A new study on the association between the consumption of soft drinks (sweetened or diet) and risk of death is making some headlines. But caution is necessary in interpreting these results. The study itself, “Association Between Soft Drink Consumption and Mortality in 10 European Countries“, is fine in terms of its methodology. This is what they found:
Of this total, 451 743 (86.7%) were included in the study, with a mean (SD) age of 50.8 (9.8) years and with 321 081 women (71.1%). During a mean (range) follow-up of 16.4 (11.1 in Greece to 19.2 in France) years, 41 693 deaths occurred. Higher all-cause mortality was found among participants who consumed 2 or more glasses per day (vs consumers of <1 glass per month) of total soft drinks (hazard ratio [HR], 1.17; 95% CI, 1.11-1.22; P < .001), sugar-sweetened soft drinks (HR, 1.08; 95% CI, 1.01-1.16; P = .004), and artificially sweetened soft drinks (HR, 1.26; 95% CI, 1.16-1.35; P < .001). Positive associations were also observed between artificially sweetened soft drinks and deaths from circulatory diseases (≥2 glasses per day vs <1 glass per month; HR, 1.52; 95% CI, 1.30-1.78; P < .001) and between sugar-sweetened soft drinks and deaths from digestive diseases (≥1 glass per day vs <1 glass per month; HR, 1.59; 95% CI, 1.24-2.05; P < .001).
This is a cohort study, which means they evaluated subjects on entry into the study and sorted them into various groups, then followed those groups over the duration of the study to assess for death from all causes. They then calculated the risk of death over the period of the study. This is a large study, so obtaining very low P-values is not difficult. For sugared drinks they found a relative risk of dying of 1.08%, with a 95% confidence range from 1.01-1.16. This is a small effect size, that almost overlaps with 1.0, which would be no change in risk. But when dealing with death as the outcome, even a small increase in risk can be meaningful.
The effect size for artificially-sweetened drinks was higher, 1.26 with a range of 1.16-1.35. This is a fairly large increase in the risk of death, and should certainly be taken seriously. There is a huge caveat in interpreting these results, however – this is an observational study. That means it can find correlations only, and not, by itself, determine cause and effect. It would require other types of data to establish a specific cause for this increased risk of death.
The obvious confounding factor here is that people who are at higher risk of vascular disease are more likely to drink diet soft drinks, not the other way around. If you are overweight or obese and trying to lose weight, you may avoid sugary drinks (which is actually a good thing). This is a classic sick-user effect.
This is similar to research showing that taking a daily multivitamin is associated with increased risk of mortality. The prevailing interpretation, supported by some evidence, is that people who are sicker at baseline are more likely to take a multivitamin because they think they need to for their health.
In both cases there may also be another phenomenon at work, a behavioral backfire effect. Whether taking a multivitamin or drinking a zero-calorie drink, people may feel as if this gives them permission to engage in other unhealthy activity. I can eat that cheeseburger because I am pairing it with a diet drink. Or I can have a bad diet or not exercise because I take a daily vitamin.
In both cases, it is probably much more likely the correlation we are seeing is due to a sick user or backfire effect rather than direct harm from either multivitamins or zero-calorie sweeteners. The way to resolve these questions is with further research. The best type of evidence would be a randomized clinical trial, because then confounding variables could be controlled for (it would eliminate the sick user or backfire effect).
For both vitamins and low calorie sweeteners (LCS) we do have some experimental data. I recently reviewed all the data for LCS, which finds that consumption of LCS is associated with decreased calorie intake and therefore weight loss, and improved metabolic parameters. Therefore if we look at the totality of the evidence there seems to be a health advantage to drinking beverages with LCS over sugar-sweetened, and people should not be scared off because of this correlational data.
For multivitamins and other vitamins the data is a bit more complex. I reviewed this data recently as well. For multivitamins there is probably no risk or benefit to all-cause mortality. However, there does appear to be an increased risk of death from taking antioxidants. There are plausible mechanisms to explain this as well.
Both of these cases illustrate that you need to look at various kinds of scientific data before making any specific recommendations based on correlational data alone. Health behavior itself is particularly susceptible to confounding factors. The authors of the current study conclude:
the results are supportive of public health campaigns aimed at limiting the consumption of soft drinks.
I disagree. I would change that to “sugary drinks” not all soft drinks. In fact, the totality of research supports the use of soft drinks with LCS as an effective method of improving health. To their credit they also say,
For artificially-sweetened soft drinks, we now need a better understanding of the mechanisms that may underlie this association and research such as ours will hopefully stimulate these efforts.
At least they gave the “more research is needed” caveat. They also acknowledge it’s not just the mechanism, it’s the arrow of causation itself that is in doubt. There may be no mechanism to LCS increasing mortality, because they don’t. At this point we still need to focus on the proper cause of this association, which other research suggests is probably primarily a sick-user effect.
This nuance is important, because we have to carefully craft the ultimate message we send to the public. Scaring people off of beverages with LCS may have a net negative effect on public health. I am confident that the authors of this study do not want that outcome. When dealing with public health, it is extremely common to have unintended consequences. Human behavior is complex, and it is often difficult to anticipate the net effect on behavior of our messaging. People often do the opposite of what we intend. Public health messaging is also competing with a host of product marketing, which has a very different goal. There is also a lot of “wellness” pseudoscience that is largely based on fearmongering, and we don’t want to inadvertently feed that.
In the end I don’t think this study changes the bottom line recommendations based on existing research. People in general should avoid drinking lots of sugar. Low calorie sweeteners are a safe and effective way to help achieve this goal. Sure, drinking water is also healthful and a great option. But many people prefer to have a little flavor, and in fact this may help them moderate their calorie intake.