Vitamin D deficiency is common in the population with IBS which can increase depression, anxiety and low-grade inflammation in these patients. These factors lead to clinical symptoms of the disease. Serotonin (5-HT) plays an important role in the pathophysiology of IBS, that is due to increase in stress and inflammation, its production and secretion raise from the lumen and its metabolization to 5-HIAA is reduced. Consequently, we designed this study for assessment effect of vitamin D supplementation in diarrhea-predominant IBS patients.

88 IBS patients (age: 18-65) participated in a randomized, placebo-controlled trial study for 12 weeks. Subjects were randomly divided into each group that received weekly 50000 IU vitamin D3 or placebo. IBS severity score system (IBS-SSS), IBS-quality of life questionnaire (QoL), Hospital Anxiety and Depression Scale (HADs), Visceral sensitivity index (VSI) and serum 25(OH) vitamin D3, serotonin, 5-HIAA and ratio of 5-HIAA/5-HT were evaluated before and after interventions.

After 12 weeks intervention with vitamin D3, patients showed significant improvement in IBS symptoms such as abdominal pain severity and duration, bowel habit satisfaction, total IBS-SSS (mean score change: -115.90 ± 55.66 vs -45.43 ± 48.89, P˂0.0001), total QoL score (mean score change: -15.44 ± 13.17 vs -9.23 ± 18.27, P=0.049), HADs-depression (mean score change: -1.33 ± 2.98 vs -0.86 ± 3.82, P=0.023), VSI score (mean score change: -9.03 ± 8.72 vs -3.34 ± 9.44, P=0.008) as compared to placebo group. There were no significant differences in change abdominal bloating and HADs-stress scores between the two groups. Although we observed reduced in 5-HT, increased in 5-HIAA and 5-HIAA/5-HT but there were no significant statistical differences between groups.

Our results indicated that vitamin D supplementation can improve IBS symptoms, QoL, depression, visceral sensitivity related to anxiety and serum serotonin in IBS-D patients.