Healthy gut, healthy brain: Probiotic strain Bifidobacterium A6 offers dual relief for depression and constipation, study finds

• A probiotic strain (Bifidobacterium animalis subsp. Lactis A6) improves both depression and constipation in those with both conditions.
• It rebalances tryptophan processing, boosting mood-enhancing serotonin while reducing stress-linked kynurenine.
• The study, involving 300 Chinese participants, found no benefits for depression alone, emphasizing the role of gut dysfunction.
• Animal models supported the findings, showing the probiotic reverses constipation and depressive-like behaviors in rats.
• Researchers urge further global trials to confirm efficacy across diverse populations.

A groundbreaking study from China Agricultural University reveals that a specific probiotic strain could simultaneously alleviate symptoms of depression and constipation in afflicted individuals, marking a pivotal step in leveraging gut health to address mental and physical ailments. The research, led by scientists including Dr. Jian Wang and Dr. Yimei Ren, focuses on Bifidobacterium animalis subsp. Lactis A6, a microbial strain derived from a centenarian’s gut microbiome. Published in June 2025, the findings highlight the intertwined gut-brain axis and suggest tailored probiotics may revolutionize treatment for comorbid conditions.

Key mechanism: How probiotic A6 rebalances gut chemistry

The probiotic’s success hinges on its ability to recalibrate tryptophan metabolism, a pathway central to both mental well-being and digestion. Tryptophan, an amino acid in food, splits into two molecular branches: one producing serotonin (a mood regulator and digestion aid) and the other generating kynurenine (linked to stress and inflammation). In the trial, participants with both depression and constipation who took A6 saw serotonin levels rise and kynurenine levels drop.

Stool and blood tests further revealed increased populations of beneficial bacteria like Lactobacillus paracasei and Bifidobacterium animalis, aiding gut ecology. Animal studies reinforced these insights: lab rats on low-fiber diets developed depression-like behaviors and constipation, which the probiotic reversed by boosting tryptophan hydroxylase 1 (a serotonin-producing enzyme) and suppressing indoleamine 2,3-dioxygenase 1 (which spurs kynurenine).

“The gut isn’t just a passive organ — it’s actively involved in brain chemistry,” explains Dr. Shanbin Chen, co-author of the study. “When combined with constipation, depressed patients may have impaired gut-brain signaling that probiotics can repair.”

Trial results highlight specific efficacy in dual-affliction patients

The eight-week, double-blind trial randomly assigned 300 participants into four groups: Individuals with depression only and constipation only, receiving either the probiotic or a placebo. Only those with both conditions showed significant improvements. Depressive symptoms, measured by the Hamilton Depression Rating Scale, dropped from an average 12.77 to 9.48 — a reduction large enough to improve daily functioning. Constipation scores (assessed via the Patient Assessment of Constipation-Symptom Scale) fell from 1.13 to 0.97.

Notably, the probiotic had no effect on participants with depression alone. This underscores the necessity of addressing gut dysfunction for the therapy to work, aligning with growing evidence of the gut’s role in mental health comorbidities.

Broader implications and path forward: Precision medicine and more research

The study challenges traditional antidepressant strategies, which focus solely on brain chemistry. Instead, it advocates treating brain-related conditions through gut health, a concept rooted in decades of gut-brain axis research. Historically, scientists recognized the gut’s status as the “second brain” in the 1990s, though conclusive clinical applications have been elusive until now.

Yet, the findings are not without caveats. The trial included only Chinese participants, raising questions about its applicability to other demographics. Dietary factors, which profoundly shape gut microbiota, were not controlled for, and long-term effects remain unknown. Dr. Fazheng Ren, another co-author, admits, “We need larger, culturally diverse studies to confirm these benefits globally.”

The strain’s origin — a centenarian’s microbiome — hints at personalized probiotics tied to specific human populations or conditions. Most products currently on the market feed existing gut flora, but this study points to strain-specific therapies as a next frontier.

The gut as an uncharted terrain for wellness

The discovery of Probiotic A6’s dual efficacy signals a paradigm shift in viewing mental and digestive health as interconnected systems. By targeting the gut to treat depression, the research bolsters precision medicine’s promise of tailored interventions. However, widespread use hinges on scientific validation and understanding diversity in human microbiomes. For now, it offers hope to those with comorbid gastrointestinal and mental health issues—rewriting the narrative that gut health is merely about digestion.

As science continues mapping the gut-brain axis, this research stands as a beacon for future natural health solutions: It is holistic, rooted in biology and sparked by the resilience of the microbiome.

Sources for this article include:

StudyFinds.org

ScienceDirect.com

NutritionProtocol.com

Harvard.health.edu