Diet Soda Study: Unanswered Questions (so far)

Seemingly every news outlet is touting a new study that was published in Obesity on May 26th, titled The effects of water and non-nutritive sweetened beverages on weight loss during a 12-week weight loss treatment program (June 2014 issue, Volume 22, Issue 6 – see full study summary here).  The journal is clear in disclosing that the study was paid for by the American Beverage Association and consulting fees were paid by the Coca-Cola company.  The researchers involved are well respected.  In fact – full disclosure on my part:  Dr. Hill of the University of Colorado is a friend and someone I greatly respect.

With that said, while the outcomes of the study make for wonderful headlines, there are many unanswered questions tied to this study, including but certainly not limited to the following:

  1. Caffeine reduction driving replacement stimulant calories: Diet Coke is the most popular diet soda and contains 45 mg of caffeine/12 oz serving.  Study participants were required to dring 24 oz/day, providing a nice boost of 90 mg of caffeine/day over the water drinkers.  The “water drinkers” were not exclusively water drinkers prior to the study, which meant they were cutting out previous soda habits as part of this study (while the soda drinkers likely continued habits similar to their previous routine).  For anyone reading this who has cut out soda cold turkey, you know it is very likely that the water drinkers missed that caffeine-induced boost, especially in the initial 6 wks of the study. They likely replaced it with other calories (or other caffeine options – see #2, below).  However, as performed, the study likely points more to the impact of eliminating a stimulant than it does to a comparison between water and diet soda.
  2. Caffeine – part II:  However, let’s assume that caffeine intake was not an issue impacting this study’s outcome.  Afterall, the “water drinkers” were allowed other alternatives, as long as they didn’t purposely add any NNS to those alternatives. So if the previous soda drinkers needed to replace their need for the caffeine (previously received in the form of soda), maybe we can assume they did so with just one extra visit to the coffee shop/day.  If we use the nutritional information provided by Starbucks in regards to their Caffe Latte, we learn that one 16 oz cup contains 190 calories/day (which translates to 4.5 lb differential over the 12 week length of the study).
  3. Fluid intake differential:  Many nutritionists recommend drinking extra fluid 30 minutes prior to a meal in order to create a feeling of “fullness” going into the meal. In this study, water drinkers were asked to drink at least 24 oz of water but no soda.  The soda drinkers were asked to drink 24 oz of soda with no limit on water.  Once again, habits take time to change, so it’s likely those suddenly replacing previous soda habits with water would be less likely to add further to that figure.  It would be interesting to note total fluid intake/day to see if this was related to decreased calorie intake, as it likely was.

Keep in mind that the weight differential of 4 lbs over a 12 week period (Just .33 lbs/week difference), while statistically significant, is still quite small.  In fact, it would require a caloric differential of just 167 calories/day to create that 4 lb difference over the 12 week period.  The single Caffe Latte would exceed this relatively small number, as would even minimal replacement tied to #1 & #3 (ie, a single size serving of chips add approximately 155 calories/day).

It appears from the write-up that the study will continue for a year.  It’s my hope this timeline provides the researchers with the opportunity to ask and further study the difficult questions that were not effectively addressed in this initial phase.  If I know Dr. Hill and his team, I have a feeling they will address them all and the headlines will be quite different about 9 months from now.

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