Internationally, IBS is thought to affect 67% more women than men (1). However, in certain regions, such as South Asia, South America, and Africa, prevalence rates in men and women are more similar, and possibly higher in men (2). Some authors have suggested that sex differences in IBS prevalence reflect international differences in the health seeking behaviour of women (1). According to this theory, in most (but not all) parts of the world, women are more likely to seek health professional advice for their symptoms, and thus are more likely to receive a diagnosis.
IBS affects people of all ages, including children and the elderly, although it is usually diagnosed before the age of 40 and is less common in people aged over 50 (3). This declining prevalence with age suggests that IBS symptoms diminish over time, a concept supported by data showing people aged over 65 years experience milder pain than people aged less than 65 years (4). It is also contrary to the popular belief that IBS is a lifelong condition, which if true, would see prevalence rates increase or remain stable over time (1).
IBS often clusters in families, although this may be more due to learned behaviours and environmental exposure than to genetic predisposition. This is suggested by twin studies that show that having a mother or father with IBS is a stronger predictor of IBS than having a twin with IBS1, (5).
IBS is considered a chronic disease in which symptoms flare intermittently over time. This was shown in large population study, whereby 40% of people with IBS were formally diagnosed over 10 years ago, and of these, nearly 60% experienced current symptoms (6). Another study (a systematic review that pooled data from 6 studies) showed that over a median follow time of 2 years, 2-18% of patients experienced a worsening of symptoms, 30% to 50% experienced stable symptoms and 12% to 38% experienced complete symptom resolution (7). Factors that predicted poorer outcomes in this study included previous surgery, longer duration of disease, coexisting anxiety and depression, and higher somatic scores (which indicate a tendency to experience psychological distress in the form of physical symptoms).
IBS symptoms may also change over time. For instance, natural history studies show that people commonly may transition from having IBS-C or IBS-D to IBS-M. These studies show that it is less common for people to transition between IBS-C and IBS-D7, although drawing conclusions about the natural course of IBS is difficult as it can be hard to discriminate between when symptom variation that reflects the natural course of IBS, from symptom variation influenced by IBS treatments (8).