Can you imagine that title being reported throughout the media without a fiery backlash of skepticism (except for smug soda drinkers)? Yet the reporting on a new study that indeed found this in their results instead focused on its finding of a positive association between diet soda consumption and vascular events. Many popular websites trumpeted the findings as solid, or at least didn’t provide an appropriate context for interpretation. Here are some reasons why both of the above conclusions would be premature. (This is, by the way, the second time this study has gone through the media rounds, the first time prior to publication.)
The prospective study consisted of 2,564 subjects who went through interviews, physical examinations, and some measurements between 1993 and 2001 (average follow-up was 9.8 years). A food frequency questionnaire estimated their consumption of regular or diet soda consumption as well as other dietary factors. They were screened annually after this- but not for soda intake, which was only collected at baseline and correlated with future events, which is a major weakness of studies like these. Vascular events included stroke, heart attacks, or vascular death.
They performed multivariate analyses to adjust for a number of possible confounders, but it is always possible there are habits associated with diet soda consumption that aren’t controlled for. For example, before adjustment, the authors found that frequent diet soda consumption was associated with former smoking, hypertension, elevated blood sugar, lower HDL, elevated triglycerides, increased waist circumference, BMI, peripheral vascular disease, previous cardiac disease, and the metabolic syndrome. So right there we know that 1 or few studies on this topic is not going to be enough for strong conclusions because of so many potential confounders. Importantly, different study designs and potential mechanisms (of which are lacking, which also should weaken conclusions of a link) should build an evidence-base that collectively suggests that there might be a risk of diet soda and vascular disease. This one study is fine by itself- but the conclusions being drawn in the press are not.
They found no association between regular sugary soda consumption and risk of vascular events at any dose of consumption, which is surprising given previous research that has demonstrated consistent associations, and should raise the question about why fact went relatively unnoticed. In fact, light regular soda consumption was associated with a reduced risk. Diet soda was positive for vascular events at all but 1 dose (the lightest consumers) in all of the statistical models they did. Of note, however, the number of subjects and events in the group consuming the most diet drinks was quite small (163 subjects, 51 events).
As the authors point out, their study is the first to specifically examine the relationship between vascular events and diet soda consumption. But one with coronary heart disease has been studied in the nurses health cohort, with neutral results after similar statistical adjustments. In other words, there was no relationship between diet soda and heart disease. The authors note:
“Beyond the inherent differences between our NOMAS cohort and the NHS in terms of age, sex, race-ethnic composition, and sample size, reasons for the discrepant results for the association between diet soft drinks and cardiovascular disease are not immediately obvious, underscoring the need for further study in other cohorts.”
On the other hand, several other studies have found positive associations between diet soda consumption and risk of metabolic syndrome in the Framingham cohort, the MESA cohort (also increased diabetes risk), and the ARIC study. However, they all caution on interpretation- it could simply be that diet soda consumption is a marker of poor diet/lifestyle habits and residual confounding by unknown variables (there may be many) that aren’t statistically controlled for could explain the results. This is why we need more research before drawing conclusions.
An important thing to note is the weak mechanistic explanations. The relationship could be real, but it also could be an artifact; some strong mechanisms could strengthen the theory, but there aren’t any. They cite the fact that research suggests artificial sweeteners increase (or at least don’t decrease) body weight (good review here), but they controlled for that. They suggest the caramel coloring could influence but through glycation but I am skeptical and this should be apparent in the regular group as well (they didn’t analyze for it either). Overall it is not convincing.
Gardener, H., Rundek, T., Markert, M., Wright, C., Elkind, M., & Sacco, R. (2012). Diet Soft Drink Consumption is Associated with an Increased Risk of Vascular Events in the Northern Manhattan Study Journal of General Internal Medicine DOI: 10.1007/s11606-011-1968-2