Is Diet Soda Really Worse For You than Regular Soda?

A recently published study looked at this question and was all over the media.     What did the study conclude?  It concluded that drinking 2 or more sodas per day (of any type of soda) was associated with a 17% increase in mortality.    Further the study concluded that regular (non-diet) soda drinkers were 8% more likely to die at follow-up than those that consumed less than one glass per day.

Those people that drank two or more artificially sweetened sodas were 26% more likely to die at follow up than those who drank less than one diet soda per day.   Seems pretty simple right – clearly diet soda is WAY worse than regular soda and all soda is bad, but is that really the story?

When we look at the study itself in more detail the picture is not so clear!    First of all, the study involved giving people ONE questionnaire on their average daily soda intake at the beginning of a 16-year study.       This is a big problem because self-reported intake is notoriously inaccurate and assuming that people kept their soda intake fixed for 16 years is a whopper of an assumption as well.

Another problem is that the study cannot account for what are known as “confounding variables” meaning things like smoking and alcohol use or other unhealthy behaviors.   The study attempted to “guess” at the levels of these unhealthy behaviors by using statistics, but the fact is that no one knows who did what in the observational study.     

This is a big problem because it may in fact be the case that most people drinking soda were engaged in unhealthy behaviors like smoking, high alcohol consumption, and/or lack of activity.     So, it could be that people who drink sodas tend to have more unhealthy behaviors and the increased death rates may have nothing to do with soda!

Last but not least lets take a look at “absolute risk” rather than relative risk which is ALWAYS important.     Here is how that works.    In the study out of 225,543 people who reported less than a glass per month of artificially sweetened beverages at baseline, there were 21,032 deaths at follow-up.  In other words, 9.3% died.    For the 6,292 people that reportedly drank two or more glasses of Artificially Sweetened Beverages per day at baseline, there were 737 deaths at follow-up.    So in this group 11.7% died.

This works out to a RELATIVE risk increase of 26% calculated as follows: subtracting the lower death rate of 9.3% from the higher death rate of 11.7% (11.7% - 9.3% =  2.4%).   Then dividing the 2.4% absolute difference by 9.3% (2.4% divided by 9.3%= 26% difference in RELATIVE risk).

Sounds like a big deal right?   Not so fast – lets take a look at the ABSOLUTE risk or raw numbers.  The absolute risk is the difference between 11.7% of the heavy soft drinkers died vs 9.3% of those who drank less than one per day.   As above 11.7% - 9.3% is 2.4%.   So the ABSOLUTE risk is only 2.4%.     Pretty insignificant isn’t it?

So long story short soda should probably be treated like other lifestyle issues – in moderation!     By the way calories DO count and sugar has real calories, but diet soda does not.    Is this a license to drink lots of diet soda – NO!