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251 Pre-, pro- and synbiotics 12 RESULTS Figure 1 shows the article selection process. Study characteristics of all included studies are summarized in Tables 2–4. Table 5 depicts the risk of bias assessment for all studies. Prebiotics Prebiotics are non-digestible food ingredients that beneficially affect the host by selectively stimulating growth and/or activity of one or a limited number of bacteria in the colon. 6 By this definition, many dietary fibers are prebiotics. Insufficient dietary fiber intake has been reported to be associated with FC. 7 This has led to the hypothesis that a lack of intake of prebiotics may be involved in the pathogenesis of constipation. Prebiotics may be beneficial for children with FC due to: 1) increase in water content (bulking effect and softening of stools); 2) increase in microbial mass and gas from fiber fermentation; 3) modification of colonic metabolite absorption, including secondary bile acids; 4) influencing gut microbes that affect motility. 8,9 We identified four systematic reviews 10–13 describing a total of six RCTs. In their crossover trial, Loening-Baucke  et al.  evaluated the effect of glucomannan (a fiber gel polysaccharide) compared to placebo in 31 children with FC >4 years of age. 8 . Children continued using their laxatives and were instructed to conduct toilet training throughout the study. Both groups received either placebo or glucomannan for 4 weeks. No significant difference was found in defecation frequency or fecal incontinence frequency between both groups after 4 weeks. There were however significant differences in the percentages of children with <3 bowel movements per week (glucomannan: 19% vs placebo: 52%, P < .05) and abdominal pain (glucomannan: 10% vs placebo: 42%, P < .05). It is important to note that the initial daily fiber intake was low in 71% of children. Castillejo  et al. performed a study where 48 children with FC received either a cocoa husk supplement or placebo, in addition to toilet training. 9 No significant differences in defecation frequency or parental report of pain during defecation were observed. In both study groups, the mean basal dietary fiber intake was near the recommended daily allowance. In a larger RCT by Kokke  et al ., the effect of a fiber mixture was compared with lactulose in 97 children with FC. 14 There were no significant differences between both groups with respect to defecation frequency, presence of ≥1 fecal incontinence episode(s) per week or abdominal pain. Baseline fiber intake was not reported. Üstündag  et al.  investigated partially hydrolyzed guar gum (PHGG) and compared this with lactulose in 61 children. 15 The authors found a significant improvement in defecation frequency in both groups, but children receiving lactulose had significantly more bowel