| Summary: | Background: Several healthy diet indices have been associated with mortality risk. However, the ideal diet should not only be healthy but also environmentally friendly and affordable. This study aimed to determine if a new Sustainable Diet Index (SDI), which takes into account the nutritional quality, environmental impacts, and market price of diets, was associated with all-cause and cause-specific mortality. Methods: Using data from the Seguimiento Universidad de Navarra Project, a prospective cohort study of Spanish university graduates, the study included 15 492 participants (median age 34 years, IQR 27–45) recruited between Dec 1, 1999, and March 31, 2014 who were followed-up for a median of 10 years (IQR 7–14). Cox regression was used to determine the relationship of SDI and its components with all-cause and cause-specific mortality risk, where high values of SDI represent high nutritional quality, low environmental impacts, and low market price. Hazard ratios (HRs) with adjustment for several confounders were calculated. The weights for the foods contributing to the SDI were assessed with multiple regression analyses and variability with nested regression analyses. Findings: The highest quartile of the SDI scores was associated with a 59% relative reduction in all-cause mortality (HR 0·41, 95% CI 0·23–0·75) and a 79% reduction in cardiovascular mortality (0·21, 0·05–0·85). The nutritional index was itself statistically associated with a reduction in all-cause mortality, but not the environmental impact index and the market price. Nevertheless, a diet only focused on the nutritional quality would be less eco-friendly and more expensive than an overall sustainable diet. SDI was positively correlated with beans and potato consumption but negatively correlated with red meat intake. Red and processed meats, fatty dairy products, and fish consumption accounted for most of the variability in the SDI. Interpretation: Dietary patterns accounting not only for nutritional quality of the food but also the environmental impact and affordability could still provide health benefits. Such sustainable diets are rich in minimally processed plant-based foods and low in some animal-sourced products. Funding: The Seguimiento Universidad de Navarra Project has received funding from the Spanish Government—Instituto de Salud Carlos III, and the European Regional Development Fund (CIBEROBN, PI17/01795).
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