There are some very basic principles taught in a beginning college nutrition course that help students learn to differentiate between junk science and real science reports. Students are introduced to the differences in physiology between rodents and humans and why the results of a rodent study cannot be directly applied to humans; the limitations of observational studies vs. intervention studies; the Principle of Toxicology – the dose makes the poison – and the longstanding problem of the unreliability of science reporting by the media.
I state all of this because this past week I began teaching the fall semester, and a classic response from one of my students to something I did highlights the need for this column.
On my first night of class, I walked in with a wild cherry diet Dr. Pepper in my hand and left it sitting on the table in front of me. Toward the end of the class we were discussing the expected higher rates of dementia and strokes in the students’ generation later in life due to self-induced high blood pressure from their heavy use of energy drinks, pre-workout drinks (often loaded with various stimulants, both legal and illegal), as well as the increasing rates of obesity.
One of my nursing students politely stated, “But professor, I just read about a month ago that diet drinks will do the same thing.” She of course had zeroed in on my diet Dr. Pepper. I love this type of a student for two reasons. One, they read, which is not common any more, illustrating they have a real interest in learning, and two, they are young enough and naïve enough to still believe too much of what the media spews, but bright enough and interested enough that when provided more of the actual details of the “science” behind a specific topic, they recognize the misinformation.
The student was referring to the recent media reports that originated from a “study” which appeared in the journal Stroke, in April. Here are a few examples of the headlines this study generated:
- NY Times: “Diet Sodas Tied to Dementia & Stroke”
- Washington Post: “Study links diet soda to higher risk of stroke, dementia”
- NBC: “Diet Sodas May Raise Risk of Dementia & Stroke, Study Finds”
- CNN: “Diet sodas may be tied to stroke, dementia”
- USA Today: “Diet Soda can increase risk of dementia and stroke, study finds”
- US News – Health Buzz: “Drinking Diet Soda Linked to Stroke, Dementia Risk, Study Finds”
What basic science principles did the media fail to recognize that should have made a difference in their headlines?
Principle 1: To make a cause and effect relationship claim between two things, the study design must lend itself to determining it. The study published in Stroke was an observational study, which cannot determine a cause and effect relationship between two things, only a correlation. In other words, it may be true that the participants who had higher rates of strokes and dementia may have had higher rates of diet soda use, a correlation, but it does not mean that specific correlation, the diet soda, caused the increased rates. Additionally, the actual soda use by any study participant is questionable due to the way the data were collected, using of a food frequency questionnaire, which are highly unreliable. A true cause and effect relationship could only be calculated using an intervention study where one group of participants used a known amount of diet soda and comparing this population to another group consuming a placebo.
Principle 2: Don’t look beyond the obvious, stop the data dredging. The researchers did not control for obesity, a major preceptory factor for both conditions. Additionally, comments from the National Library of Medicine (NLM) report, “Behind The Headlines,” stated this regarding the study: “Overall, when taking account of all health and lifestyle factors that could have an influence … there was actually no link between artificially sweetened drinks and risk of dementia [my emphasis].” The NLM report also stated, “For stroke the links with artificially sweetened drinks were inconsistent. There were no overall links [my emphasis] when looking at longer term patterns.”
Principle 3: Provide a plausible mechanism of action. There is no plausible mechanism of action artificial sweeteners could cause stroke or dementia.
One of the most common artificial sweeteners in diet soda is aspartame, which has an extensive research illustrating its safety. As an example, aspartame is used in my diet Dr. Pepper and is a simple combination of the two amino acids aspartic acid and phenylalanine linked to a methanol, all naturally occurring compounds. All three compounds are separated into individual molecules prior to absorption and have well-established normal metabolic pathways. The body makes aspartic acid, which is also widely dispersed in food in much higher amounts than found in aspartame. Phenylalanine is an essential amino acid the body needs to make all protein molecules, and unless you have phenylketonuria (which is the inability to metabolize it), phenylalanine is metabolized no differently than from any other food in which it is naturally occurring. As with aspartic acid, it is also ingested in far greater concentrations naturally from food.
The methanol occurs naturally in food and is a normal byproduct of digestion. Some will fear the purported dangers of the formaldehyde byproduct of methanol digestion, but every cell in your body produces formaldehyde as part of its normal metabolism. There is also far more formaldehyde produced from the digestion of fruit juice than aspartame. For a detailed review of the safety of sugar substitutes, I recommend the report, “Sugar Substitutes & Your Health,” from the American Council on Science and Health (ACSH).
The actual media headlines should have read, “We dredged some data to make you fear your diet soda – but it’s all nonsense.”
Enjoy your diet soda. I sure do.