We already know there's a lot to thank our gut bacteria for. But, new research suggests those little friends might be doing even more than we realize, including messing with our moods — and that probiotics can do the same. For the small study, published online earlier this month in Brain, Behavior, and Immunity, 40 healthy participants were randomly assigned to take either probiotics or placebos for four weeks. Participants also had to fill out questionnaires assessing their moods and thought patterns before and after their four-week supplement regimen. For instance, they were asked to rate to what degree they agreed with statements like, "When I feel sad, I more often think about how my life could have been different." The results showed that participants who got real probiotic supplements had significantly different questionnaire responses after the four weeks. In particular, their responses indicating a tendency to react to depressed moods with rumination and aggression decreased. This is a particularly important measure because the way we react to feeling sad can be used to predict whether or not true clinical depression takes hold. Previous research has linked our gut bacteria to both depression and anxiety. So, supplementing what we've got with probiotics could similarly affect our mental health, but it's still unclear exactly what's going on. Although the current study didn't control for the other things participants were eating, and it relied on self-reported data, it's not totally surprising that the study's authors conclude that "probiotics intervention can influence cognitive mechanisms that are known to determine vulnerability to mood disorders." We're constantly learning about the unintended effects of our go-to vitamins and meds — the good and the bad. Consider this just another reason to be extra-careful when trying new supplements.