Combined Probiotic and Vitamin D Supplementation Improves Psychological Outcomes and Reduces Low Grade Inflammation in Patients With Irritable Bowel Syndrome

ABSTRACT Background Irritable bowel syndrome (IBS) is a disorder of gut–brain interaction frequently accompanied by anxiety, depression, and low‐grade systemic inflammation. Intestinal dysbiosis and vitamin D deficiency may contribute to disease severity. This study evaluated the effects of probiotic supplementation, vitamin D replacement, and their combination on inflammatory biomarkers, IBS symptom severity, and psychological outcomes. Methods In this single‐center retrospective cohort study, 120 adults with IBS diagnosed according to the Rome IV criteria (January 2023–December 2025) received probiotic monotherapy, vitamin D monotherapy, or combined therapy. Clinical characteristics, inflammatory biomarkers, serum 25‐hydroxyvitamin D levels, Hospital Anxiety and Depression Scale (HADS) scores, and IBS Severity Scoring System (IBS‐SSS) scores were assessed before and after treatment. Correlation and multivariable logistic regression analyses were performed. Results The mean age was 41.8 ± 12.6 years, and 62% of participants were female. Combined therapy achieved the greatest reductions in C‐reactive protein, neutrophil‐to‐lymphocyte ratio, platelet‐to‐lymphocyte ratio, anxiety, depression, and IBS symptom severity. Serum 25‐hydroxyvitamin D levels were inversely correlated with depressive symptoms ( r = −0.31, p = 0.01). Combined therapy independently predicted psychological improvement after adjustment for age, sex, body mass index, and IBS subtype (OR 1.92, 95% CI 1.14–3.26, p = 0.01). Conclusions Combined probiotic and vitamin D supplementation was associated with greater improvements in psychological symptoms and inflammatory biomarkers than either monotherapy. These findings support targeting the gut–brain axis and vitamin D deficiency as complementary therapeutic strategies for IBS. Prospective randomized multicenter studies are warranted.