Comparative Effects of Plant-Based vs. Omnivorous Diets on Lipid Profiles

Thesis Abstract

This study addresses the growing public health challenge of dyslipidemia by examining how dietary pattern—specifically plant-based versus omnivorous diets—modulates lipid profiles in adults under free-living conditions. The aim is to determine whether a plant-based diet yields superior lipid outcomes compared with a conventional omnivorous diet, and to identify mediators and moderators of any observed effects. The specific objectives are (a) to compare baseline and post-intervention lipid parameters (total cholesterol, LDL-C, HDL-C, triglycerides) between diet groups over a 12-week period; (b) to assess changes in secondary markers including apolipoprotein B, lipoprotein(a), and inflammatory biomarkers (hs-CRP, IL-6); (c) to evaluate adherence, diet quality, and physical activity as potential mediators; (d) to explore the association between dietary fiber intake, saturated fat intake, and lipid changes using multivariate regression; and (e) to examine whether body weight and body composition modify the diet-lipid relationship. A randomized controlled trial design was employed, with participants allocated to a plant-based diet (n = 150) or an omnivorous diet (n = 150) for 12 weeks. The population consisted of adults aged 25–55 years with at least one cardiovascular risk factor but without diagnosed cardiovascular disease, recruited from primary care clinics and community settings in a metropolitan region. Data collection instruments included validated food frequency questionnaires and multiple 24-hour recalls to assess dietary intake, standardized anthropometric measurements, fasting venous blood samples for lipid panels and biomarkers, and accelerometer-based physical activity monitoring. The primary data analysis used intention-to-treat principles, employing repeated-measures ANOVA to compare lipid trajectories between groups, with post-hoc contrasts to identify time-by-diet interactions. Multivariate linear regression models adjusted for age, sex, baseline BMI, medication use, physical activity, and caloric intake were conducted to identify independent associations between diet type and lipid outcomes. Mediation analysis tested whether changes in dietary fiber and saturated fat intake mediated lipid changes. Sensitivity analyses included per-protocol analyses and multiple imputation for missing data. Theoretical grounding integrated the Diet-Health Nexus framework and the Dietary Approaches to Stop Hypertension (DASH) family of models to interpret how dietary pattern shifts influence lipid metabolism, complemented by the Health Belief Model to contextualize adherence behaviors. Key expected findings include greater reductions in total cholesterol and LDL-C in the plant-based group relative to the omnivorous group, with modest increases in HDL-C and reductions in triglycerides. It is anticipated that reductions in saturated fat and increases in dietary fiber will partially mediate lipid improvements, with fiber intake explaining a meaningful portion of the LDL-C decline. The study also expects that weight stability will not fully account for lipid changes, suggesting diet composition exerts independent effects on lipid metabolism. Subgroup analyses may reveal stronger lipid responses among participants with higher baseline LDL-C and those with higher dietary fiber propensity. The study contributes to knowledge by providing robust, controlled evidence on the comparative lipid effects of plant-based versus omnivorous diets in a diverse adult population, clarifying mediation pathways via dietary constituents, and informing dietary guidelines and public health strategies for dyslipidemia prevention. The findings are expected to support nuanced dietary recommendations that emphasize fiber-rich, plant-forward patterns while acknowledging individual variability in response. The main conclusion anticipated is that transitioning to a well-planned plant-based diet can yield clinically meaningful lipid improvements beyond conventional omnivorous patterns, mediated in part by reduced saturated fat intake and increased dietary fiber. Practical recommendations include structured dietary support for adherence, emphasis on whole-food plant sources, tailored guidance for individuals with elevated LDL-C, and consideration of caloric balance to optimize lipid outcomes. Future research suggestions include longer-term follow-up to assess cardiovascular event endpoints, exploration of gut microbiome mediators, and cost-effectiveness analyses of plant-based dietary interventions in diverse settings.