While most people know soda isn’t great for our waistline or our teeth. A different and perhaps more important question is whether it might affect how we feel. In 2025, a German research team published a study in JAMA Psychiatry that tried to answer that question and to figure out if the gut bacteria might be part of the story. ¹
What the Researchers Did
The team looked at 932 adults in Germany between 18 and 65 years old. ¹ of those:
• 405 had been diagnosed with major depression through a full clinical interview
• 527 had no history of depression and served as the comparison group
Everyone filled out a detailed food questionnaire about what they ate and drank over the past year, including how often they had soda. Each person also gave a stool sample so the researchers could see which bacteria were living in their gut. Depression severity was measured with a standard symptom questionnaire.
What They Found
People who drank more soda were more likely to have depression. They also tended to have more severe symptoms. People with depression drank about 114 grams of soda per day on average, compared to about 77 grams for people without depression. That’s roughly half a glass versus a third of a glass a day. ¹
When the researchers split the results by sex, the connection was clear for women and essentially absent for men. ¹ It’s not clear why. Hormones, differences in gut bacteria, or simply the smaller number of men in the study could all play a role.
A gut bacterium called Eggerthella may be part of the picture. In women, more soda was linked with more Eggerthella in the gut. This bacterium has shown up more often in people with depression in earlier research. Women who drank more soda also had less variety in their gut bacteria overall, which is generally correlated with higher risk for chronic diseases.
But the gut bacteria piece was small. Eggerthella explained only about 4% to 5% of the connection between soda and depression. ¹ So even if this bacterium matters, most of the link comes from something else, whether that’s other bacteria, inflammation, blood sugar swings, overall diet, or factors the study didn’t measure.
Soda’s overall effect was modest. Soda accounted for only a small slice of the differences in depression symptoms between people, roughly 1% overall and about 3.5% in women.¹ Depression has many causes, so no single drink would be expected to explain much on its own.
How Much Should We Trust This Study?
Let’s start with what the researchers did well. They were clear about who could take part: adults between 18 and 65, with people who had serious neurological or medical conditions left out, so those illnesses wouldn’t muddy the results. They also described the participants in detail, including age, sex, body weight, education, and depression history, so it’s easy to see who was studied. And they didn’t just ask people whether they felt depressed. Each person sat down for a structured clinical interview, the same kind of careful evaluation used to make a real diagnosis. That’s a big step up from the quick mood surveys many diet studies rely on. The stool samples were handled the same way at both study sites, which makes the gut bacteria more trustworthy, though the testing method only identifies bacteria by broad group rather than in fine detail.
The weaker spots are mostly about soda itself. To find out how much soda people drank, the researchers asked them to think back over an entire year and estimate. Most of us would struggle to do that accurately. The bigger problem is that the questionnaire couldn’t reliably tell regular soda from diet soda, and that matters a lot for the question many readers care about.
There’s also the question of what else might explain the results. The researchers did account for several important factors, including education, body weight, total calories, and antidepressant use, and their findings held up after those adjustments. But they didn’t report on other things that affect both mood and gut health, like exercise, smoking, alcohol, sleep, overall diet quality, income, or recent antibiotic or probiotic use. Any of these could be part of the story.
There are a few other areas that concern me. Almost half of the people who signed up (869 of 1,801) were dropped because their food information was missing. If those people were different in some meaningful way, the results could be skewed. The researchers also chose to focus on women only after they noticed the effect showed up mainly in women. Following that lead makes sense, but it means the findings in women are a promising clue rather than a settled answer. Finally, because soda habits, gut bacteria, and mood were all measured at the same time, the study can’t tell us which came first. Soda might affect mood, or feeling low might lead people to reach for soda. The authors are upfront about this.
My overall take: Overall, this was a well-done study that helps to move the conversation forward on how important gut microbiome is for mental health. It falls short in some areas, and I think that is okay. It doesn’t mean invalidate the findings, and it does add to the growing body of evidence demonstrating the role our microbiota and diet may play a significant role in our mental health.
“I Drink Diet Soda. Does this affect me?”
