Irritable bowel syndrome (IBS) is a frequently encountered disorder. According to the Rome IV criteria, it is characterized by abdominal pain associated with a change in stool frequency or con-sistency, or with symptomatic improvement by defecation (Mearin 2016). Associated symptoms, such as bloating and flatulence, are frequently reported. The underlying pathophysiology remains obscure, although several pathways have been proposed. Low-grade immune activation, visceral hypersensitivity, alteration in gut microbiome have all been reported (Mearin 2016). As diet exerts an impact on all these pathophysiological mechanisms, the role of dietary intervention receives spe-cial attention, with special interest in the role played by so-called fermentable oligo-, di-, monosac-charides and polyols (FODMAPs). Multiple studies indicated the beneficial effects of the low FODMAP diet in at least part of the patients (Halmos 2014, Eswaran 2016, Staudacher 2017). As a disaccharide, lactose is part of the FODMAPs. Lactose intolerance (LI) results from lactose malabsorption (LM) secondary to insufficient hydrolysis of the disaccharide lactose into galactose and glucose (Misselwitz 2019). The undigested lactose will eventually reach the colon, resulting in fermentation from colonic bacteria with production of different compounds such as short chain fat-ty acids, carbon dioxide, H2 and methane (Catanzaro 2021). These compounds have an osmotic effect and can stimulate colonic contractions. In patients suffering from LI, these pathophysiologi-cal mechanisms generate symptoms such as abdominal pain and cramps, flatulence, diarrhea, in-creased bowel sounds, among others, similar to the mechanisms by which FODMAPs induce symp-toms of IBS. As dairy products are highly present in our Western diet, LI will often be considered in patients presenting with such symptoms and they will be referred for further testing. When LM is diagnosed, a lactose-free diet (LFD) will be advocated to alleviate symptoms. While the earlier-mentioned studies investigated symptomatic improvement by the low FODMAP diet, it remains uncertain whether this restrictive diet remains beneficial in patients without evidence of LM. In a recent study the low FODMAP diet and LFD provided comparable improvement in symptom severity (Krieger-Grübel 2020). This study aims to: * Assess the improvement in IBS symptoms and quality of life (QOL) by a low FODMAP di-et when lactose malabsorption has been previously excluded; * Compare the improvement in IBS symptoms and QOL obtained by a low FODMAP diet to a lactose free diet (data from the PreVaIL study).