Plant-Based Nutrition and Type 2 Diabetes Mellitus: Prevention, Risk, Metabolic Health and the Role of Legumes, Dietary Fiber, and Red Meat

Background: Type 2 diabetes mellitus (T2D) is one of the fastest-growing chronic diseases worldwide, affecting an estimated 589 million adults in 2024, with projections rising to 853 million by 2050. Dietary modification is a cornerstone of both T2D prevention and management. Among the various dietary approaches, plant-based dietary patterns have gained increasing attention; however, uncertainty remains regarding the optimal dietary composition and the contribution of specific food groups. Methods: This narrative review summarizes the current evidence on plant-based dietary patterns and their association with T2D prevention and metabolic control, with a particular focus on legumes, dietary fiber, and red and processed meat. Evidence from prospective cohort studies, randomized controlled trials, and meta-analyses were evaluated. Results: Higher adherence to healthy plant-based dietary patterns was consistently associated with a 23-33% lower risk of developing T2D, improved insulin sensitivity, and approximately a 0.30- 0.47% reduction in glycated hemoglobin (HbA_1c). Greater consumption of legumes and dietary fiber was linked to improved glycemic control, enhanced gut microbiota composition, reduced low-grade inflammation, and beneficial effects on body weight and lipid metabolism. In contrast, higher intake of processed meat and unprocessed red meat was consistently associated with an increased risk of T2D. Importantly, unhealthy plant-based diets characterized by refined grains, sugar-sweetened beverages, and highly processed foods provided little metabolic benefit and may even increase diabetes risk by up to 10%. The interpretation of the available evidence is limited by the predominance of observational studies, heterogeneous definitions of plant-based dietary patterns, self-reported dietary assessments, and the limited availability of long-term randomized controlled trials. Conclusions: Current evidence supports healthy plant-based dietary patterns as an effective strategy for the prevention and management of T2D. Dietary patterns emphasizing whole plant foods, including legumes, whole grains, fruits, vegetables, nuts, and seeds, while limiting processed and excessive red meat intake, are associated with improved metabolic health. Future high-quality randomized controlled trials with standardized dietary definitions are needed to further define the optimal composition of plant-based diets for diabetes prevention and treatment.