This study aims to evaluate the efficacy and safety of Bifidobacterium animalis subsp. lactis BLa36 in adults with functional constipation. In this randomized, double-blind, placebo-controlled trial, participants will receive BLa36 supplementation or placebo for 8 weeks. Clinical outcomes will be assessed using validated gastrointestinal symptom scales, stool characteristics, and quality-of-life measures. In addition, changes in inflammatory and immune markers, gut microbiota composition, metabolic indicators, and gut-brain axis-related parameters will be evaluated to comprehensively determine the effects of BLa36 on intestinal function and overall health.

Inclusion Criteria:

1. Adults aged 18-65 years;
2. Individuals diagnosed with constipation, defined as fewer than 3 bowel movements per week and/or Bristol Stool Form Scale (BSFS) types 1-2;
3. Able and willing to comply with all study procedures and complete the study;
4. Provided written informed consent prior to participation;
5. Participants (including males) must have no intention of conception from 14 days before screening until 6 months after study completion and agree to use effective contraception.

Exclusion Criteria:

1. Recent use of products with similar effects that may interfere with study outcomes; Significant dietary changes during the study period;
2. History of severe allergy or immunodeficiency;
3. Pregnant or breastfeeding women, or those planning pregnancy;
4. Presence of serious systemic diseases (e.g., cardiovascular, pulmonary, hepatic, renal, endocrine, or malignant conditions);
5. Use of antibiotics within the past 2 weeks;
6. Individuals with severe psychiatric or neurological disorders;
7. Poor compliance, including failure to take the study product or attend scheduled visits;
8. Any other condition deemed unsuitable by the investigator.