Background: Among patients with inflammatory bowel disease (IBD), age is a strong and well-known determinant of disease risk. Although peak incidence rates according to age have been widely reported, little is known about how these estimates vary according to sex. We aimed to explore the incidence of Crohn's disease (CD) and ulcerative colitis (UC) according to age and sex in population-based studies across the world.
Methods: A comprehensive literature search was conducted to identify representative population-based cohorts with IBD incidence data available for the full age spectrum starting from birth divided into 5-year or less intervals. Authors and guarantors of these cohorts were invited to share their data as collaborators. Background population data were also obtained. Incidence rates for CD and UC were calculated according to age and sex. We pooled incidence rate ratios (pIRR) for males:females (M:F) by random-effects meta-analysis according to the method of Desimonian and Laird.
Results: Data from 22 population-based or nationwide cohorts from 1988–2013 were included in the analysis. Among 269,136,094 males and 277,683,752 females, there were 69,689 total cases of CD (32,300 M/37,389 F) and 80,739 total cases of UC (41,807 M/38,932F). In CD, males younger than age 15 had a significantly higher incidence, peaking between age 10–14, compared to females (pIRR age 10–14 M:F 1.53, 95% CI: 1.39–1.68, p<0.001). There was an inversion of sex ratio after age 15, with females having increasingly higher incidence of CD, reaching statistical significance after age 40–44 (pIRR age 40–44 M:F 0.83, 95% CI: 0.74–0.94) [Figure 1]. Age of onset for UC in males and females was similar until age 40–44 (pIRR age 40–44 M:F 1.17, 95% CI: 1.0–1.36), after which males had a significantly higher incidence rate compared to females until age 75 (pIRR age 75–89 M:F 1.39, 95% CI: 1.03–1.87) [Figure 2].
Conclusions: Worldwide, there is significant variability in the sex ratio of IBD according to age at onset with males having higher incidence of CD before the age of 15, after which there is a female predominance, with an opposite trend in age at onset for UC where there is a striking male predominance after age 40. Variations in incidence rates according to sex across the globe may help identify unique, previously unrecognized genetic and environmental risk factors.