This past week headlines in both newspapers and television have been full of worrisome sounding results of a recently published study showing an association between drinking diet sodas and the risk of developing strokes and dementia.
Now, while there isn’t really anything redeeming to say about diet sodas, it’s important to understand what this study did and did not say, and you cannot tell that from the headlines. As to the headlines, here are just two examples:
CNN: Gulping down an artificially sweetened beverage not only may be associated with health risks for your body, but also possibly your brain, a new study suggests.
Washington Post: Indeed, a new study shows an association between diet soda and both stroke and dementia, with people drinking diet soda daily being almost three times as likely to develop stroke and dementia as those who consumed it weekly or less.
That sounds pretty worrisome. But is it?
Studies can either show association (more of “x” seems to be associated with more of “y”) or causation (more of “x” leads to more of “y”). To show that “x” causes “y”, you need to plan a study where some people consume “x” and some don’t, and they can’t know whether they are getting “x” or a fake. You would then measure how often “y” happens going forward over the course of time.
The published study only looked back in time, examining previously collected data which showed that when people recalled drinking more diet soda over a certain length of time, that greater amount was associated with more of those people having strokes or dementia, compared to those who did not report drinking much diet soda. The researchers tried to think of any other factors that might have been confusing the results and making adjustments for them. But could they think of every possible “confounding” factor? Probably not.
Here are the closing remarks by the authors of this study (emphasis added by me):
The observational nature of our study precludes us from inferring causal links between artificially sweetened beverage consumption and the risks of stroke and dementia.
…we did not adjust for multiple comparisons meaning that some findings may be attributable to chance.
… although we addressed confounding in numerous ways, we cannot exclude the possibility of residual confounding.
… the use of a self-report food frequency questionnaire to obtain dietary intake data may be subject to recall bias, thus, introducing error into our estimated models.
Clinical trials are needed to establish whether the consumption of artificially sweetened beverages is causally related to dementia or surrogate end points, such as cognitive decline or brain atrophy.
So, not so impressive or “sexy” if you need to lead your news article or television show with these caveats from the actual study. Also, the study didn’t say anyone “gulped” down their sodas. Not sure if that verb counts as journalistic integrity free of hype.
Drinking diet sodas MAY cause stroke and dementia. WE DON’T KNOW. That’s all we can say.
I will say that there is nothing positive I can say about diet sodas, so why drink them? They just perpetuate your cravings for sweet things, which the food industry loves and indeed strives for. Water, tea and coffee (black or with cream, no sugar) are healthier beverages.
If you’re interested in some weird “associations” check out http://www.tylervigen.com/spurious-correlations