Current food consumption undermines both human and environmental health, particularly due to the overconsumption of animal-based foods. Transitioning toward more plant-based diets presents two key challenges. First, nutritional adequacy must be maintained, with iron being of particular concern, as nonheme iron from plant foods is less readily absorbed than iron from animal sources. Second, food substitutions must be acceptable to consumers for dietary change to occur. Diet modeling is a valuable tool for designing such diets. However, most models rely on constant iron absorption factors rather than more accurate diet-dependent equations, and often assess acceptability only at food-group level, ignoring food-item level. Therefore, this thesis aimed to improve the nutritional adequacy, sustainability, and acceptability of modeled diets by evaluating and developing new methods for modeling iron absorption and food substitution. Our research showed that incorporating diet-dependent iron absorption equations more effectively increased absorbable nonheme iron than constant factors, as they allow optimization of both enhancers and inhibitors of absorption. These nonlinear equations can be integrated using nonlinear programming or piecewise linear approximation. The latter outperformed nonlinear programming in mixed-integer models in both solution quality, and computation time, while performance was similar in continuous models. Additionally, modeling food substitution at food-item level, allowing both within- and between-group substitutions, required only half as much dietary change to meet nutrition and sustainability targets compared to food-group-level modeling. We also introduced a novel data-driven substitution method using recipe completion to identify commonly combined foods. This approach achieved higher acceptability than substitutions based on broad food groups and higher nutritional adequacy than substitutions based on narrow food groups. Overall, the methods developed in this thesis support the design of diets that are healthy, sustainable, and acceptable. They are particularly valuable for future research aimed at promoting sustainable diets with reduced animal-based products, while ensuring nutritional adequacy and acceptability.