non-nutritive sweetener aspartame is a chemical compound which has SEVERE negative effects on the health of people who drink it.

It’s been implicated in everything from ‘brain fog’ and headaches to multiple sclerosis, cancer, and diabetes.

Aspartame is probably heralded as the most deadly thing in our food supply, only there because ‘they’ are hiding the truth from us, and keep pumping it into the food supply to protect profit margins and keep customers addicted. It is, according to bloggers everywhere, the worst thing you could ever ingest.

Well, it’s not. Like, at all. And here’s why…

Aspartame was discovered in 1965 by a scientist called James Schlatter who was trying to generate a new drug to fight ulcers. Halfway through the process he got some of one of the ingredients on his hand and noticed it was very sweet as he licked his finger to turn a page. Ever since then it’s been the centre of controversy.

In 1980, the FDA held an independent inquiry to examine a relationship between aspartame and brain cancer which was suggested at the time. They concluded that aspartame was safe, but that FDA approval should be withheld due to ‘unanswered questions’ over its safety. Shortly after, further analysis into available data by another group who also saw flaws in the original inquiry’s approach led the FDA to approve its use in foods (1). Of course, this original stumble to approval was picked up by the popular media and various conspiracy theorists, and the accusations started.

Most safety questions which people raise come from data garnered from studies on lab rats. In the most commonly cited study, (2) rats fed large doses of aspartame (around 20mg/kilo per day) for their entire life (which was around 150 weeks after weaning) were shown to fall victim to cancer more often than a control group. The main issue here is that were this translated to humans, 20mg per kilo for a typical, small, 60kg woman would be 1.2 grams of aspartame. Baring in mind that a litre of diet coke contains 540mg, this small woman would need to drink 2 litres of diet soda every single day for her entire life, from birth, to get close to this figure. The number would be far higher in larger adults, too.

The second issue here is a very simple one, we are not rats. In research reviews (Which are more valid than single studies in just about every single context) which are more recent than the above paper, there is no link between aspartame and Brain Cancer, Gastric Cancer, Pancreatic Cancer or Endometrial (womb) Cancer (3, 4). In rats it’s also very difficult to calculate doses when translating to humans for a number of reasons, suffice it to say it’s not as simple as scaling up from a 400g rat to an 80kg human because of relative parts per million and other factors – this serves to make rodent data even more questionable when it comes to extrapolation.

Anyway, what IS aspartame?

Aspartame is a methyl ester of the amino acids l-aspartic acid and l-phenylalanine (you’ll see on a can of diet drink that it contains a source of phenylalanine – more on this in a second). It is around 200 times sweeter than sugar which means that you can use 200 times less of it to get the same effect as sugar in a food item. Although aspartame contains the same caloric load as sugar (4kcal per gram) the minuscule amount you need to use in foods means that it is generally considered to be calorie-free.

When you consume aspartame it is broken down exactly like any other food item or ingredient that you may ingest (5) in that it is simply hydrolysed in your stomach, resulting in the three constituent parts: aspartic acid, phenylalanine and methanol. In fact, in cases of HUGE doses of aspartame consumption, absolutely none ever makes it to the bloodstream (and therefore the brain, or any other tissue) (6).

This means that concerns that aspartame damage our tissues are unfounded because aspartame doesn’t actually REACH any of them. It’s broken apart by digestion.

I’ll go through each metabolite in turn.

–         Aspartic acid is probably the most common amino acid in your diet, being present in every single protein you would ever consume.

–         Methanol is rapidly converted into formaldehyde which IS indeed a poisonous substance. When ingested it becomes formic acid, the foremost reason that methanol poisoning occurs. This sounds scary until you realise that a litre of diet coke yields less methanol than a glass of tomato juice, and in fact, the methanol load from aspartame is entirely trivial in comparison to the methanol load of a typical diet. Methanol poisoning is only really likely to occur in the ingestion of methanol itself as a standalone product. Think of it like this, there is cyanide in apple seeds, but the amount is trivial and therefore not even worth mentioning. Probably don’t neck a glass of methanol, though.

–         Phenylalanine is the final metabolite and one which is genuinely a concern for an incredibly small amount of people. Those born with the condition phenylketonuria are unable to metabolise this amino acid and should, therefore, avoid high levels of its consumption. Of course, these people should probably avoid diet drinks entirely but for everyone else, it’s a non-issue, milk contains levels of phenylalanine FAR higher than any diet beverage and yet has no negative effects.

