AIMS: The Optimal Fibre Trial investigated metabolic effects of insoluble cereal fibre in subjects with high-risk prediabetes. As the study shows dose-dependent moderate glycemic and anti-inflammatory benefits, especially in subjects with an obesity-related phenotype, the putative mechanism of action of this particular food component warrants clarification. A sub-group of the OptiFiT cohort received detailed body imaging throughout the study, permitting the analysis of effects on body fat distribution by fibre supplementation. METHODS: 180 Caucasian participants with impaired glucose tolerance (IGT) received a blinded, randomized supplementation with insoluble cereal fibre or placebo for two years. Once a year, all subjects underwent fasting blood sampling, oGTT and full anthropometric measurements. A subgroup of 47 subjects additionally provided data from magnetic resonance imaging and spectroscopy for quantification of adipose tissue distribution and liver fat content. We compare these outcomes between fibre and placebo group and assess mechanistic connections to improvements in glucose metabolism and inflammation. RESULTS: MR-assessed visceral and non-visceral body fat volume, fasting glucose, HbA1c, fasting insulin, insulin resistance and uric acid decreased in the fibre group, only. However, after adjustment for weight loss, there are no significant differences in changes of MR-derived measurements of body fat distribution between the intervention groups. There is a statistical trend for fibre-driven liver fat reduction in subjects with confirmed non-alcoholic fatty liver disease (NAFLD; n = 19). The entire MR subgroup shows the same pattern in metabolic improvements as the entire cohort. CONCLUSIONS: Data and evidence on beneficial effects of insoluble cereal fibre on visceral and hepatic fat storage is limited, but warrants further research. Targetted trials assessing the usefulness of fibre in visceral obesity and NAFLD are required.