Depression and anxiety are increasingly common worldwide. In response, researchers are examining not only medications and psychotherapy, but also nutrition and the biology of the gut as possible contributors to emotional health.
One area of sustained scientific interest is the gut microbiome, which is the community of micro-organisms that reside in the digestive tract.
Once considered relevant mainly for digestion, the microbiome is now understood to communicate bidirectionally with the brain through what is known as
This communication occurs through several well-characterized biological pathways:
- Neural signaling, particularly via the vagus nerve
- Immune signaling, including inflammatory cytokines that can influence brain function
- Hormonal signaling, such as stress-related hormones including cortisol
Preclinical research has demonstrated that these pathways are functionally significant. In animal models, transferring gut microbiota from individuals with major depressive disorder into germ-free rodents can induce behavioral and physiologic
features resembling depression(2).
These findings do not imply that microbes alone cause mood disorders, but they do establish that the gut microbiome can influence brain-related behavior under controlled conditions.
This raises an important clinical question which is, can modifying the gut microbiome improve mood in humans?
Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.
In animal studies, specific probiotic strains have been shown to:
- Reduce stress hormone levels
- Decrease pro-inflammatory cytokines
- Alter anxiety-like behavior
- Modify neural activity related to emotional regulation(3)
Human trials have yielded mixed findings. Some randomized studies report modest improvements in depressive or anxiety symptoms, particularly in individuals with existing mood disorders(4,5).
Meta-analyses suggest an overall small but statistically significant effect of probiotics on
depressive symptoms, though results are more consistent in clinical populations than in healthy volunteers(6). The present study sought to clarify these inconsistencies by employing more temporally sensitive mood assessments.
Study Design
Eighty-eight healthy young adults participated in a randomized, double-blind, placebo-controlled trial.
Participants were assigned to receive either:
- A multi-species probiotic supplement
- A placebo
- The intervention lasted four weeks
Mood and emotional functioning were assessed using three complementary approaches:
- Standardized psychological questionnaires, administered before and after the intervention
- Laboratory-based emotional processing tasks, including facial emotion recognition
- Daily mood ratings, in which participants rated their overall positive or negative mood each day on a 0–100 scale
This multi-modal approach allowed the investigators to detect both categorical changes and gradual mood shifts over time.
What the Research Found
Standard Questionnaire Limitations
Pre- and post-intervention questionnaire scores showed no statistically significant differences between probiotic and placebo groups on measures of depression,
anxiety, or emotion regulation.
If the study had relied exclusively on these conventional endpoints, the conclusion would likely have been that probiotics exert no psychological effect in healthy individuals.
Daily Mood Ratings
The daily mood data revealed a different pattern. Participants receiving probiotics experienced a gradual reduction in negative mood over the four-week period relative to placebo.
Notably:
- Positive mood did not decline.
- Divergence between groups emerged after approximately two weeks.
- The separation increased toward the end of the intervention.
The timing is clinically relevant.
Conventional antidepressants also often require several weeks before symptom improvement becomes evident(7).
Although probiotics and antidepressants act through distinct primary mechanisms, both may influence inflammatory signaling and neuroendocrine pathways, including stress hormone regulation(3,8). The observed probiotic effect was modest but consistent over time.
Why Daily Monitoring Detected Change
Traditional questionnaires typically assess mood at two discrete time points including baseline and follow-up. While validated and reliable, this method assumes that meaningful changes will produce measurable shifts in composite scores. Daily mood monitoring offers greater temporal resolution.
Rather than capturing only endpoints, it traces the trajectory of change. In this study, mood improvement appeared gradually and may not have been large enough to significantly alter standardized scale totals over a short four-week interval.
The daily question
“How positive or negative do you feel today?” provided a global assessment of emotional state without subdividing experience into predefined constructs such as irritability or distress. This broader framing may have enhanced sensitivity to subtle changes.
Individual Differences
Exploratory analyses suggested that participants with higher baseline risk aversion, which is a trait associated with cognitive vulnerability to depression, showed greater reductions in negative mood during probiotic supplementation.
Risk aversion and rumination are recognized predictors of depressive vulnerability(9). These findings align with prior research indicating that probiotic effects may be stronger in individuals with elevated baseline psychological symptoms(6).
