Clinical and mechanistic evidence on metabolic effects of oat β-glucan, rice bran, and unripe banana flour: a systematic review with mechanism-informed synthesis

Dietary fibre improves metabolic health, yet comparative evidence across different fibre sources remains unclear. This systematic review evaluates metabolic effects of oat β-glucan, rice bran, and unripe banana flour in adults, assessing glycaemic, lipid, and mechanistic effects, synthesising their physiological pathways across distinct fibre types (viscosity-driven, phytochemical-mediated, and resistant starch fermentation mechanisms). Following PRISMA 2020 guidelines, PubMed, Scopus, and Web of Science were searched up to June 2025. A total of 602 records were identified, and after removing 146 duplicates, 456 were screened. Forty-four full texts were assessed, and 15 studies were included. Risk of bias was evaluated using RoB 2.0 and Newcastle–Ottawa. Outcomes included glycaemic markers, insulin response, lipid profile, blood pressure, body composition, and mechanistic biomarkers. Studies (n = 15; sample sizes 10–154; durations 3 days–12 weeks) demonstrated heterogeneous dosing and formulations. Oat β-glucan consistently improved postprandial glucose, insulin AUC, and LDL-C, with modest effects on waist circumference and blood pressure. Rice bran improved blood lipids consistently, while glycaemic effects were mixed. UBF resistant starch was associated with reduction in fasting glucose, HOMA-IR, and body weight, although findings are based on small, short-duration trials. However, variability in study design and limited mechanistic assessments restricted cross-fibre synthesis. The available evidence suggests modest improvements in glycaemic and lipid outcomes. Differences in responsiveness likely reflect variations in fibre structure, viscosity, and fermentability, suggesting complementary physiological pathways. Standardised dosing, longer interventions, and mechanistic biomarkers – including SCFA profiles, incretin response, and bile acid signalling – are needed to clarify inter-individual variability and guide fibre-based interventions.