Diet Drinks Aren’t Killing You

How to stop worrying about that Big New Study that doesn’t mean much to your life

There’s been a bunch of yelling in the media recently about a big study published in the journal Stroke. If you’re at all interested in health or the news, you’ve probably read from a thousand sources the news that scientists have proven that diet drinks are basically killing you.

Pictured; probably poison

The New Big Study that all these articles are referring to found that people who drank more diet soft drinks were at significantly greater risk of having a stroke or being diagnosed with dementia. Cue outrage, scary stories, and a heap of people switching back to the well-known healthy option of sugary soft drinks.

The story is that some evil additive in diet drinks causes your brain to go haywire. It’s a lovely story, because it plays into our deepest fears. All those nasty chemicals in diet drinks, no wonder they’re making us lose our minds!

Pictured; pure, delicious, evil

There’s only one small snag in this story; it isn’t true.

Diet drinks aren’t that bad for you at all.

The Big Scary Study

The Big Scary Study is a wonderful media device whereby journalists take a piece of reasonably humdrum, everyday research — in this case an observational study looking for associations between diet soft drinks and stroke/dementia — and turn it into something to fear.

Headlines from around the world proclaimed diet soft drinks as a health hazard. Media outlets leapt at the chance to demonize something most people think as a healthy option.

Sadly, they were wrong.

The study itself was interesting. Basically, researchers took an existing cohort of people from a big study, and used the records of 3,000 of them to compare the risk of having a diagnosis of stroke or dementia for people who drank either no diet drinks, an arbitrarily small number of drinks (1–2 per day), or a large number of drinks (>2 per day).

They then used some cool statistical tricks to control for sources of potential bias, meaning that they adjusted their analysis to eliminate the variation caused by these factors. They actually did two of these adjusted analyses, one for common socio-demographic factors and another for common conditions that are known to cause stroke/dementia themselves.

Which produced these wonderfully illegible tables. Science communication done…not well

Read The Goddamn Study

The first thing to note is that every media scare article got it wrong about dementia.

Every. Single. One.

In fact, once the researchers took into account the effect of conditions that are known to cause dementia (for example diabetes), the risk of drinking diet soft drinks disappeared. Completely.

Red square and zooming added for emphasis; no significant results. An easy way to check is to look at the confidence intervals (the hyphenated numbers in brackets). If these cross 1 (i.e. 0.5–1.5), then the results were not statistically significant.
The risk of drinking diet soft drinks disappeared. Completely.

So every article was wrong about dementia. Based on this study there’s no increased risk of getting dementia when you drink diet soft drinks at all!

Something literally anyone could’ve noticed if they had read the actual study. It’s not long.

So what about stroke?

Well, here is where it gets a bit more interesting. The researchers in this study had set up well over 100 statistical tests. Statistical tests are like coins; if you flip enough of them you’ll eventually get the answer you are looking for.

Except unlike coins, scientists have an awful lot of them

So after doing more than 100 tests, it would be more of a surprise if there were no significant results.

This study did actually find some statistically significant results indicating that stroke might be associated with drinking diet soft drinks. They weren’t consistent, and there is a very good likelihood that they represent statistical ‘noise’ instead of an actual result, but they are technically significant.

Woohoo.

Statistical significance; the lowest of all bars to measure research importance

But before you flee, terrified of the evils of diet coke, let’s have a look at a couple of other things that could be happening here;

  1. Obesity; we know that being overweight or obese makes you more likely to have a stroke. Whilst the researchers did control for a lot of factors, they didn’t directly take obesity into account. What they did show was that people who drink more diet drinks are likely to be fatter than people who don’t. Which means that the observed effect between diet drinks and stroke could just be down to obesity.
  2. Unkown confounder; a confounder is a variable that gets in the way of your results. Obesity is one example of something that could confound this experiment, but there are literally thousands more. Maybe people who drink less diet soft drinks also have less stressful lives, something known to reduce your risk of stroke? We just don’t know. This was an observational study, which means that the researchers just couldn’t adjust for every possible confounding variable.

I could go on. Suffice to say that while stroke may have been associated with drinking lots of diet drinks, there is no evidence that it is caused by the drinks. It’s likely something else.

Media Mayhem

It’s true that anyone who actually read the study would’ve known that it didn’t show that diet soft drinks caused stroke/dementia. But reading the author’s conclusions, you could be fooled into thinking it did;

In conclusion, artificially sweetened soft drink consumption was associated with an increased risk of stroke and dementia.

It’s fascinating, because this is one case where I in no way blame journalists. Most journalists don’t have the time or expertise to comb through a study and pick out the details; that’s what we have abstracts and press releases for.

In this case, these sources were wildly misleading.

So don’t worry about drinking diet soft drinks. Water may always be the healthiest choice, but they come a close second. The artificial sweeteners we use have been tested more than almost any other product that you’ll come in contact with in your life.

They are safe. And there’s currently no good reason to think that they cause strokes and/or dementia.

Diet drinks may be full of scary chemicals but you know what? So is everything else.

Drink away.

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