While preliminary, this observation raises the possibility that probiotics may exert greater benefit in psychologically vulnerable subgroups rather than uniformly across healthy populations.
Emotional Processing Tasks
Laboratory-based assessments showed:
- No significant changes in attentional bias toward emotional stimuli
- A modest improvement in facial emotion recognition accuracy in the probiotic group
These effects were secondary and subtle but consistent with previous findings that microbiome-targeted interventions can influence neural processing of emotional information(10).
Interpretation of the research
This study does not establish probiotics as a
treatment for depression. Participants were healthy young adults, and observed effects were modest.
However, the findings contribute to the broader literature in two important ways:
- They support evidence that microbiome-targeted interventions may influence negative mood, even in nonclinical populations.
- They demonstrate that daily mood monitoring may detect psychological effects that traditional pre- and post-intervention questionnaires overlook.
Given ongoing inconsistencies in probiotic research, methodological differences
(particularly in outcome measurement) may partially explain variable results across studies.
Methodological Implications
Increasingly, psychological research is incorporating ecological momentary assessment (EMA), which is repeated real-time mood reporting to capture emotional dynamics more accurately in daily life. Such approaches have been shown to predict depressive symptom development and clinical outcomes(11).
This study suggests that frequent, simple mood reporting may enhance sensitivity in intervention research beyond the microbiome field.
Emotional change often unfolds gradually, so measurement strategies should be capable of detecting that trajectory.
In clinical research, sophistication of measurement is not synonymous with complexity. Sometimes a direct, consistently asked question such as
“How are you feeling today?” provides clinically meaningful insight that multi-item instruments may not fully capture.
PRO+FLORA PROBIOTIC
Look, there’s a war going on inside your gut, and as with any war the key to a good defense is a good offense. If you’re looking for a quick and effective way to improve your gut health, fortify your immune system, and potentially improve your mood, there may be no more efficient way than with a daily probiotic. By balancing intestinal microflora, probiotics play an important role in regulating intestinal function and digestion and more.
When it comes to fortifying your body and building an immune system of STEEL,
Pro+Flora Probiotic helps you stay on point by providing your gut flora the fuel they need to keep you healthy and strong!
References:
- Cryan JF, Dinan TG: Mind-altering microorganisms: The impact of the gut microbiota on brain and behaviour. Nat Rev Neurosci 13:701-712, 2012
- Kelly JR, Borre Y, O’Brien C, et al: Transferring the blues: Depression-associated gut microbiota induces neurobehavioural changes in the rat. J Psychiatr Res 82:109-118, 2016
- Bravo JA, Forsythe P, Chew MV, et al: Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proc Natl Acad Sci U S A 108:16050-16055, 2011
- Akkasheh G, Kashani-Poor Z, Tajabadi-Ebrahimi M, et al: Clinical and metabolic response to probiotic administration in patients with major depressive disorder: A randomized, double-blind, placebo-controlled trial. Nutrition 32:315-320, 2016
- Kazemi A, Noorbala AA, Azam K, et al: Effect of probiotic and prebiotic vs placebo on psychological outcomes in patients with major depressive disorder: A randomized clinical trial. Clin Nutr 38:522-528, 2019
- Ng QX, Peters C, Ho CYX, et al: A meta-analysis of the use of probiotics to alleviate depressive symptoms. J Affect Disord 228:13-19, 2018
- Harmer CJ, Duman RS, Cowen PJ: How do antidepressants work? New perspectives for refining future treatment approaches. Lancet Psychiatry 4:409-418, 2017
- Wallace CJK, Milev R: The effects of probiotics on depressive symptoms in humans: A systematic review. Ann Gen Psychiatry 16:14, 2017
Van der Does AJW: The Leiden Index of Depression Sensitivity (LEIDS). Psychol Med 32:143-149, 2002 - Tillisch K, Labus J, Kilpatrick L, et al: Consumption of fermented milk product with probiotic modulates brain activity. Gastroenterology 144:1394-1401, 2013
- Wichers M, Groot PC: Critical slowing down as a personalized early warning signal for depression. Psychother Psychosom 85:114-116, 2016