Randomized controlled trials of the effects of capsaicin or menthol on irritable bowel syndrome: a systematic review and meta-analysis.

Irritable bowel syndrome (IBS) is a common intestinal disease characterized by abdominal pain, abdominal distension and irregular defecation frequency, and it has had a high incidence and low cure rate in recent years. Visceral hypersensitivity (VH) is one of the main physiological indicators of IBS, and TRPV1 and TRPM8 (transient receptor potential vanilloid 1 and melastatin 8) play crucial roles in VH and are widely distributed in the intestine, significantly impacting abdominal pain in IBS patients. Under the guidance of PRISMA, four databases were systematically searched at the outset, including PubMed, Web of Science, Embase, and Cochrane Library. Randomized controlled trials (RCTs) reporting specific abdominal pain scores (rather than the incidence rate) in IBS patients receiving capsaicin or menthol (agonist of TRPV1 and TRPM8) interventions were included. A meta-analysis was conducted on the retrieved studies, which consisted of three articles on capsaicin and five articles on menthol, to compare the efficacy of capsaicin and menthol in alleviating abdominal pain in IBS patients under conditions of low heterogeneity. The results demonstrated that menthol had a significant effect in relieving abdominal pain in IBS patients. Conversely, although the effect of capsaicin was not statistically significant, two studies involving long-term capsaicin intervention suggested its potential to reduce VH and subsequently relieve abdominal pain, which may be attributed to the up-regulation of the TRPV1 receptor in the gastrointestinal tract of individuals with IBS.