- “Diet Sodas May Raise Risk of Dementia and Stroke, Study Finds”
- “A Daily Diet Soda Habit May Be Linked to Dementia–Alzheimer’s”
- “Is Diet Soda Harming Your Brain Health?”
- “Diet Sodas Tied to Dementia and Stroke”
- “Here’s Another Reason You Might Want to Quit Diet Soda”
- “Drinking Too Much Soda May Be Linked to Alzheimer’s”
- “Is Soda Bad For Your Brain? (And is Diet Soda Worse?)”
- “Diet Soft Drinks Triple the Risk of Dementia”
- “Two Things Diet Soda Does to Your Brain”
Some of these headlines were from respectable sources like The New York Times and The Washington Post. It’s not surprising that many people were alarmed by the news and assumed that diet sodas had been proven to cause dementia. Some people were frightened enough to stop drinking diet sodas.
Headlines are designed to get people’s attention so they will read the article. They are notoriously unreliable. The first thing is to check whether the information in the body of the articles matches the headlines; sometimes it doesn’t. Even when the information matches, the article may selectively report some but not all of the studies’ findings, and it may put an unwarranted spin on the meaning of the findings. These headlines all referred to a single study; I wanted to know what that study actually reported, so I read it.
What Did the Study Show?
The study, by Matthew Pase and his colleagues, was published in a reputable journal, Stroke. The subjects were 2,888 individuals in the community-based Framingham Heart Study Offspring cohort. During a 10-year period of observation, 97 subjects over the age of 45 had a stroke: 82 ischemic (restriction of blood flow) as opposed to hemorrhagic (ruptured vessel and bleeding), and 81 subjects over the age of 60 developed dementia (63 consistent with Alzheimer’s). They estimated cumulative consumption of artificially-sweetened soft drinks based on self-reports on a food-frequency questionnaire. They found that drinking one or more artificially-sweetened soft drinks a day was associated with a nearly 3-fold increase in the incidence of ischemic stroke, all-cause dementia, and Alzheimer’s disease compared to drinking none. Sugar-sweetened soft drinks were not associated with stroke or dementia.
The study authors themselves clearly identified the limitations of their study:
- Absence of ethnic minorities (so the results might not apply to them).
- The study only showed a correlation; causation cannot be inferred.
- Self-reported dietary intake is unreliable, subject to recall bias.
- There could be confounding factors.
- There was no adjustment for multiple comparisons, so some findings could be due to chance.
- They said, “future research is needed to replicate our findings and to investigate the mechanisms underlying the reported associations.”
The lead author, Pase, is not yet recommending against diet beverages based on this study. In an interview with Medscape he brought up the possibility of reverse causation: “It is not clear whether the diet sodas are causing stroke and dementia or whether unhealthy people gravitate more towards these drinks than healthier people.”
Most of the media stories failed to report those caveats. Reporters may not have read the study or may not be qualified to evaluate it; often, a reporter simply regurgitates the contents of a press release about the paper and the study.
Applying the SkepDoc’s Rule
The SkepDoc’s Rule applies here: before you accept a claim, try to find out who disagrees with it and why. A little googling easily identifies several people who disagree. Sy Mukherjee, writing in Fortune, said “Stop Freaking Out About That Study.” He went on to say that an accurate description would have read “Study determines minor observational link (but no direct cause-and-effect) between certain people who drink artificial sugar beverages, but it has a small sample size that doesn’t include minorities or account for a whole bunch of other critical factors.” But of course, that’s too long for a headline, and it isn’t sexy.
Physician Aaron Carroll, in The Incidental Economist, wrote “They did not prove that diet soda causes Alzheimer’s Disease. THEY DID NOT!” He pointed out several problems with the study:
- No ethnic minorities
- Their conclusion was based on results from one model (model 2) that adjusted for demographics, diet, physical activity, and smoking. They also applied a model 3 that adjusted for more possible confounders, but those results weren’t as dramatic, so they downplayed that model.
- Different artificial sweeteners are different molecules with likely different effects.
- Multiple comparisons, so some results might be due to chance.
- Self-reported intake is unreliable due to recall bias.
- It was an observational study that could only establish correlation, not causation.
- They emphasized relative risk (3×) rather than absolute risk (actual numbers of patients with stroke or dementia were small).
