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Although food production is a main driver of environmental pressure and resource use globally, food delivers critical nutrition to humans. In life cycle assessment (LCA) of foods, the dominant functional unit is mass, despite the ISO requirement that the LCA unit should reflect the actual function. Studies have used various dietary quality scores in environmental assessments of foods, but the consideration of the dietary context is largely missing. The main function, i.e., nutrient supply, is complex since the nutritional value of a food item depends on its dietary context. Moreover, overall nutritional value is a combination of multiple nutrients.

The aim was to combine scientific knowledge from the fields of nutrition and LCA to generate a basis for further research. The long-term aim was to help develop methods to support sustainability-based planning and decision making by food chain stakeholders.

The proposed functional unit expresses the nutrient content of individual foods in relation to the nutritional supply of the complete diet, to create a single score reflecting the nutrient quality in a given dietary context. The nutrient quality index developed was evaluated by analyzing how relationships in global warming potential (GWP) between single products differed when using as functional unit either the mass of the food product, a nutrient quality index not considering the dietary context (the Nutrient Rich Foods Index 9.3, NRF9.3) and the new dietary dependent nutrient quality index (NQI) proposed. Two dietary scenarios were explored, an average Swedish diet and a typical unhealthy diet. The products considered were: bread, apples, tomatoes, milk, hard cheese, spread and chicken fillets.

The results, calculated using bread as the reference, indicated that in both dietary contexts apples, tomatoes, and hard cheese had lower NQJadjusted GWPs compared to when GWPs were calculated using mass as the functional unit. Milk's NQJ-adjusted and mass-calculated GWPs differed little, while the chicken fillet GWPs were the same in the unhealthy diet and performed better in the average diet. The NRF9.3-adjusted GWPs differed from the NQI-adjusted ones for all analysed foods. The main conclusions were: 1) considering nutritional value in the LCA of foods improves our understanding of how the environmental impacts and nutritional functions of food are related; 2) the environmental performance of different products varies with dietary context; and 3) application of the NQI could help industry, authorities, and consumers improve products and diets. (C) 2018 Elsevier Ltd. All rights reserved.