REVIEW Dietary fiber effects in chronic kidney disease: a systematic review and meta-analysis of controlled feeding trials L Chiavaroli 1,2 , A Mirrahimi 2,3 , JL Sievenpiper 1,4,5,7 , DJA Jenkins 1,4,6,7 and PB Darling 1,2,4 BACKGROUND/OBJECTIVES: Chronic kidney disease (CKD) is a major health concern associated with increased risk of cardiovascular disease, morbidity and mortality. Current CKD practice guidelines overlook dietary fiber, which is chronically low in the renal diet. However, increasing dietary fiber has been proposed to ameliorate the progress of CKD. We therefore conducted a systematic review and meta-analysis on the effect of dietary fiber intake on serum urea and creatinine as classical markers of renal health in individuals with CKD. SUBJECTS/METHODS: We searched MEDLINE, EMBASE, CINHAL and the Cochrane Library for relevant clinical trials with a follow-up ⩾ 7 days. Data were pooled by the generic inverse variance method using random-effects models and expressed as mean difference (MD) with 95% confidence intervals (95% CIs). Heterogeneity was assessed by the Cochran Q statistic and quantified by I 2 . RESULTS: A total of 14 trials involving 143 participants met the eligibility criteria. Dietary fiber supplementation significantly reduced serum urea and creatinine levels in the primary pooled analyses (MD, - 1.76 mmol/l (95% CI, - 3.00, - 0.51), P o0.01 and MD, - 22.83 mmol/l (95% CI, - 42.63, - 3.02), P = 0.02, respectively) with significant evidence of interstudy heterogeneity only in the analysis of serum urea. CONCLUSIONS: This is the first study to summarize the potential beneficial effects of dietary fiber in the CKD population demonstrating a reduction in serum urea and creatinine, as well as highlighting the lack of clinical trials on harder end points. Larger, longer, higher-quality clinical trials measuring a greater variety of uremic toxins in CKD are required (NCT01844882). European Journal of Clinical Nutrition advance online publication, 12 November 2014; doi:10.1038/ejcn.2014.237 INTRODUCTION Despite the accumulating evidence on the beneficial effects of dietary fiber in ameliorating uremic environments, guidelines make little or no reference to dietary fiber intake for chronic kidney disease (CKD). 1–6 In fact, the renal diet has chronically been deficient in dietary fiber due to concerns over increased potassium and phosphorus intake. As a result, the benefits of fiber including adequate laxation are often overlooked in those with CKD. Stephen and Cummings 7 in the early 1980s were among the first to show that dietary fiber consumption increased fecal bacterial mass and nitrogen excretion. Their findings for dietary fiber intake have since been validated by others and support the notion of a lower uremic toxin production by bacterial degradation of dietary and secretory proteins of the gastrointest- inal tract, 8–11 as well as improved cardiovascular disease (CVD) risk factor profile and oxidative stress status. 12 A recent cohort study 13 concluded that participants with the highest dietary cereal fiber intake, compared with those with the lowest, had a 50% reduced risk for incidence of moderate CKD. Furthermore, in the recent Prevención con Dieta Mediterránea study, a significant association was found between greater fiber intake and reduced risk of CKD. 14 Unfortunately, according to NHANES III data, the average dietary fiber intake in the CKD population is about 15.4 g/day, which is much lower than the recommended 25–30 g/day intake for the general population. 15 Considering the potential benefits of dietary fiber and the very low average intake of this nutrient in the CKD population, we have conducted a systematic review and meta- analysis of controlled feeding trials to assess the effect of dietary fiber on serum urea and creatinine as clinical markers of uremia in individuals with CKD. SUBJECTS AND METHODS We conducted a systematic review and meta-analysis following the Cochrane Handbook for Systematic Reviews of Interventions 16 and have reported our findings according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. 17,18 The protocol is registered at Clinical- trials.gov (NCT01844882). Study selection We searched MEDLINE, EMBASE, CINAHL and the Cochrane Library through 1 September 2014 using the search terms '(dietary fiber OR fiber$ OR fibre$ OR polysaccharides OR psyllium$ OR metamucil OR polymers OR carbohydrate$ OR dietary carbohy- drate OR fermentable OR fructans OR Asteraceae OR fructooligo- saccharide$ OR oligofructose$ OR chicory root$ OR jerusalem artichoke$ OR inulin OR Benefiber OR Unifiber OR lactulose) AND 1 Department of Nutritional Sciences, University of Toronto, Toronto, Ontario, Canada; 2 Clinical Nutrition and Risk Factor Modification Centre, St. Michael's Hospital, Toronto, Ontario, Canada; 3 School of Medicine, Faculty of Health Sciences, Queen’s University, Kingston, Ontario, Canada; 4 Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada; 5 Department of Pathology and Molecular Medicine, Faculty of Health Sciences, McMaster University, Hamilton, Ontario, Canada; 6 Department of Medicine, Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada and 7 Division of Endocrinology, Department of Medicine, St. Michael's Hospital, Toronto, Ontario, Canada. Correspondence: Dr PB Darling, Li Ka Shing Knowledge Institute, St Michael's Hospital, 209 Victoria Street, 3rd floor, Toronto, M5B 1W8 Ontario, Canada. E-mail: pauline.darling@utoronto.ca This study was presented in part at the Canadian Nutrition Society conference, St. John’s Newfoundland, 5–7 June 2014. Received 25 June 2014; revised 11 September 2014; accepted 21 September 2014 European Journal of Clinical Nutrition (2014), 1 – 8 © 2014 Macmillan Publishers Limited All rights reserved 0954-3007/14 www.nature.com/ejcn