Melatonin Regulation as a Possible Mechanism for Probiotic (VSL#3) in Irritable Bowel Syndrome: A Randomized Double-Blinded Placebo Study

Probiotics have treatment efficacy in irritable bowel syndrome (IBS), but the exact mechanism remains obscure. One hypothesis is the mediation of melatonin levels, leading to changes in IBS symptoms.

The purpose of this study was to evaluate the effects of a probiotic, VSL#3, on symptoms, psychological and sleep parameters, and pain sensitivity in IBS, and relate these parameters to in vivo melatonin levels.

Forty-two IBS patients were randomly assigned to receive VSL#3 or placebo for 6 weeks. Subjects completed bowel and psychological questionnaires, underwent rectal sensitivity testing and saliva melatonin assays.

Abdominal pain duration and distension intensity decreased significantly in the probiotic group, along with an increase in rectal distension pain thresholds. A correlation between increase in pain tolerance and improvement in abdominal pain scores (r = 0.51, p = 0.02) was seen with probiotic. There was an increase in salivary morning melatonin levels in males treated with VSL#3, which correlated (r = 0.61) with improved satisfaction in bowel habits. When grouped based on baseline diurnal melatonin levels, patients with normal diurnal fluctuations showed an increase in morning melatonin levels with VSL#3 treatment, which significantly correlated with improved satisfaction in bowel habits (r = 0.68). They also had reduced symptom severity scores and abdominal pain duration when treated with VSL#3, as well as satisfaction with bowel movements and quality-of-life.

VSL#3 improved symptoms and increased rectal pain thresholds. Symptom improvement correlated with a rise in morning melatonin, significant in males and subjects with normal circadian rhythm. This suggests that probiotics may act by influencing melatonin production, hence modulating IBS symptoms, in individuals with a normal circadian rhythm.