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The pathogenesis of chronic diarrhoeal diseases remains incompletely understood, so it is exciting when novel mechanistic insights emerge. In their recent Gut publication, Bannon et al 1 report a large elevation of the promotility enteroendocrine hormone insulin-like peptide 5 (INSL5) in the serum of patients with bile salt diarrhoea (BSD) and in a subset of patients with Irritable bowel syndrome with diarrhoea (IBS-D) without overt bile salt malabsorption, using a modified liquid chromatography-tandem mass spectrometry assay with increased sensitivity that they developed. This finding opens a new direction into how colonic hormones may regulate bowel function and contribute to diarrhoeal pathophysiology.