This is the question I imagine I would hear from a large portion of my patients and the honest answer is: this study can’t tell you.
The researchers lumped regular and diet sodas together because people had trouble remembering, and labels didn’t make clear which type they drank. When they tried to look at artificially sweetened drinks separately, they didn’t find a clear link to depression symptoms. However, they said their data simply wasn’t detailed enough to answer the question either way. ¹ Much of the explanation they offer, such as sugar feeding certain gut bacteria, applies to sugary soda specifically.
Other research gives us a few more clues:
• A large US study of more than 260,000 older adults found that people who drank four or more cans of soda a day had a higher risk of later being diagnosed with depression. The link looked somewhat stronger for diet versions than for regular. ²
• A study following more than 30,000 women found that those who consumed the most artificially sweetened drinks and sweeteners had a higher risk of developing depression over time.³
• A small, carefully controlled trial found that some artificial sweeteners changed people’s gut bacteria. How much they changed varied a lot from person to person.⁴
• The World Health Organization advises against relying on non-sugar sweeteners for weight control, mostly because of uncertain long-term effects. ⁵
All of these studies show associations, not proof. People who drink a lot of diet soda are often already trying to manage weight, blood sugar, or other health issues, and those can affect mood too.
Here’s what I’d tell you in the office:
• One diet soda doesn’t put you on a path to depression. If there’s any risk, it appears small and probably depends on how much you drink and what the rest of your diet looks like.
• If you’re drinking several cans a day, cutting back is a low-risk experiment. Try swapping some of them for sparkling water, water with a squeeze of citrus, or unsweetened tea. Pay attention to how your energy, sleep, and mood feel over a few weeks.
• Feed the good bacteria. Vegetables, beans, whole grains, nuts, and fermented foods like yogurt or kefir all support a more varied gut, and that variety was lower in heavy soda drinkers in this study.
• Mood is bigger than any one drink. Sleep, movement, stress, relationships, genetics, and medical factors all matter more than what’s in your glass.
The Bottom Line
This study adds to growing evidence that what we drink may matter for mental health, especially for women, and that gut bacteria may be one small part of how that works. It isn’t proof that soda causes depression, and it can’t answer the diet soda question. Cutting back on soda of any kind is a cheap, low risk change that’s good for your body and may help your mood too.
If you’re struggling with low mood, please don’t try to fix it with diet changes alone. Talk with your clinician. Nutrition works best as one piece of a fuller plan, not a replacement for care.
This post is for general education and isn’t a substitute for personal medical advice.
References
1. Edwin Thanarajah S, Ribeiro AH, Lee J, et al. Soft drink consumption and depression mediated by gut microbiome alterations. JAMA Psychiatry. 2025;82(11):1095-1102. doi:10.1001/jamapsychiatry.2025.2579. https://doi.org/10.1001/jamapsychiatry.2025.2579
2. Guo X, Park Y, Freedman ND, et al. Sweetened beverages, coffee, and tea and depression risk among older US adults. PLoS One. 2014;9(4):e94715. doi:10.1371/journal.pone.0094715. https://doi.org/10.1371/journal.pone.0094715
3. Samuthpongtorn C, Nguyen LH, Okereke OI, et al. Consumption of ultraprocessed food and risk of depression. JAMA Netw Open. 2023;6(9):e2334770. doi:10.1001/jamanetworkopen.2023.34770. https://doi.org/10.1001/jamanetworkopen.2023.34770
4. Suez J, Cohen Y, Valdés-Mas R, et al. Personalized microbiome-driven effects of non-nutritive sweeteners on human glucose tolerance. Cell. 2022;185(18):3307-3328.e19. doi:10.1016/j.cell.2022.07.016. https://doi.org/10.1016/j.cell.2022.07.016
5. World Health Organization. Use of Non-Sugar Sweeteners: WHO Guideline. World Health Organization; 2023. Accessed October 9, 2026. https://www.who.int/publications/i/item/9789240073616
6. JBI. Critical appraisal tools: checklist for analytical cross sectional studies. Accessed October 9, 2026. https://jbi.global/critical-appraisal-tools