Of course, this is not to say that there aren’t some studies which could cause you to doubt this – when the papers write that 2 Diet Cokes per day causes early death they aren’t just making stuff up…but they ARE still wrong and it’s pretty easy to explain why. These kinds of headlines come exclusively from one area of science: epidemiology. This just means that researchers will take a massive group of people and, in this instance, ask them how many diet drinks they consume (we’ll forget for now that people lie a lot). The group can then be split into drinkers and non-drinkers and these groups can be compared for outcomes like cancer and all-cause mortality. When you do this, diet drink drinkers tend to have worse outcomes.

But here’s the thing: diet soda consumption is associated with a lot of other lifestyle factors, such as overeating, smoking, drinking more alcohol and consuming more processed foods. Think about it – most people drink diet drinks, and those who avoid them by and large do so at least in part because for decades the messaging has been that diet drinks are bad for you.

In short, those who drink diet drinks have worse health outcomes because those who are the most health-conscious tend to avoid them (even though they don’t need to). You’re not comparing diet soda drinking and diet soda avoiding, you’re comparing normal people to really health-conscious people, so OF COURSE, they have different health outcomes.

But wait, there’s more…

People claim that aspartame can ‘trick your body’ into thinking that it has consumed sugar. This, they say, causes insulin levels to rise in much the same way as sugar, and therefore increases fat storage.

My first issue with this is that this is not how insulin ‘works’. That’s a (very long) article in and of itself, but for now, I’ll just say that insulin is massively misunderstood in the fitness industry and it cannot be looked at as a standalone hormone. For healthy people, an insulin rise is not a problem whatsoever and, in fact, it stops you from dying.

The second reason that you can throw this one out is very simple – your tongue doesn’t cause insulin spikes. What causes that is your body detecting high levels of blood glucose which need to be regulated by an insulin release. If you could trick your body and thusly alter insulin levels by tasting sweet things, all we would need to do for those who have chronically elevated insulin is feed them lemons.

Finally, there are some who say that diet drinks make them overeat by causing cravings. This one is where I’ll say that your results may vary. For myself, and the majority of people I’ve coached, a diet drink is a fantastic way to curb hunger pangs whilst trying to lose weight, because of the temporary fullness you get from a carbonated beverage, and it also serves to satisfy intense cravings for sweet foods which happen when carbs are low. There are those who say the opposite, though, and if this is you, I would recommend you keep diet sodas to an infrequent indulgence, just like anything that can trigger overeating.

So, my recommendations: Drink diet drinks if you want to – they are tasty, refreshing, calorie-free, fantastic mixers for spirits if you are having a night out and wanting to stay within your calorie needs, and (as I’ve shown) harmless if you don’t mainline them in huge doses. I do recommend you keep it to 1-2 cans per day for the simple fact that most diet beverages are acidic and therefore bad for your teeth, and I also don’t believe that you should use them as a crutch to make up for the fact that you are consuming an unsustainable diet.

And of course, if you have phenylketonuria or you find that drinking a can of diet coke makes you then inhale a pack of HobNobs, then it’s also probably a good idea to stick to water with some fruit in it.

For the rest of you, bottom’s up!




(2) Belpoggi et al (2005) ‘’First Experimental Demonstration of the Multipotential Carcinogenic Effects of Aspartame Administered in the Feed to Sprague-Dawley Rats’’. Environmental Health Perspective


(3) Cabaniols et al (2011). ‘’ Links between private habits, psychological stress and brain cancer: a case-control pilot study in France’’. Journal of Neuro-Oncology


(4) Bosetti et al (2009). ‘’ Artificial sweeteners and the risk of gastric, pancreatic, and endometrial cancers in Italy’’. Cancer Epidemiology Biomarkers and Prevention


(5) Ranney et al (1976). ‘’ Comparative metabolism of aspartame in experimental animals and humans’’. Journal of Toxicology and Environmental Health


(6) Magnuson et al (2007). ‘’ Aspartame: a safety evaluation based on current use levels, regulations, and toxicological and epidemiological studies’’. Critical Reviews in Toxicology


"All in or all out" Rachael is a vibrant, no-bullshit-talking Scottish nutrition geek and coach helping women to lose weight without giving up their confidence OR their favourite foods.

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