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Could Diet Soda Cause Clinical Depression?

A new study suggests a link, but it's important to remember the difference between causation and correlation

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A new study suggests a link between diet soda and depression, but it’s important to remember the difference between causation and correlation. Image via Flickr user DavidMartynHunt

Despite a lack of evidence in the scientific literature, we’ve seen aspartame—the calorie-free sugar sweetener found in diet drinks—blamed for a variety of health problems, everything from multiple sclerosis to migraine headaches. But here’s a new one: clinical depression?

In a preliminary release of a study to be published by Honglei Chen and colleagues from the National Institutes of Health, a survey of 263,925 adults nationwide indicated that consumption of sweetened drinks—especially diet sodas—was associated with an increased chance of a depression diagnosis. The authors, who will present their work at the American Academy of Neurology’s annual meeting in March, released only a summary of their study today.

To come to the findings, the researchers combined old data with new. They began by examining a survey originally conducted in 1995 and 1996 in which adults between the ages of 50 and 71 recorded their daily soda, tea, fruit punch and coffee consumption. Then, for this study, they returned to the same survey participants more than a decade later and asked if they had been diagnosed with depression in the years since 2000.

They found that those who drank four or more cans of sweetened drinks (whether soda, diet soda or fruit punch) had a significantly higher chance of being among the 11,311 study participants who were later diagnosed with clinical depression that those who didn’t. For sodas as a whole, there was a 30 percent greater chance of depression, but diet sodas carried a further 22 percent increase as compared to regular ones. Interestingly, regular coffee consumption was associated with a 10 percent lower chance of depression.

Does this mean you should stop drinking diet Coke and starting chugging coffee immediately? Probably not. This type of suggested link between two seemingly unrelated factors is an ideal time to bring up the difference between causation and correlation. Do the ingredients in both diet sodas and normally-sweetened drinks trigger changes in brain chemistry that lead to depression? Or are people with the tendency to become depressed simply more likely to drink these beverages in the first place?

Without the full paper, it’s hard to know for sure—we don’t know if the study’s authors controlled for all relevant factors, making sure to compare study participants who were alike in all ways except for their beverage consumption. As a result, a third, unrelated factor may cause people to both drink more soda and become depressed more frequently. Since the study is backward-looking, it’s especially hard to rule this out: The researchers can’t go back to 1996 and make sure to ask the participants every potentially relevant question to ensure that all potentially important factors have been taken into consideration.

Additionally, the fact that an association was found for both regular and diet sodas makes a causative link seem less likely. For that to be the case, either both sugar and aspartame must trigger depression, but at different frequencies, or a third ingredient in both sodas is responsible, but is somehow modulated by the presence of the natural or artificial sweetener.

So, what’s the simplest explanation? Those who like to drink a lot of diet soda are more likely to already be at risk of developing depression. And people who like to drink a lot of coffee are already less likely to be among this group. Perhaps, then, your underlying preference for how you get your energy buzz—whether through coffee or sweet drinks—may reflect something about your mental state.

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