Impact of Dietary Fiber Type on Glycemic Control in Adults with Type 2 Diabetes: A Community-Based Trial

Thesis Abstract

This study addresses the rising burden of poor glycemic control among adults with type 2 diabetes in community settings and investigates whether the type of dietary fiber consumed differentially influences glycemic outcomes, independent of total fiber intake. It aims to determine whether soluble, insoluble, or mixed dietary fiber exerts distinct effects on fasting plasma glucose (FPG), HbA1c, postprandial glucose, and insulin resistance, and to elucidate adherence, feasibility, and acceptability of fiber-type recommendations in real-world diets. Specific objectives are to (1) compare changes in HbA1c over 12 weeks among participants randomized to high soluble fiber, high insoluble fiber, and mixed fiber diets within a community-based intervention; (2) assess differential effects on FPG, postprandial glucose (PPG), and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR); (3) evaluate changes in lipid profiles, inflammatory markers (CRP), and body weight; (4) explore adherence determinants and acceptability of fiber-type interventions using qualitative interviews; and (5) identify mediating factors such as gut microbiota-derived short-chain fatty acids (SCFAs) where feasible. The study is grounded in the Theory of Planned Behavior and the Dietary Acceptance Model to explain behavioral uptake and maintenance of fiber-type dietary patterns. A parallel-group, randomized controlled trial will be conducted in adults aged 40–70 years with diagnosed type 2 diabetes recruited from four primary health clinics serving diverse urban and peri-urban communities. A sample of 360 participants (120 per arm) will be randomly allocated to high soluble fiber (HSF), high insoluble fiber (HIF), or mixed fiber (MF) dietary interventions for 12 weeks, with a 6-week follow-up. All participants will receive standard diabetes education and continue usual pharmacotherapy; dietary interventions will emphasize target fiber types through personalized meal plans, food lists, and monthly counseling sessions delivered by trained dietitians. Data collection will include baseline, 6-week, 12-week, and 18-week follow-ups. Primary outcomes are changes in HbA1c and FPG; secondary outcomes include PPG, HOMA-IR, lipid panel (LDL-C, HDL-C, TG), CRP, blood pressure, weight, and waist circumference. Dietary intake will be monitored with 3-day weighed food records at each time point, and adherence will be assessed via urinary alkylresorcinol biomarkers for whole-grain intake and dietary fiber intake estimation. Optional sub-study in a sub-cohort (n=60) will analyze stool samples for microbial diversity and SCFA concentrations to explore mechanistic pathways. Analytical approaches will include intention-to-treat and per-protocol analyses. Primary outcomes will be analyzed using mixed-effects linear models with fixed effects for time, group, and time-by-group interaction, adjusting for age, sex, baseline BMI, diabetes duration, physical activity, and concomitant medications. Post-hoc pairwise comparisons will use Bonferroni correction. Secondary outcomes will be analyzed via generalized estimating equations for repeated measures. Mediation analyses will assess whether changes in BMI and CRP mediate glycemic changes. Qualitative data from semi-structured interviews (n=30 across groups) will be analyzed using thematic analysis to identify perceived barriers, facilitators, and cultural considerations, with triangulation to enhance validity. The trial will be registered, and ethical approval will be obtained from the university’s ethics committee; informed consent will be secured from all participants. Expected findings anticipate that the HSF group will exhibit greater reductions in HbA1c (mean difference ~0.5–0.8%), FPG, and HOMA-IR compared with MF and HIF groups, owing to superior fermentation and SCFA production associated with soluble fibers. The MF group may show intermediate improvements, while lipid reductions and inflammatory marker improvements are anticipated across groups, influenced by overall dietary quality and weight change. The study will contribute to knowledge by providing empirical evidence on the differential metabolic effects of dietary fiber types in a community setting, informing diabetes dietary guidelines, and identifying practical barriers to fiber-type adoption in real-world diets. Based on findings, recommendations will include tailored fiber-type dietary strategies integrated into standard diabetes care, with emphasis on cultural tailoring, affordable food choices, and strategies to sustain long-term adherence. The main conclusion is that fiber type, independent of total fiber intake, has modifiable effects on glycemic control and cardiometabolic risk in adults with type 2 diabetes, warranting incorporation of fiber-type guidance into community-based nutrition interventions.