This press release, reporting on this article, reveals that "Increasing diet soda intake is directly linked to greater abdominal obesity in adults 65 years of age and older." I think that the adjective "directly" is meant to specify that the correlation is positive, but it could be confused to imply the absence of mediators.
In any case, the importance of this correlation is described as follows: "Findings raise concerns about the safety of chronic diet soda consumption, which may increase belly fat and contribute to greater risk of metabolic syndrome and cardiovascular diseases." Moreover, the authors recommend that "older individuals who drink diet soda daily, particularly those at high cardiometabolic risk, should try to curb their consumption of artificially sweetened drinks."
The authors are proposing that diet soda makes people fatter. The obvious alternative hypothesis, which seems substantially more plausible to me, is that people who are gaining weight drink more diet soda as an attempt to mitigate this trend. It could even be that this strategy is successful, and that without diet soda, they would have gained even more weight. The authors measured reported diet soda consumption and anthropometrics simultaneously at each follow-up wave of the study, and no lags were employed to account for this possibility of reverse causation.
To their credit, in the "limitations" section of the paper, the authors do question this association: "Whether [diet soda index] exacerbated the [waist circumference] gains observed in SALSA participants is unclear; the analyses include adjustment for anthropometric measures and other characteristics at the outset of each follow-up interval, but other factors—including family history and perceived personal weight-gain and health-risk trajectories —that increased [change in waist circumference] but were not captured in the analyses may have influenced participant decisions to use [diet soda]." But the adjustment for baseline doesn't solve the potential reverse causation here because baseline anthropometrics are measured at the same time as soda consumption. And in any case, this doubt about the interpretation of the effect is completely lost by the time the authors reach the conclusion section, and no caveats whatsoever are expressed in the press release, which ends with the recommendation to curb consumption of artificially sweetened drinks. They don't specify what people should be drinking instead.
In any case, the importance of this correlation is described as follows: "Findings raise concerns about the safety of chronic diet soda consumption, which may increase belly fat and contribute to greater risk of metabolic syndrome and cardiovascular diseases." Moreover, the authors recommend that "older individuals who drink diet soda daily, particularly those at high cardiometabolic risk, should try to curb their consumption of artificially sweetened drinks."
The authors are proposing that diet soda makes people fatter. The obvious alternative hypothesis, which seems substantially more plausible to me, is that people who are gaining weight drink more diet soda as an attempt to mitigate this trend. It could even be that this strategy is successful, and that without diet soda, they would have gained even more weight. The authors measured reported diet soda consumption and anthropometrics simultaneously at each follow-up wave of the study, and no lags were employed to account for this possibility of reverse causation.
To their credit, in the "limitations" section of the paper, the authors do question this association: "Whether [diet soda index] exacerbated the [waist circumference] gains observed in SALSA participants is unclear; the analyses include adjustment for anthropometric measures and other characteristics at the outset of each follow-up interval, but other factors—including family history and perceived personal weight-gain and health-risk trajectories —that increased [change in waist circumference] but were not captured in the analyses may have influenced participant decisions to use [diet soda]." But the adjustment for baseline doesn't solve the potential reverse causation here because baseline anthropometrics are measured at the same time as soda consumption. And in any case, this doubt about the interpretation of the effect is completely lost by the time the authors reach the conclusion section, and no caveats whatsoever are expressed in the press release, which ends with the recommendation to curb consumption of artificially sweetened drinks. They don't specify what people should be drinking instead.
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