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Dietary choices, a main driver of food production, play a significant role within the climate change arena. Consequently, there is a growing trend on publishing research assessing the environmental impacts of diets and dietary shifts, mainly following the life cycle assessment (LCA) methodology. However, several methodological issues still bring a challenge, especially in the definition of the function and the quantification of the functional unit (FU). The FU is the reference unit of an LCA study, and it is the basis for allowing comparison among different systems. This short communication defines the function of diets as the supply of the daily required amount of calories and nutrients, and it proposes a novel FU that accounts for the energy intake and the nutritional quality of the diet. In order to compare the performance of the proposed FU to the most commonly ones used for diet LCAs (mass-based and isocaloric), dietary scenarios within the Spanish context are assessed. On the one hand, using a mass-based FU, greenhouse gas (GHG) emissions are underestimated, since the nutrition properties of food are not considered, and, on the other hand, the isocaloric substitution does not allow comparison among diets with different levels of energy intake. In contrast, the proposed caloric- and nutrient-corrected FU allows to compare diets that differ in energy and nutritional quality in a fairer way. Finally, it is recommended to use this FU for future diet LCAs. (C) 2019 Elsevier B.V. All rights reserved.