Putting the “Die” in Diet Soda

by Christine Gary

Are diet sodas harmful to your health? Research findings displayed at the 2011 Conference for the American Stroke Association (ASA) showed individuals who drank diet soda daily were 61% more likely to have a stroke as opposed to those who drank no diet soda. Cue media frenzy. Virally propagated via online sources, print media, and nightly news, the story captured headlines worldwide.

As incoming students to the Tufts Friedman School of Nutrition Science and Policy, you will have to decide which scientific discoveries are worth our concern and warrant nutrition recommendations. Let’s look more at this study that linked diet soda to elevated stroke risk. When met with a health scare like this, ask some questions before you run along with the chicken littles.

1. What caliber of science is this?

When you read health news, think about the source. The most reliable evidence emerges from randomized controlled trials of phase 3 studies. The least dependable information comes from publications that are neither peer-reviewed nor endorsed by scientific experts within the field. The findings that implicated diet coke in elevated stroke occurrence were of the latter class. They were not even presented orally as a feature at the conference. They were merely printed on a poster, hanging on a wall. The researchers themselves admitted that future investigations were necessary before condemning diet soda conclusively. This was a caveat statement that journalists hunting for sensational stories conveniently ignored.

2. Are the study subjects representative of the general population?

There were 2564 participants in this study (N=2564, 36% men, mean age 69 ± 10 years, 20% white non-Hispanic, 23% black non-Hispanic, 53% Hispanic). The mean age was 69. According to the US Census Bureau, only 12.6% of Americans are over 65 years of age. Doesn’t seem like a viable subset of the population to be drawing inferences for the other 77.4%.

The racial breakdown in this study presents another possible threat to generalizability. According to the American Heart Association, Blacks and Hispanics have a higher rate of vascular disorders including stroke. The participants in this study were predominantly minorities, meaning 76% of their study population was already at high risk for the outcome (regardless of soda intake).

3. How was information managed?  

Based on their soda consumption, participants were divided into different groups. There were those who consumed soda rarely (<1/mo), moderately (1 mo-6/wk), or daily. Look at the classification for moderate drinker. One a month to six per week? That’s a very wide range! There is a big difference between someone who sips a soda once a month and someone who sips sodas regularly throughout the week, and yet these people were in the same category.

Participants self-reported their soda consumption at the time of enrollment via a questionnaire. They were followed for nearly a decade and never again asked about their consumption habits. It’s hard to imagine that every participant’s intake of soda would stay constant over nine years.

4. What was controlled for?

What if a participant had gained 30 lbs over the nine-year study period? Perhaps that excess weight, and not the diet coke, accounted for the increased stroke risk. Researchers did not take weight or family history of disease into consideration, both of which are critical to the development of the outcome of interest. They simply did not collect this information. Nor did they collect information on sodium intake. They also lacked data on the brands and varieties of diet and regular soda consumed. Perhaps changes in coloring and sweeteners over time may have played a role, and they have no way of analyzing this.

5. Is association being mistaken for causation?

While intriguing, these findings prove only an association, not cause and effect. It’s important to bear in mind that people who drink soda daily, be it regular or diet, may also partake in other unhealthy habits, like consuming high amounts of fatty, cholesterol-laden foods or leading a sedentary lifestyle.

Overweight individuals who are already at risk for health complications, may be more likely to drink diet soda. Dr. Frank Hu, professor of nutrition and epidemiology at Harvard School of Public Health, looked at the association between artificially sweetened beverages and the risk of developing diabetes. He found that after controlling for diabetes risk factors like weight, blood pressure, and cholesterol, there was no association between diet colas and diabetes risk. This confirms the idea that health outcomes have more to do with the type of person who drinks diet soda to begin with, and not the diet soda itself.

So is diet soda off the hook? ABC News Chief Health and Medical Editor Dr. Richard Besser acknowledged regular soda to be a health risk, but felt that the study presented at the ASA conference that condemned diet soda was not sound science on which to base recommendations of behavior change. However, he went on to voice a viewpoint shared by The Mayo Clinic Women’s HealthSource: While diet sodas may be calorie-less, the artificial sweeteners in them may increase sugar cravings and encourage poor food choices. In an interview with Time magazine, Dr. Frank Hu said that while diet soda is perhaps not the best alternative to regular soda, drinking diet soda in moderation should not cause appreciable harm.

Christine Gary is completing a Masters Degree in Nutrition Communication at the Tufts Friedman School of Nutrition. She’s an international marathon runner devoted to raising funds for charity: http://www.christinegary.com

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