Background: Risk factors of pediatric inflammatory bowel disease (PIBD) are incompletely understood. Recently, we reported that among patients with PIBD, a previous diagnosis of cow's milk allergy (CMA) and asthma occurred more frequently than among age- and sex-matched peers. We tested at a national level the hypothesis that CMA is associated with the risk of contracting PIBD.

Methods: A nationwide birth cohort (n = 225,041), including all Finnish children born between 1999 and 2002, was followed up until July 1, 2014. We identified all children with a diagnosis of CMA, asthma, and PIBD from a national register; the data were based on certificates, including diagnostic criteria. The 95% confidence intervals (CI) for the incidence rates per 100,000 person-years were calculated assuming a Poisson distribution.

Results: We identified 7,910 infants with CMA yielding a cumulative incidence of 3.5% by 2 years of age. The cumulative incidence of PIBD was 0.14% (n = 316) and that of asthma 6.6% (14,807). Children with CMA were more likely to develop PIBD than non-CMA children, incidence ratio 2.6 (95% CI 1.7-3.8). Children with a diagnosis of CMA contracted PIBD at a younger age than the respective non-CMA group (9 vs. 11 years, p < 0.05). The risk was more evident for ulcerative colitis than for Crohn's disease. The association between CMA and asthma was stronger than that between CMA and PIBD.

Conclusions: CMA in infancy is associated with subsequent development of asthma and PIBD. This suggests that in a subgroup of patients, CMA may share underlying background with PIBD, warranting thorough follow-up.