- Even if there is an association, there’s no evidence that changing your behavior (drinking fewer diet sodas) will reduce your risk of stroke or dementia.
A story in Science Daily further pointed out that the study did not differentiate between different types of artificial sweeteners and only reported intake of diet sodas; it did not account for other possible sources of artificial sweeteners (in coffee and other foods).
The new findings differed from the findings of previous studies. The Nurses’ Health Study reported only a slightly increased risk of stroke (relative risk 1.16) associated with drinking one or more diet sodas a day; the risk was the same for sugar-sweetened soda. The Northern Manhattan Study reported an increased risk of all vascular events, including stroke, with daily diet soda consumption, but the relative risk was only 1.43, and there was no association with regular soft drinks or light diet soda consumption. There were no previous studies on dementia.
Do Artificial Sweeteners Make People Gain Weight?
Artificial sweeteners are intended to replace sugar. There is a correlation between sugar consumption and various health problems. Gary Taubes goes overboard, arguing that sugar is the cause of obesity and most chronic diseases. It’s not that bad, but everyone agrees that limiting sugar in the diet is advisable. Sugar provides calories but has no other nutritive value; we refer to it as “empty calories.” So it seems only logical that replacing sugar with artificial sweeteners would reduce total calorie intake, help people lose weight, and have other health benefits because of the reduced sugar intake.
Paradoxically, recent studies have suggested that intake of non-nutritive sweeteners may be associated with increased weight. Again, this is correlation, not causation, and the media reports have been very misleading. There are three possibilities with a correlation between A and B: A causes B, B causes A, or some other factor causes both A and B. Being overweight may lead people to use artificial sweeteners, rather than artificial sweeteners causing people to be overweight. Weight loss is complicated, and individual success depends on a lot of factors. And there is a psychological backfire effect: when you have virtuously limited yourself to diet soda, you may rationalize that you can afford to reward yourself by eating dessert.
Steven Novella covered this subject on Science-Based Medicine. He says the research shows that:
- Replacing high-calorie sugary drinks with low calorie drinks offers a modest health benefit.
- There is no “backfire” effect where artificial sweeteners somehow trick the brain into eating more.
What About All Those Alarming Reports of Side Effects?
The Internet is full of dire warnings about several artificial sweeteners. Saccharin causes bladder cancer in rats! The FDA wanted to ban it; but subsequent research exonerated it. It causes cancer only in male rats, and it does so by a mechanism not found in humans. It is safe for human consumption.
Aspartame has been demonized in a concerted campaign by scaremongers, epitomized by the book Sweet Poison. It has been accused of causing headaches, seizures, Alzheimer’s, cancer, diabetes, multiple sclerosis, birth defects, tinnitus, memory loss, and all kinds of other problems. It doesn’t cause any of those things. Hundreds of studies have been published that examined and dismissed those claims. Aspartame has been evaluated far more extensively than any other food additive. It is safe for everyone except the few people with the genetic disorder phenylketonuria.
No, You Don’t Need to Stop Drinking Diet Sodas
The recent study only showed a modest correlation with dementia and stroke. Even the lead author of the study has said it does not constitute a reason to avoid diet sodas. Its findings have not been confirmed by other studies, and the findings only showed correlation, not causation. More importantly:
- There is no evidence that if you stop drinking diet sodas, your risk of stroke or dementia will decrease.
- There is good reason to believe that switching from diet sodas to sugar-sweetened drinks will result in worse health outcomes.
There are a number of artificial sweeteners on the market. There is no credible evidence that any of them are harmful. There is good evidence that high sugar intake is harmful.
The Dangers of Reading Headlines
The moral of the story: diet sodas are not dangerous, but reading headlines can be dangerous. Media reports of scientific studies, not just headlines, often give the wrong impression. They put a slant on results to make them more newsworthy. They tend to make preliminary research sound like definitive proof. Early studies are often superseded by later studies with the opposite findings. We should never trust a single study; we must look at the total weight of all published findings. Most published research findings turn out to be false. That might sound discouraging, but it shouldn’t be. Science is a self-correcting endeavor. Don’t believe the headlines, but stay tuned for further developments. 
This article was originally published as a SkepDoc column in Skeptic magazine.
