For decades probiotics—also known as flora or beneficial bacteria—have been associated with digestive health. They promote regularity, cure bloating, restore gut balance after antibiotics, and support the immune system. But now, thanks to evolving research, another amazing health benefit is emerging: Brain health. The concept of a gut-brain connection was once considered fringe; however, it is now gaining traction even with the medical establishment. This unique biological interlink is most commonly referred to as the “gut–brain axis.” Today experts in neuroscience, psychology and gastroenterology alike have recognized the gut-brain axis. Put simply, it is idea that the gut microbiome is an influential and inseparable partner in how the brain works. Some health experts call the gut a second brain—because the gut is teeming with microbial “messengers” that can subtly steer your mood, memory, and even your outlook on life. This sophisticated and innate communication happens via the vagus nerve, the immune system, and hormone signaling. With all this in play, some scientists are asking if we can fix, or at least assuage, depression and other cognitive issues by fortifying the gut. Numerous studies have pointed to consistent gut microbiome defects in patients with anxiety, depression and bipolar disorder. Learn more about this in our earlier post Similar gut microbiome deficiencies found in patients with anxiety, depression and bipolar disorder. But these findings are just associations. They don't necessarily prove that the literal action of swallowing probiotics—whether from food or supplements—will affect depression and other disorders. But for many a new study has settled the debate. A study with a new approach Right from the start the researchers involved in the new study acknowledged that there is a debate on the issue. Even though animal studies have repeatedly shown that probiotics can influence brain chemistry and behavior, the human studies have often been less conclusive. Some trials showed promise, while others did not. To settle the question, the researchers, based at Leiden University in The Netherlands, designed a more sensitive, nuanced study—one that could detect emotional shifts as they unfolded in real time. In the new study design, instead of relying solely on standard psychological questionnaires, the researchers included daily mood reports. According to the researchers, gathering data through standard psychological profiling can often miss the mark when it comes to subtle and numerous emotional changes. In essence, the new design allowed participants to track their feelings "in the moment." This created a much clearer and more dynamic picture of how their emotions evolved over time. Study details The study, published in Nature in April 2025, involved 84 healthy adults who participated in a randomized, double-blind, placebo-controlled trial. The participants were split into two groups, with one group taking a probiotic supplement for 28 days while the other group served as a control. The probiotic supplements contained familiar strains such as Lactobacillus and Bifidobacterium. The placebo group received an identical-looking capsule but without the active cultures. After just two weeks consistent changes were already noticed: participants who took the probiotics reported fewer negative feelings. These included reduced levels of stress, anxiety, fatigue and depressive tendencies. Notably, these changes weren't reported by the placebo group. And that wasn't the end of the goods news: The probiotics didn’t flatten all emotions, as is the tendency with prescription antidepressants. Instead, the probiotic "prescription" selectively reduced negative moods, leaving positive feelings untouched. “It is striking that by simply asking participants how they were feeling each day, we could detect the beneficial effects of probiotics on mood,” the researchers wrote in the study findings. “In contrast, the standard psychological questionnaires that are common in this field were not sensitive enough to pick up these changes.” Fighting the depression epidemic The good news contained in the study findings cannot be understated. Depression is one of world's major health challenges. In the U.S. approximately 14% of all adults aged 18 and over take antidepressant medications. An estimated 25% of those "antidepressant consumers" are long-term users; and thanks to these long-term users antidepressant sales are a $6 billion a year industry in the U.S. alone. Hence, the potential for probiotics is significant. Even with scientists emphasizing that probiotics would be a compliment to pharmaceuticals, not a replacement, the timeline of beneficial effects is similar to that of common antidepressants. That time frame is generally around two weeks. But unlike drugs, probiotics are generally well-tolerated, have few side effects, and don't require a prescription. Earlier study The new 2025 study built on a previous 2017 mouse study that helped lay some ground work. In the 2017 study, researchers at the University of Virginia School of Medicine fed mice a Lactobacillus probiotic via yogurt. The team found that the rodents’ depression-like symptoms were largely reversed by consuming the yogurt. The findings were published that year in Scientific Reports. Could this be the beginning of a new era in treating depression? There's already a name for the application of probiotics in treating cognitive function—psychobiotics. Perhaps one day psychobiotics will stand side-by-side with therapy sessions and prescription drugs in the sphere of treating depression; however, past history tells us it takes a long time to convince medical authorities to amend long-established practices. Until that time, consumers who take probiotics to support immune function will know they're receiving an additional research-backed benefit: cognitive health support. Sources for this article include: Nature, Scientific Reports.
