Strengths and limitations
The study population is one of the major strengths of our study.We followed up a homogeneous population in which the initial diagnosis of functional constipation was made based on standardized Rome III criteria [3]. While different interventions were used in the original studies, both were similar in nature, i.e., focused on modification of gut microbiota, Hence, our decision to present the results of both cohorts in a single report. As initially no differences were found between the experimental and the control groups, our main focus was on long-term outcomes and not on the differences between groups. The response rate to the invitation to this follow-up study was high. This was particularly true for the GNN study that was carried out exclusively at our center. In regard to the B. lactis study, the response rate to this follow-up study was also high; how-ever, we only invited children living in Poland to participate.Thus, the results from the B. lactis study should be interinpreted with caution. Still, the findings compare well with those of the GNN study, which reduces the risk of attrition bias.