Despite the growing discourse on sustainable diets, a critical gap persists in the integrated assessment of nutritional quality and environmental impact. While most analyses treat these dimensions in isolation, few provide a unified framework for their simultaneous evaluation. This study develops and applies a scalable framework for jointly assessing nutrient density and environmental impact using 119 commonly consumed foods and beverages in Spain. Nutrient density was measured with the Nutrient Rich Factor (NRF11.3) index, while environmental impact was quantified as kg CO2e/kg food based on Life Cycle Assessment. To evaluate how various analytical frameworks influence sustainability assessments, results were compared across three classification systems: food origin, a conventional food group classification, and the WHO-Nutrient-Profile-Model. The integrated analysis reveals nutritional-environmental patterns often overlooked in environmental assessments. Beyond the well-documented high impact of red meat, several sweet, highly processed products, such as chocolate, cookies, and confectionery, show CO2 emissions comparable to animal-based foods while providing very low nutrient density. In contrast, plant-based and minimally processed foods consistently combine high nutrient density with lower environmental impact across classification systems. By demonstrating how sustainability outcomes vary depending on nutritional, environmental, and food classification frameworks, this study underscores the value of integrated assessments for evidence-based dietary sustainability evaluations. Although Spain serves as the case study, this framework is globally transferable. It provides actionable insights for dietary guidelines, reformulation targets, labeling systems, and procurement policies designed to align public health with environmental targets, ultimately fostering more evidence-based food policy.