Quick Facts

• Higher gut bacteria diversity was associated with stronger stress hormone responses in 74 healthy adults
• Participants with elevated butyrate-producing bacteria showed increased stress reactivity
• Researchers say stronger acute stress response enables flexible adaptation to challenges

New research challenges the common belief that more gut bacteria always equals better health outcomes. A University of Vienna study found that people with higher gut microbiome diversity actually showed stronger stress responses.

The study, published in Neurobiology of Stress in April 2026, examined 74 healthy adults between ages 18 and 34. Researchers collected stool samples from participants, then exposed half to the established Trier Social Stress Test while the other half completed a neutral task.

The results surprised researchers. Higher microbial diversity correlated with stronger hormonal and subjectively perceived stress reactivity. Participants with elevated butyrate-producing bacteria showed increased stress responses, while those with higher propionate production capacity had lower stress reactivity.

Study leader Thomas Karner, a psychologist at the University of Vienna, explained the findings. “A stronger acute stress response is not necessarily detrimental,” Karner said. “Appropriate activation of the stress system enables flexible adaptation to challenges and threats. A greater diversity of gut bacteria, as well as certain metabolic products, could play a supportive role in this process.”

The research marks the first time scientists have linked gut bacteria diversity and metabolite production capacity to acute stress response in healthy adults. Previous studies focused mainly on chronic stress and disease states.

Participants in the study showed concerning dietary patterns. Nearly 70 percent ate only one to two meals daily, and 71 percent consumed fast or processed foods one to two times weekly. Gastrointestinal problems affected 56 percent of participants.

Two-thirds of participants believed stress influenced their digestion, and 74 percent experienced diet changes affecting their gut health. These findings highlight the bidirectional relationship between stress and gut function.

The gut-brain axis operates through multiple pathways including the vagus nerve, immune signaling, and bacterial metabolites. The hypothalamic-pituitary-adrenal axis coordinates stress responses and serves as a key communication route in this system.

Dr. Arpana Gupta, co-director of the UCLA Goodman-Luskin Microbiome Center, emphasized the therapeutic potential. “If we can identify what a healthy resilient brain and microbiome look like, then we can develop targeted interventions to those areas to reduce stress,” Gupta said.

The University of Vienna team is developing a non-invasive stool test to measure microbial stress-resilience markers. This tool could enable personalized mental health care within five years.

A 2024 meta-analysis in JAMA Psychiatry found that people with major depressive disorder had significantly lower microbial richness than healthy controls. Probiotic interventions improved symptoms in 32 percent of cases, matching some pharmaceutical treatments for mild-to-moderate depression.

The research suggests that moderate, time-limited stress responses indicate a flexible, adaptive system. The goal is developing appropriate responses followed by efficient recovery.

Read more: Did You Know Your Gut Affects Stress Response? New Research Explains