• 5-hydroxytryptamine
• 5-hydroxyindole acetic acid
• diarrhoea
• irritable bowel syndrome serotonin
• postprandial symptoms

Postprandial symptoms are a common feature in patients with irritable bowel syndrome (IBS). In one study, one half of patients presenting with IBS reported symptom occurrence or exacerbation following a meal.¹ This effect of meals on gastrointestinal symptoms has been attributed to an increased colonic contractile response to meals in IBS patients. This colonic response has several components.

The first and most rapid component occurs within a few minutes of distension of the stomach by the meal and is mediated by gastric mechanoreceptors that evoke colonic contraction through a vagally mediated afferent pathway.

A second phase, mediated by chemoreceptors in the small intestine, results in colonic contraction that may last up to two hours after meal ingestion.² Prolonged manometry³ and barostat studies⁴ demonstrated that the increase in colonic motility after meals was almost immediate, and subsequently we and others reported that patients with diarrhoea and urgency predominant IBS experienced these symptoms in association with repetitive high amplitude propagated contractions that induce mass movements in the colon.^5,6

The third phase of the colonic contraction after the meal results from ileal stimulation by chyme and has been documented best in animals as it occurs 2–6 hours post-meal ingestion,⁷ a time when humans are often ingesting another meal and stimulating the first two components!

The first two phases of the colonic response to food involve serotonergic pathways: thus, antagonism of the serotonin (5-hydroxytryptamine (5-HT₃)) receptor reduces both components of the colonic response to meal ingestion.⁸

In this issue of Gut, Houghton and colleagues⁹ provide further support for the role of serotonin in mediating this response [see page 663]. They report increased postprandial serotonin levels in patients with diarrhoea predominant IBS and …