Plant-based diets have gained increasing attention for their potential role in preventive medicine, particularly in reducing the risk of chronic diseases such as type 2 diabetes, cardiovascular disease, obesity, and metabolic syndrome. This systematic review synthesizes evidence from 32 longitudinal studies to evaluate the impact of plant-based diets on disease prevention and health outcomes. The review identifies consistent patterns, including improved metabolic health, weight management, cardiovascular risk reduction, and positive effects on gut microbiome composition and inflammation. However, inconsistencies arise due to variability in diet definitions, mixed findings on specific outcomes, and heterogeneity in study populations. Critical gaps in the literature include the lack of long-term studies, limited mechanistic insights, underrepresentation of diverse populations, and a need for more rigorous intervention studies and personalized nutrition approaches. Identified research gaps highlight the need for long-term studies, deeper exploration of mechanistic pathways, and greater inclusivity of diverse populations. These insights underscore the significance of plant-based diets as a cornerstone of preventive medicine while emphasizing the necessity for targeted interventions and personalized approaches to maximize their benefits. The findings contribute to a growing body of evidence supporting the integration of plant-based dietary strategies into public health policies and clinical practices.