Diets have become increasingly monotonous and high in ultra-processed foods (UPFs), contributing to poor health outcomes and environmental degradation. While sustainable diets, food biodiversity, and food processing levels have each been linked to nutritional and environmental outcomes, their combined impact has not been assessed.

To examine whether food biodiversity, intakes of UPFs, and adherence to the EAT-Lancet diet can simultaneously optimize nutrient adequacy while reducing environmental impacts.

Using data from 368,733 adults in the European Prospective Investigation into Cancer and Nutrition, we assessed associations and interactions between dietary species richness (DSR; disaggregated into DSR_Plant and DSR_Animal), food processing levels (Nova categories; %g/day), and adherence to EAT-Lancet recommendations (Healthy Reference Diet (HRD) score; 0-140 points) with the Probability of Adequate Nutrient Intake Diet (PANDiet) score, dietary greenhouse gas emissions (GHGe; kg CO₂-eq/day), and land use (m²/day). Regression models subsequently informed multi-objective optimization to identify optimal dietary patterns balancing nutritional and environmental outcomes.

Compared to observed diets, optimal diets showed on average: HRD score increased by 13.91 (95% CI: 13.89, 13.93) points; DSR_Plant by 1.36 (1.35, 1.37) species, while 12.44 (12.40, 12.49) percentage points substitution of UPFs with unprocessed or minimally processed foods was observed. Correspondingly, on average, PANDiet score increased by 4.12 (4.10, 4.14) percentage points, while GHGe and land use reduced by 1.07 (1.05, 1.09) kg CO₂-eq/day and 1.43 (1.41, 1.45) m²/day, respectively.