Diet soda and heart, stroke risk: A link doesn’t prove cause and effect

A study just presented at the American Stroke Assn.’s International Stroke Conference reported a link between the amount of diet soda someone drinks and the risk of having a stroke or heart attack.

Really?

Here’s the outline of the study, which was started in 2003, and what it found:

A total of 2,564 people in the study were asked about their intake of sodas (among other questions) at the start of the study. After nine years, 559 cardiovascular events had occurred, and those who had reported drinking diet soda every day had a 60% higher rate of these events, which included various forms of stroke as well as heart attacks.

The scientists adjusted for certain factors, such as age, sex, race, smoking, exercise, alcohol and daily calories. When they added additional factors to do with heart disease risk, such as metabolic syndrome, the risk was still 48% higher for the daily-diet-soda-drinking group. (Metabolic syndrome is a group of factors that can include extra weight around the waist and the inability to efficiently process blood sugar.)

“If our results are confirmed with future studies, then it would suggest that diet soda may not be the optimal substitute for sugar-sweetened beverages for protection against vascular outcomes,” noted the study lead author, Hannah Gardener of the University of Miami School of Medicine, in a release from the stroke association. She made that point also in news reports, such as one written up by WebMD.

It’s worth noting, as some scientists did, that this is a link, not proof of cause and effect. After all, there are many things that people who slurp diet sodas every day are apt to do – like eat a lousy diet — and not all of these can be adjusted for, no matter how hard researchers try. Maybe those other factors are responsible for the stroke and heart attack risk, not the diet drinks. (Those who drink daily soda of any stripe, diet or otherwise, are probably not the most healthful among us.)

Here’s one comment from Dr. Steven Greenberg, a professor of neurology at Harvard Medical School and vice-chairman of the stroke-meeting conference committee: “You try to control for everything but you can’t.” (It’s from the report at WebMD.)

Second, the link depended on self-reported food intake – and self reports are not always that reliable.

Third, the self-report was at the start of the study. Habits do change over time.

This isn’t the first report suggesting a potential link to heart health, however, as fellow Times blogger Karen Kaplan noted in an item in January. The famous Framingham heart study has also reported a link between people who drank sodas -- diet or otherwise – and the risk for metabolic syndrome. I guess we’ll learn more as the months and years unfold -- if future studies can tease apart all the lifestyle factors that may be linked to a soda-guzzling habit.

In the meantime, water doesn’t prime us to expect cloying sweetness with every mouthful. And sodas --diet or otherwise -- can certainly do that.