Objective: This study aimed to identify diets with improved nutrient quality and environmental impact within the boundaries of dietary practices.Design:We used Data Envelopment Analysis to benchmark diets for improved adherence to food-based dietary guidelines (FBDG). We then optimised these diets for dietary preferences, nutrient quality and environmental impact. Diets were evaluated using the Nutrient Rich Diet score (NRD15.3), diet-related greenhouse gas emission (GHGE) and a diet similarity index that quantified the proportion of food intake that remained similar as compared with the observed diet.Setting:National dietary surveys of four European countries (Denmark, Czech Republic, Italy and France). Subjects: Approximately 6500 adults, aged 18-64 years. Results: When dietary preferences were prioritised, NRD15·3 was ∼6 % higher, GHGE was ∼4 % lower and ∼85 % of food intake remained similar. This diet had higher amounts of fruit, vegetables and whole grains than the observed diet. When nutrient quality was prioritised, NRD15·3 was ∼16 % higher, GHGE was ∼3 % lower and ∼72 % of food intake remained similar. This diet had higher amounts of legumes and fish and lower amounts of sweetened and alcoholic beverages. Finally, when environmental impact was prioritised, NRD15·3 was ∼9 % higher, GHGE was ∼21 % lower and ∼73 % of food intake remained similar. In this diet, red and processed meat partly shifted to either eggs, poultry, fish or dairy.Conclusions:Benchmark modelling can generate diets with improved adherence to FBDG within the boundaries of dietary practices, but fully maximising health and minimising GHGE cannot be achieved simultaneously.