Diet Soda Is Bumming You Out

Looking for a little afternoon pick-me-up? Grab a Diet Coke. On second thought, maybe don’t.

Turns out that diet drink on your office desk may be dragging you down more than it’s picking you up. Preliminary findings from a far-reaching study of American adults funded by the National Institutes of Health (NIH), the National Institute of Environmental Health Sciences, and the National Cancer Institute suggest there is a link between sugary drinks—particularly so-called “diet” drinks—and depression.

Led by Dr. Honglei Chen, an investigator with the NIH and a member of the American Academy of Neurology, a research team examined the beverage consumption habits of 263,925 adults from 1995 to 1996, all of whom were between 50 and 71 years-old. About a decade later, more than 11,300 of those subjects reported having experienced depression since 2000.

Researchers crunched the numbers and found a strong correlation between drinking sugary drinks and later onsets of depression. Specifically, subjects who drank four or more cups of soft drinks of any kind were 30 percent more likely to develop depression later in life than those who didn’t. Those who drank similar amounts of fruit punches were 38 percent more likely.

Broken down further, diet drinks were worst of all. Diet soda drinkers were 31 percent more likely to report depression than non-soda drinkers, compared to 22 percent for regular soda drinkers. Among fruit punch drinkers, the difference was even more striking: Diet drinkers versus non-diet drinkers were respectively 51 percent and 8 percent more likely to develop depression than non-fruit punch drinkers.

In contrast, coffee drinkers were nearly 10 percent less likely to report a later onset of depression than subjects who didn’t drink coffee. Iced tea consumption was likewise examined and the results were similar, suggesting a link between caffeine and decreased depression. Diet iced tea drinkers were 25 percent more likely to report depression than non-iced tea drinkers. But, like coffee drinkers, consumers of regular iced tea were 6 percent less likely to report depression than non-iced tea drinkers.

Assuming that many of the soft drinks consumed by subjects also contained caffeine, the correlations suggest that depression associated with sugary drinks—both real sugar and synthetic, like aspartame—is stronger than whatever positive mood benefits may result from regular caffeine consumption.

Of course, this assumes there’s any causality at all—something researchers have yet to determine. It’s conceivable that heavy coffee drinkers are simply less likely to be depressed than people who crave sugary drinks—which, like a late night pint of Haagen Dazs during a nasty break-up, may provide the brain with what amounts to a therapeutic shot of glucose (along with an ensuing crash and oh, so much regret).

With regard to diet drinks specifically, it’s also possible that people who feel compelled to drink diet drinks have lower self-esteem and are more likely to report depression.

Results of the study were controlled for factors like age, sex, education, and—perhaps most importantly—weight and exercise. But outside the constructs of the study, the data may offer another strike against the diet drink industry. In the real world, many diet drinkers are also overweight. A University of Texas study from 2005, for example, found that diet soda drinkers tend to have bigger waistlines than non-diet drinkers. Authors of that study were careful to note that diet soda drinkers may be more likely to start out overweight, drinking diet as a (apparently futile) weight loss measure. But they also note that every time someone drinks a diet soft drink, it’s at the expense of a healthier drink—like water or fresh juice.

Other studies suggest that artificial sweeteners may actually cause obesity directly, if unexpectedly. One theory says they (like real sugars) affect the bacteria in our guts, which may interfere with proper metabolism and our ability to feel full. Other studies suggest that, unlike real sugars, artificial sweeteners may make us feel hungrier and may fail to activate the brain’s natural reward system—the same one activated by sex and drugs.

The real-world implication is clear. Meta-analyses of studies examining weight and depression (like this one, cited by the NIH) confirm what we already intuitively know: Being overweight or obese has a strong, reciprocal relationship with depression. In that context, sugary drinks—and diet drinks, specifically—could be a double-whammy. They might make you depressed and fat, which, in turn, makes you more depressed.

At the very least, all of these explanations suggest that there is some kind of vicious cycle driven by obesity, artificial sweeteners and depression. One hopes the exact relationship will continue to be teased out by researchers in coming years.