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October 15, 2019

Why the diet soda controversy may never die

Daily Briefing
    Editor's note: This popular story from the Daily Briefing's archives was republished on February 26, 2021.

    Despite the "exceedingly good" odds that drinking too much soda won't kill you, in recent years, there's been a seemingly endless slew of research highlighting its vices, Aaron Carroll, a health economist and pediatrics professor at Indiana University, writes. Here are a few reasons, according to Carroll, why the studies just keep coming.

    Cheat sheets: Evidence-based medicine 101

    The latest big soda study

    Just last month, a study into the risks of drinking soda received widespread media attention, Carroll writes. The 450,000-person study, published in JAMA Internal Medicine, showed that drinking soft drinks every day—including sugar-free diet sodas—is associated with a greater risk of premature death.

    The researchers found that, after accounting for factors that could increase risk of death like body mass index (BMI) or smoking, participants who consumed two or more glasses of soft drinks each day were 17% more likely to die early than those who drank less than a single glass per month.

    Additionally, people who consumed two or more glasses of sugar-sweetened soft drinks or artificially sweetened soft drinks each day were 8% and 26% more likely to die early than those who drank less than one glass per month.

    Neil Murphy, a scientist at the International Agency for Research on Cancer, and co-author on the study, said the results "provide further support to limit consumption [of soft drinks] and to replace them with other healthier beverages, preferably water."

    But according to Carroll, "[a]s usual, the study (and some of the articles) lacked some important context and caused more worry than was warranted."

    As such, Carroll offers a few reasons why "this cycle" of soda studies that lack appropriate context receiving widespread media coverage "is unlikely to end." 

    People are more health conscious

    For one, as people have become more worried about their health, soda has become an "easy target," Carroll writes.

    It is true that no one "needs" to drink soda, but just because we don't need it, that doesn't mean it can't be consumed in moderation, Carroll writes. "We don't need ice cream or pie, but for a lot of people, life would be less enjoyable without those things," according to Carroll.

    Further, Carroll notes that the more health conscious people become, the more they assume that artificial things, or anything created in a lab, cannot be good for our bodies, according to Carroll.

    But just because some ingredients "occur naturally" and others "are coaxed into existence," that doesn't mean that one is "inherently better than another," according to Carroll. In fact, he notes that some research demonstrates that artificial sugars could be better to consumer over added sugars.

    The tenure track dilemma

    Another reason why there's so much research interest in soda is because young academics need to publish research to get tenure, Carroll writes.

    "I'm a professor on the research tenure track, and I'm here to tell you that… You need funding to survive, and you need to publish to get funding," Carroll writes.

    According to Carroll, the easiest way to publish is usually to demonstrate a correlation between a factor and an outcome using a large data set. "This kind of research is rampant. That's how we hear year after year that everyone is dehydrated. … It's how we hear that coffee is affecting health in this way or that," he adds.

    And often, Carroll notes these studies are being produced by prestigious universities that have the resources and funding necessary to build the large data sets that researchers need to analyze. As such, they attract "more media attention" when the results are published, Carroll writes.

    As long as "the culture of science" encourages this level of output from prestigious universities, "we'll continue to read stories about how diet soda will kill us," according to Carroll.

    The limitations of observational studies

    Another reason why the diet soda catches attention is because many people fail to understand the limitations of observational studies and treat them as more conclusive than they are, Carroll states.

    When people see "increased risk," they usually assume the risk is causing a bad outcome, Carroll notes. However, observational studies, like the JAMA study published last month, can only determine "if two things are related, not whether one is to blame for the other," Carroll writes.

    As a result, we shouldn't assume that diet sodas cause premature death—but "you shouldn't assume that diet sodas cause better health either," Carroll adds.

    He concludes, "None of this should be taken as a license to drink cases of soda a week. A lack of evidence of danger at normal amounts doesn't mean that consuming any one thing is huge amounts is a good idea." But Carroll adds, "It would probably be a public service if we stopped repeating a lot of this research—and stopped reporting on it breathlessly" (Carroll, "The Upshot," New York Times, 10/14).

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