Diabetes mellitus is a chronic metabolic disorder characterized by impaired glucose regulation due to defects in insulin secretion, insulin action, or both. Nutrition plays a fundamental role in both the prevention and management of diabetes, influencing disease progression, glycemic control, complications, and overall patient outcomes. This review provides a comprehensive synthesis of the current literature on the interplay between nutrition and diabetes, focusing on macronutrient and micronutrient metabolism, dietary patterns, evidence-based nutritional interventions, and emerging perspectives in nutrigenomics and microbiome science. Special emphasis is given to type 1 diabetes, type 2 diabetes, and gestational diabetes, highlighting tailored nutritional strategies and their physiological underpinnings. The review also discusses controversies regarding low-carbohydrate, ketogenic, Mediterranean, and plant-based diets, as well as the role of intermittent fasting. Moreover, it includes future directions for personalized nutrition in diabetes care. By bridging mechanistic insights and clinical evidence, this work underscores the importance of nutrition as both a preventive and therapeutic cornerstone in diabetes management.