This randomized, double-blind, placebo-controlled study evaluates the efficacy and safety of an 8-week probiotic supplementation (Lactobacillus plantarum LP140 and Bifidobacterium breve BB010 in total daily dose 1 × 10¹⁰ CFU or matching placebo) in reducing perceived stress and anxiety, and improvement related psychological parameters in healthy university students during the examination period. The primary outcome is the change in Perceived Stress Scale (PSS-10) score at week 5 and week 8 of interevention compared to placebo. The secondary outcomes are the changes in DASS-21 subscales (Stress, Anxiety, Depression), State-Trait Anxiety Inventory - State version scale (STAI-S), Sense of Coherence Scale (SOC-29), Pittsburgh Sleep Quality Index (PSQI), self-reported gastrointestinal symptoms (Likert scale) and incidence of adverse events during the 8-week lasting interventional study.

The gut-brain axis enables bidirectional communication between the gastrointestinal tract and the central nervous system. Emerging evidence suggests that selected probiotic strains (psychobiotics) may modulate stress responses and emotional regulation. University examination periods constitute a predictable model of transient psychological stress in otherwise healthy individuals.

This study investigates whether daily supplementation with a probiotic combination of Lactobacillus plantarum LP140 and Bifidobacterium breve BB010 reduces perceived stress and improves related psychological parameters in healthy students exposed to exam-related stress. Two hundred male and female students aged 18-40 years with at least moderate stress and mild anxiety, but without clinical depression, will be enrolled at the Medical University of Lublin, Poland. After providing written informed consent and confirming eligibility criteria, participants will be randomized in a 1:1 ratio to receive either a probiotic formulation containing Lactobacillus plantarum LP140 and Bifidobacterium breve BB010 (total daily dose of 1 × 10¹⁰ colony-forming units) or a matching placebo.

The investigational product and placebo will be identical in appearance and administered orally once daily (two capsules) for 8 weeks. Randomization will be computer-generated, and both participants and investigators will remain blinded to treatment allocation throughout the study. The intervention will begin approximately six weeks before the first scheduled academic examination. Study assessments will be conducted at baseline, at week 5 (approximately one week before the first examination), and at week 8 (±3 days before a subsequent examination). Psychological and functional parameters will be collected using validated self-report instruments administered electronically, with selected assessments performed during on-site visits. Participants will be asked to maintain their usual diet, physical activity, and caffeine intake during the study period. Safety and tolerability will be monitored throughout the intervention, and adverse events will be recorded using an electronic reporting system with daily reminders.

The study is powered to detect clinically meaningful differences between groups in perceived stress levels and will be analyzed using appropriate covariance-based statistical models adjusted for baseline values and relevant covariates. All participant data will be coded to ensure confidentiality and handled in accordance with applicable data protection regulations.

Inclusion Criteria:

• Male or female students aged 18-40 years
• At least moderate stress (DASS-21 Stress score ≥10)
• Mild anxiety (DASS-21 Anxiety score 6-13)
• No clinical depression (DASS-21 Depression score ≤9)
• Willingness to maintain usual diet, physical activity, and intake of caffeinated beverages during the study
• Written informed consent

Exclusion Criteria:

• Irritable bowel syndrome (Rome IV criteria) or any chronic intestinal disease (including celiac disease)
• Known food allergies, immunodeficiency disorders, or immunosuppressive treatment
• Antibiotic use within 4 weeks prior to screening
• Any psychiatric illness within the past 5 years
• Any malignant disease or autoimmune or allergic diseases (e.g., atopic dermatitis, asthma)
• Use of systemic psychotropic medications or steroids; use of topical corticosteroid creams within 4 weeks prior to screening
• Pregnancy, breastfeeding, or plans to become pregnant during the study
• Probiotic or prebiotic consumption or planned dietary changes within 4 weeks prior to screening
• Heavy caffeine consumption (>4 cups of 300 mL coffee per day)
• Drug or alcohol addiction or regular use of recreational drugs
• Smoking more than 10 cigarettes per day