A 42-year-old woman presents with a 6-month history of abdominal pain, bloating and alternating diarrhoea and constipation. Blood tests including FBC, CRP and coeliac serology are normal. Abdominal and rectal examination are unremarkable. She meets Rome criteria for irritable bowel syndrome (IBS). She has already made general dietary changes but symptoms persist. What is the most appropriate next dietary advice?