“Our data suggest that the relation between soft drinks and depressive symptoms arises via the influence of the microbiome,” says study leader Sharmili Edwin Thanarajah, PhD, a psychiatry researcher at the University Hospital Frankfurt in Germany. The microbiome describes the collection of trillions of bacteria, fungi, parasites, and viruses that cohabitate in the human body, mostly in the gut.
“Our findings well align with increasing evidence that soft drink consumption is linked to both poor metabolic and mental health,” says Dr. Thanarajah.
Women Who Drank More Soda Had a Higher Risk of Depression
Thanarajah and her team wanted to see if people who regularly drank sugar-sweetened soda had higher levels of certain bacteria known to inflame the intestines, possibly leading to poor nutrient absorption — and whether that was connected to depression risk.
The scientists analyzed medical details from more than 900 German adults between ages 18 and 65. Participants filled out food questionnaires and also provided stool samples so researchers could analyze their gut microbiomes.
The authors looked at dietary habits and gut bacteria composition in about 400 people diagnosed with major depressive disorder, comparing them with about 500 healthy controls (people without major depressive disorder). Roughly 65 percent of each group was female.
The results, published this month in the journal JAMA Psychiatry, showed a significant link between self-reported soft drink consumption and depression diagnosis, as well as depression severity — but only among women, not men.
Women who drank the most soda had a 17 percent higher risk of depression compared with women who didn’t regularly drink it.
Women who had more soda also tended to have more severe self-reported depression symptoms. Again, this link was not seen among men. These more severe symptoms included intense sadness, suicidal thoughts, self-hatred, severe fatigue, and significant difficulty functioning.
As to why this effect only occurs in women, Thanarajah and her colleagues have no clear answer, but speculate that hormonal differences or sex-related reactions of the immune system may possibly play a role.
Hennis Tung, RD, a registered dietitian based in Hong Kong who started her career as a clinical dietitian in New York City, warns that just because researchers observed these effects only in women doesn’t mean that men can consume soft drinks without any possible consequences.
“Excessive sugary drink intake can still have numerous negative health effects for all, including weight gain and increased risk of chronic diseases,” says Tung, who was not involved in this study.
A Specific Type of Gut Bacteria May Play a Role
In this study, researchers observed that women who regularly drank sugary soft drinks experienced a significant increase in levels of a specific intestinal bacteria called Eggerthella.
“Excessive simple sugars in soft drinks disrupt the gut microbiome by fostering pro-inflammatory bacteria, weakening gut barrier integrity, and impairing mucosal immunity [immune response that occurs in the internal lining of the intestines],” wrote Thanarajah and coauthors.
This inflammation can extend to the central nervous system, which includes the brain and spinal cord. The inflammatory response, in turn, may trigger depressive symptoms, the researchers wrote.
“Essentially, higher levels of this bacteria disrupt the gut microbiome and contribute to inflammation that negatively impacts well-being,” says Tung.
Are Diet Sodas Any Safer?
“Additives in some soft drinks, such as artificial sweeteners and preservatives, may further disrupt gut microbial balance and potentially impact mental health,” says Uma Naidoo, MD, a psychiatrist and director of Nutritional and Metabolic Psychiatry at Massachusetts General Hospital in Boston who was also not involved in the new research.
The Study Had Some Limitations
This investigation had strengths in that it drew on a large group with clinically defined major depressive disorder, and research analyzed biological samples demonstrating a possible link between specific bacteria and depression.
However, participants were entirely from Germany, dietary data was self-reported, and the design of the research was largely observational, so it did not show if changes in diet, for example, could affect outcomes in terms of depression.
Thanarajah and her collaborators wrote that more research is needed to understand how targeted nutritional therapies might help relieve depression symptoms.
Dr. Naidoo, who is a member of the American Psychiatric Association, points out that while an unhealthy diet may affect mental health, poor mental health can also lead to bad food choices.
“Depression may increase the preference for high-sugar foods and drinks, creating a vicious cycle,” she says.
Breaking the Cycle With Healthier Choices
The study results highlight the need to break unhealthy patterns that can send people spiraling, the authors wrote.
Dietary changes, particularly reducing sugary drink intake, could be a helpful step, according to Tung.
If you’re looking to cut back on a soda habit, she recommends setting realistic goals and making small, gradual changes instead of quitting cold turkey.
“Look for alternatives to see what you like most and can follow through with, such as unsweetened tea, infused water, sparkling water, or seltzer,” she says.
