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260 Chapter 12 Synbiotics Synbiotics contain both pre- and probiotics, usually the prebiotic compound selectively favors the probiotic compound. 6 We identified one RCT using synbiotics; a combination of probiotic strains of L. casei , L. rhamnosus, S. thermophilus, B. breve, L. acidophilus, B. infantis , and fructooligosaccharide as prebiotic. This study investigated the following three interventions in 97 children: liquid paraffin oil + placebo, synbiotics + placebo and liquid paraffin oil + synbiotics. 27 There was a significant difference in defecation frequency between the three groups after treatment; no separate analyses between the groups were performed, but children who received liquid paraffin oil + synbiotics experienced the largest increase in defecation frequency. DISCUSSION Studies were highly heterogeneous with respect to study design, study population, interventions used, dosages of treatment, study duration and follow-up. It is therefore difficult to draw firm conclusions. Although some studies found significant differences for pre- or probiotics with respect to defecation frequency, fecal incontinence or abdominal pain, these studies should be interpreted with caution due to the high risk of bias across studies. In addition, studies with the lowest risk of bias reported mostly non-significant results. 9,16,19 Defecation frequency was the only outcome measure that was used in all studies. Of these studies, 3/6 on prebiotics and 4/6 on probiotics reported a significant increase in defecation frequency compared to controls. However, it is important to take into account that this outcome measure was used differently between studies; some studies reported the mean or median defecation frequency at the end of the study (per day or per week) while others reported defecation frequency as the percentage of children having more than three bowel movements per week. As recently published, 28,29 there is a need for defining uniform outcome measures and using the same definitions in order to compare results between studies. Furthermore, it is also important to realize that small, statistically significant differences may not always be clinically relevant; this is especially true for the assessment of defecation frequency. The human microbiome is a fascinating and thrilling area of research that is likely to change our view on health and disease in the near future. However, for now, the precise role of the human microbiota in children with FC is not clear. In order to improve our understanding of the role of the gut microbiota in pediatric FC, a better understanding of the gut microbiota in healthy children is needed. A recently published study has identified a core gut